Climate change mitigation

Fossil fuel related CO2 emissions compared to five of IPCC's emissions scenarios. The dips are related to global recessions. Data from IPCC SRES scenarios; Data spreadsheet included with International Energy Agency's "CO2 Emissions from Fuel Combustion 2010 - Highlights"; and Supplemental IEA data. Image source: Skeptical Science

Global mean surface temperature change since 1880, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. Source: NASA GISS. Global dimming, from sulfate aerosol air pollution, between 1950 and 1980 is believed to have mitigated global warming somewhat.

Global carbon dioxide emissions from human activities 1800–2007.^[1]

Greenhouse gas emissions by sector. See World Resources Institute for a detailed breakdown.

Global public support for energy sources, based on a survey by Ipsos (2011).^[2]

Climate change mitigation consists of actions to limit the magnitude or rate of long-term climate change.^[3] Climate change mitigation generally involves reductions in human (anthropogenic) emissions of greenhouse gases (GHGs).^[4] Mitigation may also be achieved by increasing the capacity of carbon sinks, e.g., through reforestation.^[4] Mitigation policies can substantially reduce the risks associated with human-induced global warming.^[5]

"Mitigation is a public good; climate change is a case of ‘the tragedy of the commons’" Effective climate change mitigation will not be achieved if each agent (individual, institution or country) acts independently in its own selfish interest, (See International Cooperation and Emissions Trading) suggesting the need for collective action. Some adaptation actions, on the other hand, have characteristics of a private good as benefits of actions may accrue more directly to the individuals, regions, or countries that undertake them, at least in the short term. Nevertheless, financing such adaptive activities remains an issue, particularly for poor individuals and countries."^[6]

Examples of mitigation include switching to low-carbon energy sources, such as renewable and nuclear energy, and expanding forests and other "sinks" to remove greater amounts of carbon dioxide from the atmosphere.^[4] Energy efficiency may also play a role,^[7] for example, through improving the insulation of buildings.^[8] Another approach to climate change mitigation is climate engineering.^[9]

Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC).^[10] The ultimate objective of the UNFCCC is to stabilize atmospheric concentrations of GHGs at a level that would prevent dangerous human interference of the climate system.^[11] Scientific analysis can provide information on the impacts of climate change, but deciding which impacts are dangerous requires value judgments.^[12]

In 2010, Parties to the UNFCCC agreed that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.^[13] This may be revised with a target of limiting global warming to below 1.5 °C relative to pre-industrial levels. The current trajectory of global greenhouse gas emissions does not appear to be consistent with limiting global warming to below 1.5 or 2 °C, relative to pre-industrial levels.^[14] Other mitigation policies have been proposed, some of which are more stringent^[15] or modest^[16][17] than the 2 °C limit.

Background

Greenhouse gas concentrations and stabilization

Stabilizing CO₂ emissions at their present level would not stabilize its concentration in the atmosphere.^[18]

Stabilizing the atmospheric concentration of CO₂ at a constant level would require emissions to be effectively eliminated.^[18]

One of the issues often discussed in relation to climate change mitigation is the stabilization of greenhouse gas concentrations in the atmosphere. The United Nations Framework Convention on Climate Change (UNFCCC) has the ultimate objective of preventing "dangerous" anthropogenic (i.e., human) interference of the climate system. As is stated in Article 2 of the Convention, this requires that greenhouse gas (GHG) concentrations are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can proceed in a sustainable fashion.^[19]

There are a number of anthropogenic greenhouse gases. These include carbon dioxide (chemical formula: CO₂), methane (CH
4), nitrous oxide (N
2O), and a group of gases referred to as halocarbons.^[20] The emissions reductions necessary to stabilize the atmospheric concentrations of these gases varies.^[18] CO₂ is the most important of the anthropogenic greenhouse gases (see radiative forcing).^[21]

There is a difference between stabilizing CO₂ emissions and stabilizing atmospheric concentrations of CO₂.^[22] Stabilizing emissions of CO₂ at current levels would not lead to a stabilization in the atmospheric concentration of CO₂. In fact, stabilizing emissions at current levels would result in the atmospheric concentration of CO₂ continuing to rise over the 21st century and beyond (see the graphs opposite).

The reason for this is that human activities are adding CO₂ to the atmosphere far faster than natural processes can remove it (see carbon dioxide in Earth's atmosphere for a more complete explanation).^[18] This is analogous to a flow of water into a bathtub.^[23] So long as the tap runs water (analogous to the emission of carbon dioxide) into the tub faster than water escapes through the plughole (the natural removal of carbon dioxide from the atmosphere), then the level of water in the tub (analogous to the concentration of carbon dioxide in the atmosphere) will continue to rise.

According to some studies, stabilizing atmospheric CO₂ concentrations would require anthropogenic CO₂ emissions to be reduced by 80% relative to the peak emissions level.^[24] An 80% reduction in emissions would stabilize CO₂ concentrations for around a century, but even greater reductions would be required beyond this.^[18][24] Other research has found that, after leaving room for emissions for food production for 9 billion people and to keep the global temperature rise below 2 °C, emissions from energy production and transport will have to peak almost immediately in the developed world and decline at ca. 10% per annum until zero emissions are reached around 2030. In developing countries energy and transport emissions would have to peak by 2025 and then decline similarly.^{[25][26][27][28]}

Stabilizing the atmospheric concentration of the other greenhouse gases humans emit also depends on how fast their emissions are added to the atmosphere, and how fast the GHGs are removed. Stabilization for these gases is described in the later section on non-CO₂ GHGs.

Projections

Projections of future greenhouse gas emissions are highly uncertain.^[29] In the absence of policies to mitigate climate change, GHG emissions could rise significantly over the 21st century.^[30]

Numerous assessments have considered how atmospheric GHG concentrations could be stabilized.^[31] The lower the desired stabilization level, the sooner global GHG emissions must peak and decline.^[32] GHG concentrations are unlikely to stabilize this century without major policy changes.^[30]

Projected carbon dioxide emissions and atmospheric concentrations over the 21st century for reference and mitigation scenarios.

Rate of world energy usage per day, from 1970 to 2010. Every fossil fuel source has increased in large amounts between 1970 and 2010, dominating all other energy sources. Hydroelectricity has increased at a slow steady rate over this same period, nuclear entered a period of rapid growth between 1970 and 1990 before levelling off. Other Renewables, between 2000 and 2010 have, having started from a low usage rate, began to enter into a period of rapid growth. 1000TWh=1PWh.[33]

Energy consumption by power source

"Hydropower-Internalised Costs and Externalised Benefits"; Frans H. Koch; International Energy Agency (IEA)-Implementing Agreement for Hydropower Technologies and Programmes; 2000.

To create lasting climate change mitigation, the replacement of high carbon emission intensity power sources, such as conventional fossil fuels—oil, coal and natural gas—with low-carbon power sources is required. Fossil fuels supply humanity with the vast majority of our energy demands, and at a growing rate. In 2012 the IEA noted that coal accounted for half the increased energy use of the prior decade, growing faster than all renewable energy sources.^[34] Both hydroelectricity and nuclear power together provide the majority of the generated low-carbon power fraction of global total power consumption.

Fuel type	Average total global power consumption in TW[35]
	1980	2004	2006
Oil	4.38	5.58	5.74
Gas	1.80	3.45	3.61
Coal	2.34	3.87	4.27
Hydroelectric	0.60	0.93	1.00
Nuclear power	0.25	0.91	0.93
Geothermal, wind, solar energy, wood	0.02	0.13	0.16
Total	9.48	15.0	15.8
Source: The USA Energy Information Administration

Change and use of energy, by source, in units of (PWh) in that year.[36]
	Fossil	Nuclear	All renewables	Total
1990	83.374	6.113	13.082	102.569
2000	94.493	7.857	15.337	117.687
2008	117.076	8.283	18.492	143.851
Change 2000–2008	22.583	0.426	3.155	26.164

Methods and means

This graph shows the projected contribution of various energy sources to world primary electricity consumption (PEC).^[37] It is based on a climate change mitigation scenario, in which GHG emissions are substantially reduced over the 21st century. In the scenario, emission reductions are achieved using a portfolio of energy sources, as well as reductions in energy demand. Also available in greyscale.

Assessments often suggest that GHG emissions can be reduced using a portfolio of low-carbon technologies.^[38] At the core of most proposals is the reduction of greenhouse gas (GHG) emissions through reducing energy waste and switching to low-carbon power sources of energy. As the cost of reducing GHG emissions in the electricity sector appears to be lower than in other sectors, such as in the transportation sector, the electricity sector may deliver the largest proportional carbon reductions under an economically efficient climate policy.^[39]

"Economic tools can be useful in designing climate change mitigation policies." "While the limitations of economics and social welfare analysis, including cost–benefit analysis, are widely documented, economics nevertheless provides useful tools for assessing the pros and cons of taking, or not taking, action on climate change mitigation, as well as of adaptation measures, in achieving competing societal goals. Understanding these pros and cons can help in making policy decisions on climate change mitigation and can influence the actions taken by countries, institutions and individuals."^[6]

Other frequently discussed means include energy conservation, increasing fuel economy in automobiles (which includes the use of electric hybrids), charging plug-in hybrids and electric cars by low-carbon electricity, making individual-lifestyle changes^[40] (e.g., cycling instead of driving),^[41] and changing business practices. Many fossil fuel driven vehicles can be converted to use electricity, the U.S. has an estimated capacity of supporting 73% light duty vehicles (LDV). In terms of transportation, the net result would be a 27% total reduction in emissions of the greenhouse gases carbon dioxide, methane, and nitrous oxide, a 31% total reduction in nitrogen oxides, a slight reduction in nitrous oxide emissions, an increase in particulate matter emissions, the same sulfur dioxide emissions, and the near elimination of carbon monoxide and volatile organic compound emissions (a 98% decrease in carbon monoxide and a 93% decrease in volatile organic compounds). The emissions would be displaced away from street level, where they have "high human-health implications."^[42]

A range of energy technologies may contribute to climate change mitigation.^[43] These include nuclear power and renewable energy sources such as biomass, hydroelectricity, wind power, solar power, geothermal power, ocean energy, and; the use of carbon sinks, and carbon capture and storage. For example, Pacala and Socolow of Princeton^[44] have proposed a 15 part program to reduce CO₂ emissions by 1 billion metric tons per year − or 25 billion tons over the 50-year period using today's technologies as a type of Global warming game.^[45]

Another consideration is how future socio-economic development proceeds. Development choices (or "pathways") can lead differences in GHG emissions.^[46] Political and social attitudes may affect how easy or difficult it is to implement effective policies to reduce emissions.^[47]

Alternative energy sources

Renewable energy

The 22,500 MW nameplate capacity Three Gorges Dam in the Peoples Republic of China, the largest hydroelectric power station in the world.

The Shepherds Flat Wind Farm is an 845 megawatt (MW) nameplate capacity, wind farm in the U.S. state of Oregon, each turbine is a nameplate 2 or 2.5 MW electricity generator.

The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity for 7.5 hours after the sun has stopped shining.^[48]

Solar cookers use sunlight as energy source for outdoor cooking.

Renewable energy flows involve natural phenomena such as sunlight, wind, rain, tides, plant growth, and geothermal heat, as the International Energy Agency explains:^[49]

Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources.

Climate change concerns^[50][51][52] and the need to reduce carbon emissions are driving increasing growth in the renewable energy industries.^[53][54][55] Low-carbon renewable energy replaces conventional fossil fuels in three main areas: power generation, hot water/ space heating, and transport fuels.^[56] In 2011, the share of renewables in electricity generation worldwide grew for the fourth year in a row to 20.2%.^[57] Based on REN21's 2014 report, renewables contributed 19% to supply global energy consumption. This energy consumption is divided as 9% coming from burning biomass, 4.2% as heat energy (non-biomass), 3.8% hydro electricity and 2% as electricity from wind, solar, geothermal, and biomass thermal power plants.^[58]

Renewable energy use has grown much faster than anyone anticipated.^[59] The Intergovernmental Panel on Climate Change (IPCC) has said that there are few fundamental technological limits to integrating a portfolio of renewable energy technologies to meet most of total global energy demand.^[60] At the national level, at least 30 nations around the world already have renewable energy contributing more than 20% of energy supply.

As of 2012, renewable energy accounts for almost half of new electricity capacity installed and costs are continuing to fall.^[61] Public policy and political leadership helps to "level the playing field" and drive the wider acceptance of renewable energy technologies.^[62] As of 2011, 118 countries have targets for their own renewable energy futures, and have enacted wide-ranging public policies to promote renewables.^[63][64] Leading renewable energy companies include BrightSource Energy, First Solar, Gamesa, GE Energy, Goldwind, Sinovel, Suntech, Trina Solar, Vestas and Yingli.^[65][66]

The incentive to use 100% renewable energy has been created by global warming and other ecological as well as economic concerns.^[59] Mark Z. Jacobson says producing all new energy with wind power, solar power, and hydropower by 2030 is feasible and existing energy supply arrangements could be replaced by 2050. Barriers to implementing the renewable energy plan are seen to be "primarily social and political, not technological or economic". Jacobson says that energy costs with a wind, solar, water system should be similar to today's energy costs.^[67] According to a 2011 projection by the (IEA)International Energy Agency, solar power generators may produce most of the world's electricity within 50 years, dramatically reducing harmful greenhouse gas emissions.^[68] Critics of the "100% renewable energy" approach include Vaclav Smil and James E. Hansen. Smil and Hansen are concerned about the variable output of solar and wind power, NIMBYism, and a lack of infrastructure.^[69]

Economic analysts expect market gains for renewable energy (and efficient energy use) following the 2011 Japanese nuclear accidents.^[70][71] In his 2012 State of the Union address, President Barack Obama restated his commitment to renewable energy and mentioned the long-standing Interior Department commitment to permit 10,000 MW of renewable energy projects on public land in 2012.^[72] Globally, there are an estimated 3 million direct jobs in renewable energy industries, with about half of them in the biofuels industry.^[73]

Some countries, with favorable geography, geology and weather well suited to an economical exploitation of renewable energy sources, already get most of their electricity from renewables, including from geothermal energy in Iceland (100 percent), and Hydroelectric power in Brazil (85 percent), Austria (62 percent), New Zealand (65 percent), and Sweden (54 percent).^[74] Renewable power generators are spread across many countries, with wind power providing a significant share of electricity in some regional areas: for example, 14 percent in the U.S. state of Iowa, 40 percent in the northern German state of Schleswig-Holstein, and 20 percent in Denmark. Solar water heating makes an important and growing contribution in many countries, most notably in China, which now has 70 percent of the global total (180 GWth). Worldwide, total installed solar water heating systems meet a portion of the water heating needs of over 70 million households. The use of biomass for heating continues to grow as well. In Sweden, national use of biomass energy has surpassed that of oil. Direct geothermal heating is also growing rapidly.^[74] Renewable biofuels for transportation, such as ethanol fuel and biodiesel, have contributed to a significant decline in oil consumption in the United States since 2006. The 93 billion liters of biofuels produced worldwide in 2009 displaced the equivalent of an estimated 68 billion liters of gasoline, equal to about 5 percent of world gasoline production.^[74]

Some of the world's largest solar power stations: Ivanpah (CSP) and Topaz (PV)

Nuclear power

Blue Cherenkov light being produced near the core of the Fission powered Advanced Test Reactor

Since about 2001 the term "nuclear renaissance" has been used to refer to a possible nuclear power industry revival, driven by rising fossil fuel prices and new concerns about meeting greenhouse gas emission limits.^[75] However, in March 2011 the Fukushima nuclear disaster in Japan and associated shutdowns at other nuclear facilities raised questions among some commentators over the future of nuclear power.^[76][77][78] Platts has reported that "the crisis at Japan's Fukushima nuclear plants has prompted leading energy-consuming countries to review the safety of their existing reactors and cast doubt on the speed and scale of planned expansions around the world".^[79]

The World Nuclear Association has reported that nuclear electricity generation in 2012 was at its lowest level since 1999.^[80] Several previous international studies and assessments,^[81][82][83] suggested that as part of the portfolio of other low-carbon energy technologies, nuclear power will continue to play a role in reducing greenhouse gas emissions. Historically, nuclear power usage is estimated to have prevented the atmospheric emission of 64 gigatonnes of CO2-equivalent as of 2013.^[84] Public concerns about nuclear power include the fate of spent nuclear fuel, nuclear accidents, security risks, nuclear proliferation, and a a concern that nuclear power plants are very expensive.^[85][86][87] Of these concerns, nuclear accidents and disposal of long-lived radioactive waste have probably had the greatest public impact worldwide.^[85]

Nuclear power, with a 10.6% share of world electricity production as of 2013, is second only to hydroelectricity as the largest source of low-carbon power.^[88] Over 400 reactors generate electricity in 31 countries.^[89]

A Yale University review published in the Journal of Industrial Ecology analyzing CO₂ life cycle assessment emissions from nuclear power determined that: "The collective LCA literature indicates that life cycle GHG emissions from nuclear power are only a fraction of traditional fossil sources and comparable to renewable technologies."^[90] Uncertainty surrounding the future GHG emissions of nuclear power have to do with the potential for a declining uranium ore grade without a corresponding increase in the efficiency of enrichment methods. In a scenario analysis of future global nuclear development, as it could be effected by a decreasing global uranium market of average ore grade, the analysis determined that depending on conditions, median life cycle nuclear power GHG emissions could be between 9 and 110 g CO₂-eq/kWh by 2050.^[90]

Although this future analyses deals with extrapolations for present Generation II reactor technology, the same paper also summarizes the literature on "FBRs"/Fast Breeder Reactors, of which two are in operation as of 2014 with the newest being the BN-800, for these reactors it states that the "median life cycle GHG emissions ... [are] similar to or lower than [present] LWRs and purports to consume little or no uranium ore.^[90]

In their 2014 report, the IPCC comparison of energy sources global warming potential per unit of electricity generated, which notably included albedo effects, found that nuclear power's median CO2-equivalent value of 12 g CO2-eq/kWh is the lowest global warming forcing of all baseload power sources, with comparable low carbon power baseload sources, such as hydropower and biomass, both producing substantially more global warming forcing 24 and 230 g CO2-eq/kWh respectively.^[91]

In 2014, Brookings Institution published The Net Benefits of Low and No-Carbon Electricity Technologies which states, after performing an energy and emissions cost analysis, that "The net benefits of new nuclear, hydro, and natural gas combined cycle plants far outweigh the net benefits of new wind or solar plants", with the most cost effective low carbon power technology being determined to be nuclear power.^[92][93][94]

During his presidential campaign, Barack Obama stated, "Nuclear power represents more than 70% of our noncarbon generated electricity. It is unlikely that we can meet our aggressive climate goals if we eliminate nuclear power as an option."^[95]

This graph illustrates nuclear power is the USA's largest contributor of non-greenhouse-gas-emitting electric power generation, comprising nearly three-quarters of the non-emitting sources.

Nuclear power may be uncompetitive compared with fossil fuel energy sources in countries without a carbon tax program, and in comparison to a fossil fuel plant of the same power output, nuclear power plants take a longer amount of time to construct.^{[96][97][98][99]}

Two new, first of their kind, EPR reactors under construction in Finland and France have been delayed and are running over-budget.^{[100][101][102]} However learning from experience, two further EPR reactors under construction in China are on, and ahead, of schedule respectively.^[103] As of 2013, according to the IAEA and the European Nuclear Society, worldwide there were 68 civil nuclear power reactors under construction in 15 countries.^[104][105] China has 29 of these nuclear power reactors under construction, as of 2013, with plans to build many more,^[105][106] while in the US the licenses of almost half its reactors have been extended to 60 years,^[107] and plans to build another dozen are under serious consideration.^[108] There are also a considerable number of new reactors being built in South Korea, India, and Russia. At least 100 older and smaller reactors will "most probably be closed over the next 10-15 years".^[109] This is probable only if one does not factor in the ongoing Light Water Reactor Sustainability Program, created to permit the extension of the life span of the USA's 104 nuclear reactors to 60 years. The licenses of almost half of the USA's reactors have been extended to 60 years as of 2008.^[107] Two new AP1000 reactors are, as of 2013, being constructed at Vogtle Electric Generating Plant.

Public opinion about nuclear power varies widely between countries.^[110][111] A poll by Gallup International (2011)^[112] assessed public opinion in 47 countries. The poll was conducted following a tsumani and earthquake which caused an accident at the Fukushima nuclear power plant in Japan. 49% stated that they held favourable views about nuclear energy, while 43% held an unfavourable view.^[113] Another global survey by Ipsos (2011)^[114] assessed public opinion in 24 countries. Respondents to this survey showed a clear preference for renewable energy sources over coal and nuclear energy (refer to graph opposite).^[2] Ipsos (2012)^[115] found that solar and wind were viewed by the public as being more environmentally friendly and more viable long-term energy sources relative to nuclear power and natural gas. However, solar and wind were viewed as being less reliable relative to nuclear power and natural gas. In 2012 a poll done in the UK found that 63% of those surveyed support nuclear power, and with opposition to nuclear power at 11%.^[116] In Germany, strong anti-nuclear sentiment led to eight of the seventeen operating reactors being permanently shut down following the March 2011 Fukushima nuclear disaster.^[117]

Nuclear fusion research, in the form of the International Thermonuclear Experimental Reactor is underway. Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However, the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production was still believed to be unlikely before 2050.^[118]

Coal to gas fuel switching

Most mitigation proposals imply—rather than directly state—an eventual reduction in global fossil fuel production. Also proposed are direct quotas on global fossil fuel production.^[119][120]

Natural gas emits far fewer greenhouse gases (i.e. CO₂ and methane—CH₄) than coal when burned at power plants, but evidence has been emerging that this benefit could be completely negated by methane leakage at gas drilling fields and other points in the supply chain.

A study performed by the Environmental Protection Agency (EPA) and the Gas Research Institute (GRI) in 1997 sought to discover whether the reduction in carbon dioxide emissions from increased natural gas (predominantly methane) use would be offset by a possible increased level of methane emissions from sources such as leaks and emissions. The study concluded that the reduction in emissions from increased natural gas use outweighs the detrimental effects of increased methane emissions. More recent peer-reviewed studies have challenged the findings of this study, with researchers from the National Oceanic and Atmospheric Administration (NOAA) reconfirming findings of high rates of methane (CH4) leakage from natural gas fields.

A 2011 study^[121] by noted climate research scientist, Tom Wigley,^[122] found that while carbon dioxide (CO₂) emissions from fossil fuel combustion may be reduced by using natural gas rather than coal to produce energy, it also found that additional methane (CH4) from leakage adds to the radiative forcing of the climate system, offsetting the reduction in CO₂ forcing that accompanies the transition from coal to gas. The study looked at methane leakage from coal mining; changes in radiative forcing due to changes in the emissions of sulfur dioxide and carbonaceous aerosols; and differences in the efficiency of electricity production between coal- and gas-fired power generation. On balance, these factors more than offset the reduction in warming due to reduced CO₂ emissions. When gas replaces coal there is additional warming out to 2,050 with an assumed leakage rate of 0%, and out to 2,140 if the leakage rate is as high as 10%. The overall effects on global-mean temperature over the 21st century, however, are small. Petron et al. (2013)^[123] and Alvarez et al. (2012)^[124] note that estimated that leakage from gas infrastructure is likely to be underestimated. These studies indicate that the exploitation of natural gas as a "cleaner" fuel is questionable. A 2014 meta-study of 20 years of natural gas technical literature shows that methane emissions are consistently underestimated but on a 100-year scale, the climate benefits of coal to gas fuel switching are likely larger than the negative effects of natural gas leakage.^[125]

Heat pump

Outside unit of an air-source heat pump.

A heat pump is a device that provides heat energy from a source of heat to a destination called a "heat sink". Heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses some amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.

While air conditioners and freezers are familiar examples of heat pumps, the term "heat pump" is more general and applies to many HVAC (heating, ventilating, and air conditioning) devices used for space heating or space cooling. When a heat pump is used for heating, it employs the same basic refrigeration-type cycle used by an air conditioner or a refrigerator, but in the opposite direction—releasing heat into the conditioned space rather than the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground.^[126] In heating mode, heat pumps are three to four times more efficient in their use of electric power than simple electrical resistance heaters.

It has been concluded that heat pumps are the single technology that could reduce the greenhouse gas emissions of households better than every other technology that is available on the market. With a market share of 30% and (potentially) clean electricity, heat pumps could reduce global CO2 emissions by 8% annually.^[127] Using ground source heat pumps could reduce around 60% of the primary energy demand and 90% of CO2 emissions in Europe in 2050 and make handling high shares of renewable energy easier.^[128] Using surplus renewable energy in heat pumps is regarded as the most effective household means to reduce to reduce global warming and fossil fuel depletion.^[129]

With significant amounts of fossil fuel used in electricity production, demands on the electrical grid also generate greenhouse gases. Without a high share of low-carbon electricity, a domestic heat pump will produce more carbon emissions than using natural gas.^[130]

Fossil fuel phase-out: carbon neutral and negative fuels

Fossil fuel may be phased-out with carbon neutral and carbon negative pipeline and transportation fuels created with power to gas and gas to liquids technologies.^{[131][132][133][134][135]} Carbon dioxide from fossil fuel flue gas can be used to produce plastic lumber allowing carbon negative reforestation.^[136]

Demand side management

Energy efficiency and conservation

A spiral-type integrated compact fluorescent lamp, use has grown among North American consumers since its introduction in the mid-1990s.^[137]

Efficient energy use, sometimes simply called "energy efficiency", is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature. Installing fluorescent lights or natural skylights reduces the amount of energy required to attain the same level of illumination compared to using traditional incandescent light bulbs. Compact fluorescent lights use two-thirds less energy and may last 6 to 10 times longer than incandescent lights.^[138]

Energy efficiency has proved to be a cost-effective strategy for building economies without necessarily growing energy consumption. For example, the state of California began implementing energy-efficiency measures in the mid-1970s, including building code and appliance standards with strict efficiency requirements. During the following years, California's energy consumption has remained approximately flat on a per capita basis while national U.S. consumption doubled. As part of its strategy, California implemented a "loading order" for new energy resources that puts energy efficiency first, renewable electricity supplies second, and new fossil-fired power plants last.^[139]

Energy conservation is broader than energy efficiency in that it encompasses using less energy to achieve a lesser energy service, for example through behavioural change, as well as encompassing energy efficiency. Examples of conservation without efficiency improvements would be heating a room less in winter, driving less, or working in a less brightly lit room. As with other definitions, the boundary between efficient energy use and energy conservation can be fuzzy, but both are important in environmental and economic terms. This is especially the case when actions are directed at the saving of fossil fuels.^[140]

Reducing energy use is seen as a key solution to the problem of reducing greenhouse gas emissions. According to the International Energy Agency, improved energy efficiency in buildings, industrial processes and transportation could reduce the world's energy needs in 2050 by one third, and help control global emissions of greenhouse gases.^[141]

Demand side switching sources

Fuel switching on the demand side refers to changing the type of fuel used to satisfy a need for an energy service. To meet deep decarbonization goals, like the 80% reduction by 2050 goal being discussed in California and the European Union, many primary energy changes are needed.^[142][143] Energy efficiency alone may not be sufficient to meet these goals, switching fuels used on the demand side will help lower carbon emissions.^[144][145] Progressively coal, oil and eventually natural gas for space and water heating in buildings will need to be reduced. For an equivalent amount of heat, burning natural gas produces about 45 per cent less carbon dioxide than burning coal.^[146] There are various ways in which this could happen, and different strategies will likely make sense in different locations. While the system efficiency of a gas furnace may be higher than the combination of natural gas power plant and electric heat, the combination of the same natural gas power plant and an electric heat pump has lower emissions per unit of heat delivered in all but the coldest climates. This is possible because of the very efficient coefficient of performance of heat pumps.

At the beginning this century 70% of all electricity was generated by fossil fuels, and as carbon free sources eventually make up half of the generation mix, replacing gas or oil furnaces and water heaters with electric ones will have a climate benefit. In areas like Norway, Brazil and Quebec that have abundant hydroelectricity, electric heat and hot water is common.

The economics of switching the demand side from fossil fuels to electricity for heating, will depend on the price of fuels vs electricity and the relative prices of the equipment. The EIA Annual Energy Outlook 2014 suggests that domestic gas prices will rise faster than electricity prices which will encourage electrification in the coming decades.^[147] Electrifying heating loads may also provide a flexible resource that can participate in demand response. Since thermostatically controlled loads have inherent energy storage, electrification of heating could provide a valuable resource to integrate variable renewable resources into the grid.

Alternatives to electrification, include decarbonizing pipeline gas through power to gas, biogas, or other carbon neutral fuels. A 2015 study by Energy+Environmental Economics shows that a hybrid approach of decarbonizing pipeline gas, electrification, and energy efficiency can meet carbon reduction goals at a similar cost as only electrification and energy efficiency in Southern California.^[148]

Demand side grid management

Expanding intermittent electrical sources such as wind power, creates a growing problem balancing grid fluctuations. Some of the plans include building pumped storage or continental super grids costing billions of dollars. However instead of building for more power,there are a variety of ways to affect the size and timing of electricity demand on the consumer side. Designing for reduced demands on a smaller power grid is more efficient and economic than having extra generation and transmission for intermittentcy, power failures and peak demands. Having these abilities is one of the chief aims of a smart grid.

Time of use metering is a common way to motivate electricity users to reduce their peak load consumption. For instance, running dishwashers and laundry at night after the peak has passed, reduces electricity costs.

Dynamic demand plans have devices passively shut off when stress is sensed on the electrical grid. This method may work very well with thermostats, when power on the grid sags a small amount, a low power temperature setting is automatically selected reducing the load on the grid. For instance millions of refrigerators reduce their consumption when clouds pass over solar installations. Consumers would need to have a smart meter in order for the utility to calculate credits.

Demand response devices could receive all sorts of messages from the grid. The message could be a request to use a low power mode similar to dynamic demand, to shut off entirely during a sudden failure on the grid, or notifications about the current and expected prices for power. This would allow electric cars to recharge at the least expensive rates independent of the time of day. The vehicle-to-grid suggestion would use a car's battery or fuel cell to supply the grid temporarily.

Sinks and negative emissions

A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period, such as a growing forest. A negative carbon dioxide emission on the other hand is a permanent removal of carbon dioxide out of the atmosphere, such as directly capturing carbon dioxide in the atmosphere and storing it in geologic formations underground.

The Antarctic Climate and Ecosystems Cooperative Research Centre (ACE-CRC) notes that one third of humankind’s annual emissions of CO₂ are absorbed by the oceans. The oceans act as a carbon sink, that is, a reservoir that accumulates and stores carbon via its physicochemical and biological processes.^[149] Unfortunately, this "vital service comes with the cost of ocean acidification". The ecological effects of ocean acidification are still largely unknown. Research so far has focussed on how acidification lowers pH and the level of carbonate ions available for calcifying organisms to form their shells. These organisms include plankton species that contribute to the foundation of the Southern Ocean food web. However acidification may impact on a broad range of other physiological and ecological processes, such as fish respiration, larval development and changes in the solubility of both nutrients and toxins.^[150] According to the CSIRO the Southern Ocean is absorbing increasing amounts of carbon dioxide, with potentially significant impacts on marine life.^[151]

Agriculture

According to the EPA, agricultural soil management practices can lead to production and emission of nitrous oxide (N2O), a major greenhouse gas and air pollutant. Activities that can contribute to N
2O emissions include fertilizer usage, irrigation and tillage. The management of soils accounts for over half of the emissions from the Agriculture sector. Cattle livestocks account for one third of emissions, through methane emissions. Manure management and rice cultivation also emit emissions.^[152]

Methods that significantly enhance carbon sequestration in soil include no-till farming, residue mulching, cover cropping, and crop rotation, all of which are more widely used in organic farming than in conventional farming.^[153][154] Because only 5% of US farmland currently uses no-till and residue mulching, there is a large potential for carbon sequestration.^[155]

A 2015 study found that farming can deplete soil carbon and render soil incapable of supporting life. Instead the study showed that conservation farming can protect carbon in soils, and repair damage over time.^[156]

The farming practise of cover crops has been recognized as climate-smart agriculture by the White House.^[157]

Reforestation and avoided deforestation

Managed grazing methods are argued to be able to restore grasslands, thereby significantly decreasing atmospheric CO₂ levels.^[158]

Almost 20% (8 GtCO₂/year) of total greenhouse-gas emissions were from deforestation in 2007. The Stern Review found that, based on the opportunity costs of the landuse that would no longer be available for agriculture if deforestation were avoided, emission savings from avoided deforestation could potentially reduce CO₂ emissions for under $5/tCO₂, possiblly as little as $1/tCO₂. Afforestation and reforestation could save at least another 1GtCO₂/year, at an estimated cost of $5/tCO₂ to $15/tCO₂.^[159] The Review determined these figures by assessing 8 countries responsible for 70% of global deforestation emissions. Pristine temperate forest has been shown to store three times more carbon than IPCC estimates took into account, and 60% more carbon than plantation forest.^[160] Preventing these forests from being logged would have significant effects. Further significant savings from other non-energy-related-emissions could be gained through cuts to agricultural emissions, fugitive emissions, waste emissions, and emissions from various industrial processes.^[159] Using evidence from Mozambique, a typical low income country where agriculture is the dominant provider of income for most citizens, researchers from the Overseas Development Institute found a positive correlation between increased production intensification and reduced land conversion, and crop returns, economic growth and food security.^[161]

Restoring grasslands store CO₂ from the air into plant material. Grazing livestock, usually not left to wander, would eat the grass and would minimize any grass growth. However, grass left alone would eventually grow to cover its own growing buds, preventing them from photosynthesizing and the dying plant would stay in place.^[162] A method proposed to restore grasslands uses fences with many small paddocks and moving herds from one paddock to another after a day a two in order to mimick natural grazers and allowing the grass to grow optimally.^{[162][163][164]} Additionally, when part of leaf matter is consumed by a herding animal, a corresponding amount of root matter is sloughed off too as it would not be able to sustain the previous amount of root matter and while most of the lost root matter would rot and enter the atmosphere, part of the carbon is sequestered into the soil.^[162] It is estimated that increasing the carbon content of the soils in the world’s 3.5 billion hectares of agricultural grassland by 1% would offset nearly 12 years of CO₂ emissions.^[162] Allan Savory, as part of holistic management, claims that while large herds are often blamed for desertification, prehistoric lands supported large or larger herds and areas where herds were removed in the United States are still desertifying.^[158]

Carbon capture and storage

Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a coal-fired plant.

Carbon capture and storage (CCS) is a method to mitigate climate change by capturing carbon dioxide (CO₂) from large point sources such as power plants and subsequently storing it away safely instead of releasing it into the atmosphere. The Intergovernmental Panel on Climate Change says CCS could contribute between 10% and 55% of the cumulative worldwide carbon-mitigation effort over the next 90 years. The International Energy Agency says CCS is "the most important single new technology for CO₂ savings" in power generation and industry.^[165] Though it requires up to 40% more energy to run a CCS coal power plant than a regular coal plant, CCS could potentially capture about 90% of all the carbon emitted by the plant.^[165] Norway, which first began storing CO₂, has cut its emissions by almost a million tons a year, or about 3% of the country's 1990 levels.^[165] As of late 2011, the total CO₂ storage capacity of all 14 projects in operation or under construction is over 33 million tonnes a year. This is broadly equivalent to preventing the emissions from more than six million cars from entering the atmosphere each year.^[166]

Negative carbon dioxide emissions

Creating negative carbon dioxide emissions literally removes carbon from the atmosphere. Examples are direct air capture, biochar, bio-energy with carbon capture and storage and enhanced weathering technologies. These processes are sometimes considered as variations of sinks or mitigation,^[167][168] and sometimes as geoengineering.^[169]

In combination with other mitigation measures, sinks in combination with negative carbon emissions are considered crucial for meeting the 350 ppm target,^[170][171] and even the less conservative 450 ppm target.^[172]

Geoengineering

Geoengineering is seen by some as an alternative to mitigation and adaptation, but by others as an entirely separate response to climate change. In a literature assessment, Barker et al. (2007) described geoengineering as a type of mitigation policy.^[173] IPCC (2007) concluded that geoengineering options, such as ocean fertilization to remove CO₂ from the atmosphere, remained largely unproven.^[174] It was judged that reliable cost estimates for geoengineering had not yet been published.

Chapter 28 of the National Academy of Sciences report Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992) defined geoengineering as "options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry."^[175] They evaluated a range of options to try to give preliminary answers to two questions: can these options work and could they be carried out with a reasonable cost. They also sought to encourage discussion of a third question — what adverse side effects might there be. The following types of option were examined: reforestation, increasing ocean absorption of carbon dioxide (carbon sequestration) and screening out some sunlight. NAS also argued "Engineered countermeasures need to be evaluated but should not be implemented without broad understanding of the direct effects and the potential side effects, the ethical issues, and the risks.".^[175] In July 2011 a report by the United States Government Accountability Office on geoengineering found that "[c]limate engineering technologies do not now offer a viable response to global climate change."^[176]

Carbon dioxide removal

Carbon dioxide removal has been proposed as a method of reducing the amount of radiative forcing. A variety of means of artificially capturing and storing carbon, as well as of enhancing natural sequestration processes, are being explored. The main natural process is photosynthesis by plants and single-celled organisms (see biosequestration). Artificial processes vary, and concerns have been expressed about the long-term effects of some of these processes.^[177]

It is notable that the availability of cheap energy and appropriate sites for geological storage of carbon may make carbon dioxide air capture viable commercially. It is, however, generally expected that carbon dioxide air capture may be uneconomic when compared to carbon capture and storage from major sources — in particular, fossil fuel powered power stations, refineries, etc. In such cases, costs of energy produced will grow significantly. However, captured CO₂ can be used to force more crude oil out of oil fields, as Statoil and Shell have made plans to do.^[178] CO₂ can also be used in commercial greenhouses, giving an opportunity to kick-start the technology. Some attempts have been made to use algae to capture smokestack emissions,^[179] notably the GreenFuel Technologies Corporation, who have now shut down operations.^[180]

Solar radiation management

The main purpose of solar radiation management seek to reflect sunlight and thus reduce global warming. The ability of stratospheric sulfate aerosols to create a global dimming effect has made them a possible candidate for use in climate engineering projects.^[181]

Non-CO₂ greenhouse gases

CO₂ is not the only GHG relevant to mitigation,^[182] and governments have acted to regulate the emissions of other GHGs emitted by human activities (anthropogenic GHGs). The emissions caps agreed to by most developed countries under the Kyoto Protocol regulate the emissions of almost all the anthropogenic GHGs.^[183] These gases are CO₂, methane (chemical formula: CH₄), nitrous oxide (N₂O), the hydrofluorocarbons (abbreviated HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆).

Stabilizing the atmospheric concentrations of the different anthropogenic GHGs requires an understanding of their different physical properties. Stabilization depends both on how quickly GHGs are added to the atmosphere and how fast they are removed. The rate of removal is measured by the atmospheric lifetime of the GHG in question (see the main GHG article for a list). Here, the lifetime is defined as the time required for a given perturbation of the GHG in the atmosphere to be reduced to 37% of its initial amount.^[18] Methane has a relatively short atmospheric lifetime of about 12 years, while N₂O's lifetime is about 110 years. For methane, a reduction of about 30% below current emission levels would lead to a stabilization in its atmospheric concentration, while for N₂O, an emissions reduction of more than 50% would be required.^[18]

Methane is a significantly more potent greenhouse gas than carbon dioxide in the amount of heat it can trap, especially in the short term.^[184] Burning one molecule of methane generates one molecule of carbon dioxide, indicating there may be no net benefit in using gas as a fuel source.^[121][123] Reducing the amount of waste methane produced in the first place and moving away from use of gas as a fuel source will have a greater beneficial impact, as might other approaches to productive use of otherwise-wasted methane. In terms of prevention, vaccines are in the works in Australia to reduce significant global warming contributions from methane released by livestock via flatulence and eructation.^[185]

Another physical property of the anthropogenic GHGs relevant to mitigation is the different abilities of the gases to trap heat (in the form of infrared radiation). Some gases are more effective at trapping heat than others, e.g., SF₆ is 22,200 times more effective a GHG than CO₂ on a per-kilogram basis.^[186] A measure for this physical property is the global warming potential (GWP), and is used in the Kyoto Protocol.^[187]

Although not designed for this purpose, the Montreal Protocol has probably benefitted climate change mitigation efforts.^[188] The Montreal Protocol is an international treaty that has successfully reduced emissions of ozone-depleting substances (e.g., CFCs), which are also greenhouse gases.

By sector

Transport

The Tesla Roadster emits no tailpipe emissions, uses lithium ion batteries to achieve 220 mi (350 km) per charge, while also capable of going 0-60 in under 4 seconds.

Bicycles have almost no carbon footprint compared to cars, and canal transport may represent a positive option for certain types of freight in the 21st century^[189]

Modern energy-efficient technologies, such as plug-in hybrid electric vehicles, and development of new technologies, such as hydrogen cars, may reduce the consumption of petroleum and emissions of carbon dioxide. A shift from air transport and truck transport to electric rail transport would reduce emissions significantly.^[190][191] For electric vehicles, the reduction of carbon emissions will improve further if the way the required electricity is generated is low-carbon power in origin.

Urban planning

Effective urban planning to reduce sprawl would decrease Vehicle Miles Travelled (VMT), lowering emissions from transportation. Increased use of public transport can also reduce greenhouse gas emissions per passenger kilometer. Between 1982 and 1997, the amount of land consumed for urban development in the United States increased by 47 percent while the nation's population grew by only 17 percent.^[192] Inefficient land use development practices have increased infrastructure costs as well as the amount of energy needed for transportation, community services, and buildings.

At the same time, a growing number of citizens and government officials have begun advocating a smarter approach to land use planning. These smart growth practices include compact community development, multiple transportation choices, mixed land uses, and practices to conserve green space. These programs offer environmental, economic, and quality-of-life benefits; and they also serve to reduce energy usage and greenhouse gas emissions.

Approaches such as New Urbanism and Transit-oriented development seek to reduce distances travelled, especially by private vehicles, encourage public transit and make walking and cycling more attractive options. This is achieved through "medium-density", mixed-use planning and the concentration of housing within walking distance of town centers and transport nodes.

Smarter growth land use policies have both a direct and indirect effect on energy consuming behavior. For example, transportation energy usage, the number one user of petroleum fuels, could be significantly reduced through more compact and mixed use land development patterns, which in turn could be served by a greater variety of non-automotive based transportation choices.

Building design

Emissions from housing are substantial,^[193] and government-supported energy efficiency programmes can make a difference.^[194]

For institutions of higher learning in the United States, greenhouse gas emissions depend primarily on total area of buildings and secondarily on climate.^[195] If climate is not taken into account, annual greenhouse gas emissions due to energy consumed on campuses plus purchased electricity can be estimated with the formula, E=aS^b, where a =0.001621 metric tonnes of CO₂ equivalent/square foot or 0.0241 metric tonnes of CO₂ equivalent/square meter and b = 1.1354.^[196]

New buildings can be constructed using passive solar building design, low-energy building, or zero-energy building techniques, using renewable heat sources. Existing buildings can be made more efficient through the use of insulation, high-efficiency appliances (particularly hot water heaters and furnaces), double- or triple-glazed gas-filled windows, external window shades, and building orientation and siting. Renewable heat sources such as shallow geothermal and passive solar energy reduce the amount of greenhouse gasses emitted. In addition to designing buildings which are more energy-efficient to heat, it is possible to design buildings that are more energy-efficient to cool by using lighter-coloured, more reflective materials in the development of urban areas (e.g. by painting roofs white) and planting trees.^[197][198] This saves energy because it cools buildings and reduces the urban heat island effect thus reducing the use of air conditioning.

Societal controls

Another method being examined is to make carbon a new currency by introducing tradeable "Personal Carbon Credits". The idea being it will encourage and motivate individuals to reduce their 'carbon footprint' by the way they live. Each citizen will receive a free annual quota of carbon that they can use to travel, buy food, and go about their business. It has been suggested that by using this concept it could actually solve two problems; pollution and poverty, old age pensioners will actually be better off because they fly less often, so they can cash in their quota at the end of the year to pay heating bills, etc.

Population

Population density by country

Various organizations promote population control as a means for mitigating global warming.^{[199][200][201][202][203]} Proposed measures include improving access to family planning and reproductive health care and information, reducing natalistic politics, public education about the consequences of continued population growth, and improving access of women to education and economic opportunities.

Population control efforts are impeded by there being somewhat of a taboo in some countries against considering any such efforts.^[204] Also, various religions discourage or prohibit some or all forms of birth control.

Population size has a different per capita effect on global warming in different countries, since the per capita production of anthropogenic greenhouse gases varies greatly by country.^[205]

Costs and benefits

Costs

The Stern Review proposes stabilising the concentration of greenhouse-gas emissions in the atmosphere at a maximum of 550ppm CO₂e by 2050. The Review estimates that this would mean cutting total greenhouse-gas emissions to three quarters of 2007 levels. The Review further estimates that the cost of these cuts would be in the range −1.0 to +3.5% of World GDP, (i.e. GWP), with an average estimate of approximately 1%.^[159] Stern has since revised his estimate to 2% of GWP.^[206] For comparison, the Gross World Product (GWP) at PPP was estimated at $74.5 trillion in 2010,^[207] thus 2% is approximately $1.5 trillion. The Review emphasises that these costs are contingent on steady reductions in the cost of low-carbon technologies. Mitigation costs will also vary according to how and when emissions are cut: early, well-planned action will minimise the costs.^[159]

One way of estimating the cost of reducing emissions is by considering the likely costs of potential technological and output changes. Policy makers can compare the marginal abatement costs of different methods to assess the cost and amount of possible abatement over time. The marginal abatement costs of the various measures will differ by country, by sector, and over time.^[159]

Benefits

Total extreme weather cost and number of events costing more than $1 billion in the United States from 1980 to 2011.

Yohe et al. (2007) assessed the literature on sustainability and climate change.^[208] With high confidence, they suggested that up to the year 2050, an effort to cap greenhouse gas (GHG) emissions at 550 ppm would benefit developing countries significantly. This was judged to be especially the case when combined with enhanced adaptation. By 2100, however, it was still judged likely that there would be significant effects of global warming. This was judged to be the case even with aggressive mitigation and significantly enhanced adaptive capacity.

Sharing

One of the aspects of mitigation is how to share the costs and benefits of mitigation policies. There is no scientific consensus over how to share these costs and benefits (Toth et al., 2001).^[209] In terms of the politics of mitigation, the UNFCCC's ultimate objective is to stabilize concentrations of GHG in the atmosphere at a level that would prevent "dangerous" climate change (Rogner et al., 2007).^[210]

GHG emissions are an important correlate of wealth, at least at present (Banuri et al., 1996, pp. 91–92).^[211] Wealth, as measured by per capita income (i.e., income per head of population), varies widely between different countries. Activities of the poor that involve emissions of GHGs are often associated with basic needs, such as heating to stay tolerably warm. In richer countries, emissions tend to be associated with things like cars, central heating, etc. The impacts of cutting emissions could therefore have different impacts on human welfare according to wealth.

Distributing emissions abatement costs

There have been different proposals on how to allocate responsibility for cutting emissions (Banuri et al., 1996, pp. 103–105):^[211]

• Egalitarianism: this system interprets the problem as one where each person has equal rights to a global resource, i.e., polluting the atmosphere.
• Basic needs and Rawlsian criteria: this system would have emissions allocated according to basic needs, as defined according to a minimum level of consumption. Consumption above basic needs would require countries to buy more emission rights. This can be related to Rawlsian philosophy. From this viewpoint, developing countries would need to be at least as well off under an emissions control regime as they would be outside the regime.
• Proportionality and polluter-pays principle: Proportionality reflects the ancient Aristotelian principle that people should receive in proportion to what they put in, and pay in proportion to the damages they cause. This has a potential relationship with the "polluter-pays principle", which can be interpreted in a number of ways:
  • Historical responsibilities: this asserts that allocation of emission rights should be based on patterns of past emissions. Two-thirds of the stock of GHGs in the atmosphere at present is due to the past actions of developed countries (Goldemberg et al., 1996, p. 29).^[212]
  • Comparable burdens and ability to pay: with this approach, countries would reduce emissions based on comparable burdens and their ability to take on the costs of reduction. Ways to assess burdens include monetary costs per head of population, as well as other, more complex measures, like the UNDP's Human Development Index.
  • Willingness to pay: with this approach, countries take on emission reductions based on their ability to pay along with how much they benefit from reducing their emissions.

Specific proposals

• Ad hoc: Lashof (1992) and Cline (1992) (referred to by Banuri et al., 1996, p. 106),^[211] for example, suggested that allocations based partly on GNP could be a way of sharing the burdens of emission reductions. This is because GNP and economic activity are partially tied to carbon emissions.
• Equal per capita entitlements: this is the most widely cited method of distributing abatement costs, and is derived from egalitarianism (Banuri et al., 1996, pp. 106–107). This approach can be divided into two categories. In the first category, emissions are allocated according to national population. In the second category, emissions are allocated in a way that attempts to account for historical (cumulative) emissions.
• Status quo: with this approach, historical emissions are ignored, and current emission levels are taken as a status quo right to emit (Banuri et al., 1996, p. 107). An analogy for this approach can be made with fisheries, which is a common, limited resource. The analogy would be with the atmosphere, which can be viewed as an exhaustible natural resource (Goldemberg et al., 1996, p. 27).^[212] In international law, one state recognized the long-established use of another state's use of the fisheries resource. It was also recognized by the state that part of the other state's economy was dependent on that resource.

Governmental and intergovernmental action

Many countries, both developing and developed, are aiming to use cleaner technologies (World Bank, 2010, p. 192).^[213] Use of these technologies aids mitigation and could result in substantial reductions in CO₂ emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. It is often argued that the results of climate change are more damaging in poor nations, where infrastructures are weak and few social services exist. The Commitment to Development Index is one attempt to analyze rich country policies taken to reduce their disproportionate use of the global commons. Countries do well if their greenhouse gas emissions are falling, if their gas taxes are high, if they do not subsidize the fishing industry, if they have a low fossil fuel rate per capita, and if they control imports of illegally cut tropical timber.

Kyoto Protocol

The main current international agreement on combating climate change is the Kyoto Protocol, which came into force on 16 February 2005. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). Countries that have ratified this protocol have committed to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases.

Temperature targets

The graph on the right shows three "pathways" to meet the UNFCCC's 2 °C target, labelled "global technology", "decentralised solutions", and "consumption change". Each pathway shows how various measures (e.g., improved energy efficiency, increased use of renewable energy) could contribute to emissions reductions. Image credit: PBL Netherlands Environmental Assessment Agency.^[214]

Actions to mitigate climate change are sometimes based on the goal of achieving a particular temperature target. One of the targets that has been suggested is to limit the future increase in global mean temperature (global warming) to below 2 °C, relative to the pre-industrial level.^[215][216] The 2 °C target was adopted in 2010 by Parties to the United Nations Framework Convention on Climate Change.^[217] Most countries of the world are Parties to the UNFCCC.^[218] The target had been adopted in 1996 by the European Union Council.^[219]

Feasibility of 2 °C

Temperatures have increased by 0.8 °C compared to the pre-industrial level, and another 0.5–0.7 °C is already committed.^[220] The 2 °C rise is typically associated in climate models with a carbon dioxide equivalent concentration of 400–500 ppm by volume; the current (January 2015) level of carbon dioxide alone is 400 ppm by volume, and rising at 1-3 ppm annually. Hence, to avoid a very likely breach of the 2 °C target, CO₂ levels would have to be stabilised very soon; this is generally regarded as unlikely, based on current programs in place to date.^[221][222] The importance of change is illustrated by the fact that world economic energy efficiency is improving at only half the rate of world economic growth.^[223]

Views in the literature

There is disagreement among experts over whether or not the 2 °C target can be met.^[224][225] For example, according to Anderson and Bows (2011),^[226] "there is little to no chance" of meeting the target. On the other hand, according to Alcamo et al. (2013):^[227]

• Policies adopted by parties to the UNFCCC are too weak to meet a 2 or 1.5 °C target. However, these targets might still be achievable if more stringent mitigation policies are adopted immediately.
• Cost-effective 2 °C scenarios project annual global greenhouse gas emissions to peak before the year 2020, with deep cuts in emissions thereafter, leading to a reduction in 2050 of 41% compared to 1990 levels.^[228]

Other targets

Scientific analysis can provide information on the impacts of climate change and associated policies, such as reducing GHG emissions. However, deciding what policies are best requires value judgements.^[12] For example, limiting global warming to 1 °C relative to pre-industrial levels may help to reduce climate change damages more than a 2 °C limit.^[229] However, a 1 °C limit may be more costly to achieve than a 2 °C limit.^[230]

According to some analysts, the 2 °C "guardrail" is inadequate for the needed degree and timeliness of mitigation.^[15] On the other hand, some economic studies suggest more modest mitigation policies.^[16] For example, the emissions reductions proposed by Nordhaus (2010)^[17] might lead to global warming (in the year 2100) of around 3 °C, relative to pre-industrial levels.

Encouraging use changes

Emissions tax

An emissions tax on greenhouse gas emissions requires individual emitters to pay a fee, charge or tax for every tonne of greenhouse gas released into the atmosphere.^[231] Most environmentally related taxes with implications for greenhouse gas emissions in OECD countries are levied on energy products and motor vehicles, rather than on CO₂ emissions directly.^[231]

Emission taxes can be both cost-effective and environmentally effective.^[231] Difficulties with emission taxes include their potential unpopularity, and the fact that they cannot guarantee a particular level of emissions reduction.^[231] Emissions or energy taxes also often fall disproportionately on lower income classes. In developing countries, institutions may be insufficiently developed for the collection of emissions fees from a wide variety of sources.^[231]

Subsidies

According to Mark Z. Jacobson, a program of subsidization balanced against expected flood costs could pay for conversion to 100% renewable power by 2030.^[232] Jacobson, and his colleague Mark Delucchi, suggest that the cost to generate and transmit power in 2020 will be less than 4 cents per kilowatt hour (in 2007 dollars) for wind, about 4 cents for wave and hydroelectric, from 4 to 7 cents for geothermal, and 8 cents per kwh for solar, fossil, and nuclear power.^[233]

Investment

Another indirect method of encouraging uses of renewable energy, and pursue sustainability and environmental protection, is that of prompting investment in this area through legal means, something that is already being done at national level as well as in the field of international investment.^[234]

Carbon emissions trading

With the creation of a market for trading carbon dioxide emissions within the Kyoto Protocol, it is likely that London financial markets will be the centre for this potentially highly lucrative business; the New York and Chicago stock markets may have a lower trade volume than expected as long as the US maintains its rejection of the Kyoto.^[235]

However, emissions trading may delay the phase-out of fossil fuels.^[236]

In The North East United States a successful Cap and Trade] program has shows potential for this solution.^[237]

The European Union Emission Trading Scheme (EU ETS)^[238] is the largest multi-national, greenhouse gas emissions trading scheme in the world. It commenced operation on 1 January 2005, and all 25 member states of the European Union participate in the scheme which has created a new market in carbon dioxide allowances estimated at 35 billion Euros (US$43 billion) per year.^[239] The Chicago Climate Exchange was the first (voluntary) emissions market, and is soon to be followed by Asia's first market (Asia Carbon Exchange). A total of 107 million metric tonnes of carbon dioxide equivalent have been exchanged through projects in 2004, a 38% increase relative to 2003 (78 Mt CO₂e).^[240]

Twenty three multinational corporations have come together in the G8 Climate Change Roundtable, a business group formed at the January 2005 World Economic Forum. The group includes Ford, Toyota, British Airways and BP. On 9 June 2005 the Group published a statement^[241] stating that there was a need to act on climate change and claiming that market-based solutions can help. It called on governments to establish "clear, transparent, and consistent price signals" through "creation of a long-term policy framework" that would include all major producers of greenhouse gases.

The Regional Greenhouse Gas Initiative is a proposed carbon trading scheme being created by nine North-eastern and Mid-Atlantic American states; Connecticut, Delaware, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Vermont. The scheme was due to be developed by April 2005 but has not yet been completed.

Implementation

Implementation puts into effect climate change mitigation strategies and targets. These can be targets set by international bodies or voluntary action by individuals or institutions. This is the most important, expensive and least appealing aspect of environmental governance.^[242]

Funding

Implementation requires funding sources but is often beset by disputes over who should provide funds and under what conditions.^[242] A lack of funding can be a barrier to successful strategies as there are no formal arrangements to finance climate change development and implementation.^[243] Funding is often provided by nations, groups of nations and increasingly NGO and private sources. These funds are often channelled through the Global Environmental Facility (GEF). This is an environmental funding mechanism in the World Bank which is designed to deal with global environmental issues.^[242] The GEF was originally designed to tackle four main areas: biological diversity, climate change, international waters and ozone layer depletion, to which land degradation and persistent organic pollutant were added. The GEF funds projects that are agreed to achieve global environmental benefits that are endorsed by governments and screened by one of the GEF’s implementing agencies.^[244]

Problems

There are numerous issues which result in a current perceived lack of implementation.^[242] It has been suggested that the main barriers to implementation are, Uncertainty, Fragmentation, Institutional void, Short time horizon of policies and politicians and Missing motives and willingness to start adapting. The relationships between many climatic processes can cause large levels of uncertainty as they are not fully understood and can be a barrier to implementation. When information on climate change is held between the large numbers of actors involved it can be highly dispersed, context specific or difficult to access causing fragmentation to be a barrier. Institutional void is the lack of commonly accepted rules and norms for policy processes to take place, calling into question the legitimacy and efficacy of policy processes. The Short time horizon of policies and politicians often means that climate change policies are not implemented in favour of socially favoured societal issues. Statements are often posed to keep the illusion of political action to prevent or postpone decisions being made. Missing motives and willingness to start adapting is a large barrier as it prevents any implementation.^[243]

The issues that arise with a system which involves international government cooperation, such as Cap and Trade, could potentially be improved with a polycentric approach where the rules are enforced by many small sections of authority as apposed to one overall enforcement agency.^[245]

Occurrence

Despite a perceived lack of occurrence, evidence of implementation is emerging internationally. Some examples of this are the initiation of NAPA’s and of joint implementation. Many developing nations have made National Adaptation Programs of Action (NAPAs) which are frameworks to prioritize adaption needs.^[246] The implementation of many of these is supported by GEF agencies.^[247] Many developed countries are implementing ‘first generation’ institutional adaption plans particularly at the state and local government scale.^[246] There has also been a push towards joint implementation between countries by the UNFCC as this has been suggested as a cost-effective way for objectives to be achieved.^[248]

Territorial policies

United States

Efforts to reduce greenhouse gas emissions by the United States include energy policies which encourage efficiency through programs like Energy Star, Commercial Building Integration, and the Industrial Technologies Program.^[249] On 12 November 1998, Vice President Al Gore symbolically signed the Kyoto Protocol, but he indicated participation by the developing nations was necessary prior its being submitted for ratification by the United States Senate.^[250]

In 2007, Transportation Secretary Mary Peters, with White House approval, urged governors and dozens of members of the House of Representatives to block California’s first-in-the-nation limits on greenhouse gases from cars and trucks, according to e-mails obtained by Congress.^[251] The U.S. Climate Change Science Program is a group of about twenty federal agencies and US Cabinet Departments, all working together to address global warming.

The Bush administration pressured American scientists to suppress discussion of global warming, according to the testimony of the Union of Concerned Scientists to the Oversight and Government Reform Committee of the U.S. House of Representatives.^[252][253] "High-quality science" was "struggling to get out," as the Bush administration pressured scientists to tailor their writings on global warming to fit the Bush administration's skepticism, in some cases at the behest of an ex-oil industry lobbyist. "Nearly half of all respondents perceived or personally experienced pressure to eliminate the words 'climate change,' 'global warming' or other similar terms from a variety of communications." Similarly, according to the testimony of senior officers of the Government Accountability Project, the White House attempted to bury the report "National Assessment of the Potential Consequences of Climate Variability and Change," produced by U.S. scientists pursuant to U.S. law.^[254] Some U.S. scientists resigned their jobs rather than give in to White House pressure to underreport global warming.^[252]

In the absence of substantial federal action, state governments have adopted emissions-control laws such as the Regional Greenhouse Gas Initiative in the Northeast and the Global Warming Solutions Act of 2006 in California.^[255]

Developing countries

In order to reconcile economic development with mitigating carbon emissions, developing countries need particular support, both financial and technical. One of the means of achieving this is the Kyoto Protocol's Clean Development Mechanism (CDM). The World Bank's Prototype Carbon Fund^[256] is a public private partnership that operates within the CDM.

An important point of contention, however, is how overseas development assistance not directly related to climate change mitigation is affected by funds provided to climate change mitigation.^[257] One of the outcomes of the UNFCC Copenhagen Climate Conference was the Copenhagen Accord, in which developed countries promised to provide US $30 million between 2010 and 2012 of new and additional resources.^[257] Yet it remains unclear what exactly the definition of additional is and the European Commission has requested its member states to define what they understand to be additional, and researchers at the Overseas Development Institute have found 4 main understandings:^[257]

1. Climate finance classified as aid, but additional to (over and above) the ‘0.7%’ ODA target;
2. Increase on previous year's Official Development Assistance (ODA) spent on climate change mitigation;
3. Rising ODA levels that include climate change finance but where it is limited to a specified percentage; and
4. Increase in climate finance not connected to ODA.

The main point being that there is a conflict between the OECD states budget deficit cuts, the need to help developing countries adapt to develop sustainably and the need to ensure that funding does not come from cutting aid to other important Millennium Development Goals.^[257]

However, none of these initiatives suggest a quantitative cap on the emissions from developing countries. This is considered as a particularly difficult policy proposal as the economic growth of developing countries are proportionally reflected in the growth of greenhouse emissions. Critics of mitigation often argue that, the developing countries' drive to attain a comparable living standard to the developed countries would doom the attempt at mitigation of global warming. Critics also argue that holding down emissions would shift the human cost of global warming from a general one to one that was borne most heavily by the poorest populations on the planet.

In an attempt to provide more opportunities for developing countries to adapt clean technologies, UNEP and WTO urged the international community to reduce trade barriers and to conclude the Doha trade round "which includes opening trade in environmental goods and services".^[258]

Non-governmental approaches

While many of the proposed methods of mitigating global warming require governmental funding, legislation and regulatory action, individuals and businesses can also play a part in the mitigation effort.

Choices in personal actions and business operations

Environmental groups encourage individual action against global warming, often aimed at the consumer. Common recommendations include lowering home heating and cooling usage, burning less gasoline, supporting renewable energy sources, buying local products to reduce transportation, turning off unused devices, and various others.

A geophysicist at Utrecht University has urged similar institutions to hold the vanguard in voluntary mitigation, suggesting the use of communications technologies such as videoconferencing to reduce their dependence on long-haul flights.^[259]

Air travel and shipment

In 2008, climate scientist Kevin Anderson raised concern about the growing effect of rapidly increasing global air transport on the climate in a paper,^[260] and a presentation,^[261] suggesting that reversing this trend is necessary to reduce emissions.

Part of the difficulty is that when aviation emissions are made at high altitude, the climate impacts are much greater than otherwise. Others have been raising the related concerns of the increasing hypermobility of individuals, whether traveling for business or pleasure, involving frequent and often long distance air travel, as well as air shipment of goods.^[262]

Business opportunities and risks

On 9 May 2005 Jeff Immelt, the chief executive of General Electric (GE), announced plans to reduce GE's global warming related emissions by one percent by 2012. "GE said that given its projected growth, those emissions would have risen by 40 percent without such action."^[263]

On 21 June 2005 a group of leading airlines, airports and aerospace manufacturers pledged to work together to reduce the negative environmental impact of aviation, including limiting the impact of air travel on climate change by improving fuel efficiency and reducing carbon dioxide emissions of new aircraft by fifty percent per seat kilometre by 2020 from 2000 levels. The group aims to develop a common reporting system for carbon dioxide emissions per aircraft by the end of 2005, and pressed for the early inclusion of aviation in the European Union's carbon emission trading scheme.^[264]

Investor response

Climate change is also a concern for large institutional investors who have a long term time horizon and potentially large exposure to the negative impacts of global warming because of the large geographic footprint of their multi-national holdings. SRI (Socially responsible investing) Funds allow investors to invest in funds that meet high ESG (environmental, social, governance) standards as such funds invest in companies that are aligned with these goals.^[265] Proxy firms can be used to draft guidelines for investment managers that take these concerns into account.^[266]

Legal action

In some countries, those affected by climate change may be able to sue major producers. Attempts at litigation have been initiated by entire peoples such as Palau^[267] and the Inuit,^[268] as well as non-governmental organizations such as the Sierra Club.^[269] Although proving that particular weather events are due specifically to global warming may never be possible,^[270] methodologies have been developed to show the increased risk of such events caused by global warming.^[271]

For a legal action for negligence (or similar) to succeed, "Plaintiffs ... must show that, more probably than not, their individual injuries were caused by the risk factor in question, as opposed to any other cause. This has sometimes been translated to a requirement of a relative risk of at least two."^[272] Another route (though with little legal bite) is the World Heritage Convention, if it can be shown that climate change is affecting World Heritage Sites like Mount Everest.^[273][274]

Legal action has also been taken to try to force the U.S. Environmental Protection Agency to regulate greenhouse gas emissions under the Clean Air Act,^[275] and against the Export-Import Bank and OPIC for failing to assess environmental impacts (including global warming impacts) under NEPA.

According to a 2004 study commissioned by Friends of the Earth, ExxonMobil and its predecessors caused 4.7 to 5.3 percent of the world's man-made carbon dioxide emissions between 1882 and 2002. The group suggested that such studies could form the basis for eventual legal action.^[276]

In 2015, Exxon, received a subpoena. According to the Washington Post and confirmed by the company, the attorney general of New York, Eric Schneiderman, opened an investigation into the possibility that the company had mislead the public and investors about the risks of climate change.^[277]

See also

• List of countries by ratio of GDP to carbon dioxide emissions
• 4 Degrees and Beyond International Climate Conference
• Alternative propulsion
• Avoiding dangerous climate change
• Biochar
• Black carbon
• Carbon diet
• Carbon negative fuel
• Climate change denial
• Climate change mitigation scenarios
• Climate bond
• Coal
• Contraction and Convergence
• Emissions reduction efforts
• Green IT
• Greenhouse gas removal
• Hell and High Water: Global Warming
• Hypermobility
• Iron fertilization
• List of climate change initiatives
• List of energy storage projects
• Low carbon diet
• Mitigation of peak oil
• Oil phase-out
• Overpopulation
• Resilience (ecology)
• Resistance (ecology)
• Stratospheric sulfur aerosols (geoengineering)
• Vulnerability
• Zero-carbon economy
• Environmental impact of the coal industry

By country

• Debate over China's economic responsibilities for climate change mitigation
• European Climate Change Programme
• Mitigation of global warming in Australia

Notes

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2. Ipsos 2011, p. 3
3. Fisher, B.S.; et al., "Ch. 3: Issues related to mitigation in the long-term context", Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, 3.5 Interaction between mitigation and adaptation, in the light of climate change impacts and decision-making under long-term uncertainty, in IPCC AR4 WG3 2007
4. IPCC, "Summary for policymakers", Climate Change 2007: Working Group III: Mitigation of Climate Change, Table SPM.3, C. Mitigation in the short and medium term (until 2030), in IPCC AR4 WG3 2007
5. Oppenheimer, M., et al., Section 19.7.1: Relationship between Adaptation Efforts, Mitigation Efforts, and Residual Impacts, in: Chapter 19: Emergent risks and key vulnerabilities (archived July 8 2014), pp.46-49, in IPCC AR5 WG2 A 2014
6. http://report.mitigation2014.org/drafts/final-draft-postplenary/ipcc_wg3_ar5_final-draft_postplenary_chapter3.pdf
7. "Sec 5.5 Technology flows and development", Climate Change 2007: Synthesis Report, in IPCC AR4 SYR 2007, p. 68
8. Levine, M.; et al., "Ch 6: Residential and commercial buildings", Climate Change 2007: Working Group III: Mitigation of Climate Change, Sec 6.4.2 Thermal envelope, in IPCC AR4 WG3 2007
9. UK Royal Society 2009
10. UNFCCC (5 March 2013), Introduction to the Convention, UNFCCC
11. UNFCCC (2002), Full Text of the Convention, Article 2: Objectives, UNFCCC
12. Oppenheimer, M., et al., FAQ 19.1, in: Chapter 19: Emergent risks and key vulnerabilities (archived July 8 2014), p.52, in IPCC AR5 WG2 A 2014
13. UNFCCC. Conference of the Parties (COP) (15 March 2011), Report of the Conference of the Parties on its sixteenth session, held in Cancun from 29 November to 10 December 2010. Addendum. Part two: Action taken by the Conference of the Parties at its sixteenth session (PDF), Geneva, Switzerland: United Nations, p.3, paragraph 4. Document available in UN languages and text format.
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External links

• Intergovernmental Panel on Climate Change – Includes the Working Group III Report "Mitigation of Climate Change" as part of the Fourth Assessment Report
• Why Black Carbon and Ozone Also Matter, in September/October 2009 Foreign Affairs with Veerabhadran Ramanathan and Jessica Seddon Wallack.
• The Climate Threat We Can Beat, in May/June 2012 Foreign Affairs with David G. Victor, Charles F. Kennel, Veerabhadran Ramanathan.
• The Radical Emission Reduction Conference

European Union

• European Union's European Climate Change Programme
• EU New Energy Policy
• European Union Greenhouse Gas Emission Trading Scheme (EU ETS)
• Community strategy to reduce CO2 from light vehicles.
• UK's Climate Change Programme
• The Stern Review on the economics of climate change – Parts III and IV of the Stern Review are on climate change mitigation

USA

• EPA: What You Can Do.
• U.S. Mayors Climate Protection Agreement signed by 178 mayors representing nearly 40 million Americans
• California Climate Change Portal.

Academic

• Rivington M, Matthews KB, Buchan K and Miller D (2005) "An integrated assessment approach to investigate options for mitigation and adaptation to climate change at the farm-scale", NJF Seminar 380, Odense, Denmark, 7–8 November 2005.
• Jacobson, M.Z.; Delucchi, M.A. (2009). "A Plan to Power 100 Percent of the Planet with Renewables" (originally published as "A Path to Sustainable Energy by 2030")". Scientific American 301 (5): 58–65. doi:10.1038/scientificamerican1109-58. PMID 19873905.
• Living On a New Earth, Scientific American April 2010
• Global Warming Newswire – published scientific studies on global warming