Coal, oil, and natural gas remain the primary global energy sources even as renewables have begun rapidly increasing.[1]
The Climate Change Performance Index ranks countries by greenhouse gas emissions (40% of score), renewable energy (20%), energy use (20%), and climate policy (20%).

Climate change mitigation consists of actions to limit the magnitude or rate of global warming and its related effects.[2] This generally involves reductions in human emissions of greenhouse gases (GHGs).[3]

Fossil fuels account for about 70% of GHG emissions.[4] The main challenge is their substitution with low-carbon energy sources. Due to massive price drops, wind power and solar photovoltaics (PV) are increasingly out-competing oil, gas and coal[5] but require energy storage and extended electrical grids. Mitigation may also be achieved by increasing the capacity of carbon sinks, for example through reforestation.[3] Other examples of mitigation include reducing energy demand by increasing energy efficiency and removing carbon dioxide from Earth's atmosphere.[3][6] Climate engineering is often controversial but might have to be used in addition to reducing GHG emissions.[7]

Almost all countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC).[8] The ultimate objective of the UNFCCC is to stabilize atmospheric concentrations of GHGs at a level that would prevent dangerous human interference with the climate system.[9] In 2010, Parties to the UNFCCC agreed that future global warming should be limited to below 2 °C (3.6 °F) relative to the pre-industrial level.[10] With the Paris Agreement of 2015 this was confirmed.

With the Special Report on Global Warming of 1.5 °C, the International Panel on Climate Change has emphasized the benefits of keeping global warming below this level, suggesting a global collective effort that may be guided by the 2015 United Nations Sustainable Development Goals.[11] Emissions pathways with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure including transport and buildings, and industrial systems.[12]

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.[13][14][15] However, globally the benefits of keeping warming under 2 °C exceed the costs.[16]

Greenhouse gas concentrations and stabilization

refer to caption and adjacent text
Stabilizing CO2 emissions at their present level would not stabilize its concentration in the atmosphere.[17]
refer to caption and adjacent text
Stabilizing the atmospheric concentration of CO2 at a constant level would require emissions to be effectively eliminated.[17]

The UNFCCC aims to stabilize greenhouse gas (GHG) concentrations 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.[18] Currently human activities are adding CO2 to the atmosphere faster than natural processes can remove it.[17] According to a 2011 US study, stabilizing atmospheric CO2 concentrations would require anthropogenic CO2 emissions to be reduced by 80% relative to the peak emissions level.[19][needs update]

The IPCC works with the concept of a fixed carbon emissions budget. If emissions remain on the current level of 42 GtCO
2
, the carbon budget for 1.5°C could be exhausted in 2028.[20] The rise in temperature to that level would occur with some delay between 2030 and 2052.[21]. Even if it was possible to achieve negative emissions in the future, 1.5°C must not be exceeded at any time to avoid the loss of ecosystems.[22]

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 about 10% each year until zero emissions are reached around 2030.[23][24][25][26][needs update]

Sources of greenhouse gas emissions

GHG emissions 2018
without land-use change
using 100 year GWP[27]

  CO
2
by fossil fuel (72%)
  CH4 methane (19%)
  N
2
O
nitrous oxide (6%)
  Fluorinated gases (3%)

CO
2
emissions by fuel type[28]

  coal (40%)
  oil (34%)
  gas (21%)
  cement (4%)
  flaring (1%)

With the Kyoto Protocol, the reduction of almost all anthropogenic greenhouse gases has been addressed.[29] These gases are CO2, methane (CH4), nitrous oxide (N2O) and fluorinated gases (F-Gases): the hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur hexafluoride (SF6). Their global warming potential (GWP) depends on their lifetime in the atmosphere. Methane has a relatively short atmospheric lifetime of about 12 years but a high immediate impact.[30] 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.[17] Estatimatons largely depend on the ability of oceans and land sinks to absorb GHGs. The risk of feedback effects in global warming leads to high uncertainties in the determination of GWP values.

Carbon dioxide (CO
2
)

  • Fossil fuel: oil, gas and coal are the major driver of anthropogenic global warming with annual emissions of 34.6 GtCO
    2
    in 2018.[28]
  • cement production is estimated 1.5 GtCO
    2
    [28]
  • Land-use change (LUC) is the imbalance of deforestation and reforestation. Estimations are very uncertain at 3.8 GtCO
    2
    [27]. Wildfires cause emissions of about 7 GtCO
    2
    [31][32]
    • Fossil fuel (33%) also accounts for most of the methane emissions including gas distribution, leakages and gas venting.[33]
    • Cattle (21%) account for two thirds of the methane emitted by livestock, followed by buffalo, sheep and goats[34]
    • Human waste and waste water (21%): When biomass waste in landfills and organic substances in domestic and industrial waste water are decomposed by bacteria in anaerobic conditions, substantial amounts of methane are generated. [33]
    • Rice cultivation (10%) on flooded rice fields is another agricultural source, where anaerobic decomposition of organic material produces methane.
      • Most emissions by agriculture, especially meat production: cattle (droppings on pasture), fertilizers, animal manure
        • Switchgear in the power sector, semi-conducture manufacture, aluminium production and a large unknown source of SF6

          Projections of future greenhouse gas emissions are highly uncertain.[36] In the absence of policies to mitigate climate change, GHG emissions could rise significantly over the 21st century.[37] Current scientific projections warn of a 4.5 degree temperature rise in decades. [38]

          Methods and means

          We cannot be radical enough in dealing with those issues that face us at the moment. The question is what is practically possible.

          David Attenborough, in testimony to the UK House of Commons Business, Energy and Industrial Strategy Committee.[39]

          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.[40]

          Economic tools can be useful in designing climate change mitigation policies.[41] Abolishing fossil fuel subsidies is very important but must be done carefully to avoid making poor people poorer.[42]

          Other frequently discussed means include efficiency, public transport, 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 changes,[43] and changing business practices. Replacing gasoline and diesel vehicles with electric means their emissions would be displaced away from street level, where they cause illness.

          Another consideration is how future socioeconomic development proceeds.[44]

Fossil fuel substitution

As most greenhouse gas emissions are due to fossil fuels, rapidly phasing out oil, gas and coal is critical.[45] The incentive to use 100% renewable energy has been created by global warming and other ecological as well as economic concerns.[46] According to the IPCC, there are few fundamental technological limits to integrating a portfolio of renewable energy technologies to meet most of total global energy demand.[47]

The global primary energy demand was 161,320 TWh in 2018.[48] This refers to electricity, transport and heating including all losses. The primary energy demand in a low-carbon economy is difficult to determine. In transport and electricity production, fossil fuel usage has a low efficiency of less than 50%. Motors of vehicles produce a lot of heat which is wasted. Electrification of all sectors and switching to renewable energy can lower the primary energy demand significantly. On the other hand, storage requirements and reconversion to electricity lower the efficiency of renewable energy.

In 2018, biomass and waste was listed with a share of 10% of primary energy, hydro power with 3%. Wind, solar energy and other renewables were at 2%.[48]

Low-carbon energy sources

Wind and sun can be sources for large amounts of low-carbon energy at competitive production costs. Solar PV module prices fell by around 80% in the 2010s, and wind turbine prices by 30–40%.[49] But even in combination, generation of variable renewable energy fluctuates a lot. This can be tackled by extending grids over large areas with a sufficient capacity or by using energy storage. According to the International Renewable Energy Agency (IRENA), the deployment of renewable energy would have to be accelerated six-fold though to stay under the 2 °C target. [50]Load management of industrial energy consumption can help to balance the production of renewable energy production and its demand. Electricity production by biogas and hydro power can follow the energy demand.

Solar energy

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.[51]
  • Solar photovoltaics has become the cheapest way to produce electric energy in many regions of the world, with production costs down to 0.015 - 0.02 US$/KWh in desert regions.[52] The growth of photovoltaics is exponential and has doubled every three years since the 1990s.
  • A different technology is concentrated solar power (CSP) using mirrors or lenses to concentrate a large area of sunlight onto a receiver. With CSP, the energy can be saved up for a few hours. Prices in Chile are expected to fall below 0.05 US$/KWh in 2020.[53]
  • The Shepherds Flat Wind Farm is an 845 megawatt (MW) nameplate capacity, wind farm in the US state of Oregon, each turbine is a nameplate 2 or 2.5 MW electricity generator.

    Regions in the higher northern and southern latitudes have the highest potential for wind power.[54] Installed capacity has reached 650 GW in 2019. Offshore wind power currently has a share of about 10% of new installations.[55] Offshore wind farms are more expensive but the units deliver more energy per installed capacity with less fluctuations.

    Hydro Power

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

    Hydroelectricity plays a leading role in countries like Brazil, Norway and China:[56] but there are geographical limits and environmental issues.[57]Tidal power can be used in coastal regions.

    Bioenergy

    Biogas plants can provide dispatchable electricity generation, and heat when needed.[58] A common concept is the co-fermentation of energy crops mixed with manure in agriculture. Burning plant-derived biomass releases CO
    2
    , but it has still been classified as a renewable energy source in the EU and UN legal frameworks because photosynthesis cycles the CO
    2
    back into new crops. How a fuel is produced, transported and processed has a significant impact on lifecycle emissions. Transporting fuels over long distances and excessive use of nitrogen fertilisers can reduce the emissions savings made by the same fuel compared to natural gas by between 15 and 50 per cent.[59] Renewable biofuels are starting to be used in aviation.

    Nuclear power

    In most 1.5 °C pathways nuclear power increases its share.[60] The main[citation needed] advantage is the ability to deliver large amounts of base load. It has been repeatedly classified as a climate change mitigation technology.[61]

    On the other hand, nuclear power comes with environmental risks which could outweigh the benefits. Apart from nuclear accidents, the disposal of radioactive waste can cause damage and costs over more than one million years. Separated plutonium could be used for nuclear weapons. [62][63] Public opinion about nuclear power varies widely between countries.[64][65]

    As of 2019 the cost of extending nuclear power plant lifetimes is competitive with other electricity generation technologies, including new solar and wind projects.[66] New projects are reported to be highly dependent on public subsidies.[67]

    Nuclear fusion research, in the form of the International Thermonuclear Experimental Reactor is underway but fusion is not likely to be commercially widespread before 2050.[68]

    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.[69][70][71]

    Natural gas

    Natural gas, which is mostly methane, is viewed as a bridge fuel since it produces about half as much CO
    2
    as burning coal.[72] Gas-fired power plants can provide the required flexibility in electricity production in combination wind and solar energy.[73] But methane is itself a potent greenhouse gas, and it currently leaks from production wells, storage tanks, pipelines, and urban distribution pipes for natural gas.[74] In a low-carbon scenario, gas-fueled power plants could still continue operation if methane was produced using power-to-gas technology with renewable energy sources.

    Energy storage

    Wind energy and photovoltaics can deliver large amounts of electric energy but not at any time and place. One approach is the conversation into storable forms of energy. This generally leads to losses in efficiency. A study by Imperial College London calculated the lowest levelised cost of different systems for mid-term and seasonal storage. In 2020, pumped hydro (PHES), compressed air (CAES) and Li-on batteries are most cost effective depending on charging rhythm. For 2040, a more significant role for Li-on and hydrogen is projected.[75]

    • Long distance power lines help to minimize storage requirements. A large network can smoothen variations of wind energy. With a global grid, even photovoltaics could be available all day and night. The strongest High-voltage direct current (HVDC) connections are quoted with losses of only 1.6% per 1000 km[83] HVDC is currently only used for point-to-point connections.

      China has built many HVDC connections within the country and supports the idea of a global, intercontinental grid as a backbone system for the existing national AC grids. [84] A super grid in the USA in combination with renewable energy could reduce GHG emissions by 80%. [85]

      Smart grid and load management

      Instead of expanding grids and storage for more power, there are a variety of ways to affect the size and timing of electricity demand on the consumer side. Identifying and shifting electrical loads can reduce power bills by taking advantage of lower off-peak rates and flatten demand peaks. Traditionally, the energy system has treated consumer demand as fixed and used centralised supply options to manage variable demand. Now, better data systems and emerging onsite storageand generation technologies can combine with advanced, automated demand control software to pro-actively manage demand and respond to energy market prices.[86]

      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 need to have a smart meter in order for the utility to calculate credits.

      Demand response devices can 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 allows electric cars to recharge at the least expensive rates independent of the time of day. Vehicle-to-grid uses a car's battery or fuel cell to supply the grid temporarily.

      Decarbonization of transport

      Between a quarter and three-quarters of cars on the road by 2050 are forecast to be electric.[87]

      Hydrogen can be a solution for long-distance transport by trucks and hydrogen-powered ships where batteries alone are too heavy.[88][89] Passenger cars using hydrogen are already produced in small numbers. While being more expensive than battery powered cars, they can refuel much faster, offering higher ranges up to 700 km.[90] The main disadvantage of hydrogen is the low efficiency of only 30%. When used for vehicles, more than twice as much energy is needed compared to a battery powered electric car. [91]

      Although aviation biofuel is used somewhat, as of 2019 decarbonisation of aviation by 2050 is claimed to be "really difficult".[92]

      Decarbonization of heating

      The buildings sector accounts for 23% of global energy-related CO2 emissions[93] About half of the energy is used for space and water heating.[94] A combination of electric heat pumps and building insolation can reduce the primary energy demand significantly. Generally, electrification of heating would only reduce GHG emissions if the electric power comes from low-carbon sources. A fossil-fuel power station may only deliver 3 units of electrical energy for every 10 units of fuel energy released. Electrifying heating loads may also provide a flexible resource that can participate in demand response to integrate variable renewable resources into the grid.

      Heat pump

      Outside unit of an air-source heat pump

      A modern heat pump typically produces around three times more thermal energy than electrical energy consumed, giving an effective efficiency of 300%, depending on the coefficient of performance. It uses an electrically driven compressor to operate a refrigeration cycle that extracts heat energy from outdoor air and moves that heat to the space to be warmed. In the summer months, the cycle can be reversed for air conditioning. In areas with average winter temperatures well below freezing, ground source heat pumps are more efficient than air-source heat pumps. The high purchase price of a heat pump compared to resistance heaters may be offset when air conditioning is also needed.

      With a market share of 30% and clean electricity, heat pumps could reduce global CO
      2
      emissions by 8% annually.[95] Using ground source heat pumps could reduce around 60% of the primary energy demand and 90% of CO
      2
      emissions of natural gas boilers in Europe in 2050 and make handling high shares of renewable energy easier.[96] Using surplus renewable energy in heat pumps is regarded as the most effective household means to reduce global warming and fossil fuel depletion.[97]

      Electric resistant heating

      Radiant heaters in households are cheap and wide-spread but less efficient than heat pumps. In areas like Norway, Brazil, and Quebec that have abundant hydroelectricity, electric heat and hot water are common. Large scale hot water tanks can be used for demand-side management and store variable renewable energy over hours or days.

Energy conservation

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.[98]

Energy efficiency

Efficiency covers a wide range of means from building insulation to public transport. The cogeneration of electric energy and district heat also improves efficiency.

Lifestyle and behavior

The IPCC Fifth Assessment Report emphasises that behaviour, lifestyle, and cultural change have a high mitigation potential in some sectors, particularly when complementing technological and structural change.[99]:20 Examples would be heating a room less or driving less. In general, higher consumption lifestyles have a greater environmental impact. Several scientific studies have shown that when relatively rich people wish to reduce their carbon footprint, there are a few key actions they can take such as living car-free (2.4 tonnes CO2), avoiding one round-trip transatlantic flight (1.6 tonnes) and eating a plant-based diet (0.8 tonnes).[100]

These appear to differ significantly from the popular advice for "greening" one's lifestyle, which seem to fall mostly into the "low-impact" category: Replacing a typical car with a hybrid (0.52 tonnes); Washing clothes in cold water (0.25 tonnes); Recycling (0.21 tonnes); Upgrading light bulbs (0.10 tonnes); etc. The researchers found that public discourse on reducing one's carbon footprint overwhelmingly focuses on low-impact behaviors, and that mention of the high-impact behaviors is almost non-existent in the mainstream media, government publications, school textbooks, etc.[100][101][102]

Scientists also argue that piecemeal behavioural changes like re-using plastic bags are not a proportionate response to climate change. Though being beneficial, these debates would drive public focus away from the requirement for an energy system change of unprecedented scale to decarbonise rapidly.[103]

Dietary change

Overall, food accounts for the largest share of consumption-based GHG emissions with nearly 20% of the global carbon footprint, followed by housing, mobility, services, manufactured products, and construction. Food and services are more significant in poor countries, while mobility and manufactured goods are more significant in rich countries.[104]:327 The widespread adoption of a vegetarian diet could cut food-related greenhouse gas emissions by 63% by 2050.[105] China introduced new dietary guidelines in 2016 which aim to cut meat consumption by 50% and thereby reduce greenhouse gas emissions by 1 billion tonnes by 2030.[106] A 2016 study concluded that taxes on meat and milk could simultaneously result in reduced greenhouse gas emissions and healthier diets. The study analyzed surcharges of 40% on beef and 20% on milk and suggests that an optimum plan would reduce emissions by 1 billion tonnes per year.[107][108]

Carbon sinks and removal

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. Carbon dioxide removal on the other hand is a permanent removal of carbon dioxide out of the atmosphere. Examples are direct air capture, enhanced weathering technologies such as storing it in geologic formations underground and biochar. These processes are sometimes considered variations of sinks or mitigation,[109][110] and sometimes as geoengineering.[111] In combination with other mitigation measures, carbon sinks and removal are crucial for meeting the 2 degree target.[112]

The Antarctic Climate and Ecosystems Cooperative Research Centre (ACE-CRC) notes that one third of humankind's annual emissions of CO
2
are absorbed by the oceans.[113] However, this also leads to ocean acidification, which may harm marine life.[114] Acidification lowers 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.[115]

Reforestation, avoided deforestation and afforestation

Transferring land rights to indigenous inhabitants is argued to efficiently conserve forests.

Almost 20 percent (8 GtCO2/year) of total greenhouse-gas emissions were from deforestation in 2007.[needs update] It is estimated that avoided deforestation reduces CO2 emissions at a rate of 1 tonne of CO2 per $1–5 in opportunity costs from lost agriculture. Reforestation, which is restocking of depleted forests, could save at least another 1 GtCO2/year, at an estimated cost of $5–15/tCO2.[116] According to research conducted at ETH Zurich, restoring all degraded forests all over the world could capture about 205 billion tons of carbon in total (which is about 2/3rd of all carbon emissions, bringing global warming down to below 2 °C[citation needed]).[117][118] Afforestation is where there was previously no forest. According to research by Tom Crowther et al., there is still enough room to plant an additional 1.2 trillion trees. This amount of trees would cancel out the last 10 years of CO2 emissions and sequester 160 billion tons of carbon.[119][120][121][122] This vision is being executed by the Trillion Tree Campaign. Other studies[123][124] have found large-scale afforestation can do more harm than good or such plantations are estimated to have to be prohibitively massive to reduce emissions.

Transferring rights over land from public domain to its indigenous inhabitants, who have had a stake for millennia in preserving the forests that they depend on, is argued to be a cost-effective strategy to conserve forests.[125] This includes the protection of such rights entitled in existing laws, such as India's Forest Rights Act.[125] The transferring of such rights in China, perhaps the largest land reform in modern times, has been argued to have increased forest cover.[126][127] Granting title of the land has shown to have two or three times less clearing than even state run parks, notably in the Brazilian Amazon.[128][129] Conservation methods that exclude humans and even evict inhabitants from protected areas (called "fortress conservation") often lead to more exploitation of the land as the native inhabitants then turn to work for extractive companies to survive.[126]

With increased intensive agriculture and urbanization, there is an increase in the amount of abandoned farmland. By some estimates, for every acre of original old-growth forest cut down, more than 50 acres of new secondary forests are growing, even though they do not have the same biodiversity as the original forests and original forests store 60% more carbon than these new secondary forests.[130][131] According to a study in Science, promoting regrowth on abandoned farmland could offset years of carbon emissions.[132] Research by the university ETH Zurich estimates that Russia, the United States and Canada have the most land suitable for reforestation.[133][134]

Avoided desertification

Restoring grasslands stores CO2 from the air in 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.[135] A method proposed to restore grasslands uses fences with many small paddocks and moving herds from one paddock to another after a day or two in order to mimic natural grazers and allowing the grass to grow optimally.[135][136][137] Additionally, when part of the leaf matter is consumed by an animal in the herd, 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.[135] 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.[135] 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.[138]

Additionally, the global warming induced thawing of the permafrost, which stores about two times the amount of the carbon currently released in the atmosphere,[139] releases the potent greenhouse gas, methane, in a positive feedback cycle that is feared to lead to a tipping point called runaway climate change. While the permafrost is about 14 degrees Fahrenheit, a blanket of snow insulates it from the colder air above which could be 40 degrees below zero Fahrenheit.[140] A method proposed to prevent such a scenario is to bring back large herbivores such as seen in Pleistocene Park, where they keep the ground cooler by reducing snow cover height by about half and eliminating shrubs and thus keeping the ground more exposed to the cold air.[141]

Protect healthy soils and recover damaged soils, can remove from the atmosphere 5.5 billion tons of carbon annually, what is approximately equal to the annual emissions of the USA.[142]

Carbon capture and storage

Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a large point source, for example burning natural gas

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 IPCC estimates that the costs of halting global warming would double without CCS.[143] The International Energy Agency says CCS is "the most important single new technology for CO2 savings" in power generation and industry.[144][better source needed] Norway's Sleipner gas field, beginning in 1996, stores almost a million tons of CO2 a year to avoid penalties in producing natural gas with unusually high levels of CO2.[145][144] According to a Sierra Club analysis, the US Kemper Project, which was due to be online in 2017, is the most expensive power plant ever built for the watts of electricity it will generate.[146]

Enhanced weathering

Enhanced weathering is the removal of carbon from the air into the earth, enhancing the natural carbon cycle where carbon is mineralized into rock. The CarbFix project couples with carbon capture and storage in power plants to turn carbon dioxide into stone in a relatively short period of two years. While this project used basalt rocks, olivine has also shown promise.[143]

Geoengineering

IPCC (2007) concluded that geoengineering options, such as ocean fertilization to remove CO2 from the atmosphere, remained largely unproven.[147] 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."[148] 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. Increasing ocean absorption of carbon dioxide (carbon sequestration) and screening out some sunlight were evaluated. 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."[148] 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."[149]

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.[150]

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. As in the case of the US Kemper Project with carbon capture, costs of energy produced will grow significantly. CO2 can also be used in commercial greenhouses, giving an opportunity to kick-start the technology.

Solar radiation management

The main purpose of solar radiation management is 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.[151]

By Sector

Agriculture

Managed grazing methods are argued to be able to restore grasslands, thereby significantly decreasing atmospheric CO2 levels.[138]

An agriculture that mitigates climate change is generally called sustainable agriculture, defined as an agriculture that "meets society's food and textile needs in the present without compromising the ability of future generations to meet their own needs".[152]

One mode of agriculture considered as relatively sustainable is regenerative agriculture.[153] It includes several methods, the main of which are: conservation tillage, diversity, rotation and cover crops, minimizing physical disturbance, minimizing the usage of chemicals. It has other benefits like improving the state of the soil and consequently yields. Some of the big agricultural companies like General Mills and a lot of farms support it.[154]

In the United States, soils account for about half of agricultural greenhouse gas emissions while agriculture, forestry and other land use emits 24%.[155] Globally, livestock is responsible for 18 percent of greenhouse gas emissions, according to FAO's report called "Livestock's Long Shadow: Environmental Issues and Options"[156][better source needed]

The US EPA says soil management practices that can reduce the emissions of nitrous oxide (N
2
O
) from soils include fertilizer usage, irrigation, and tillage. Manure management and rice cultivation also produce gaseous emissions.

Important mitigation options for reducing the greenhouse gas emissions from livestock (especially ruminants) include genetic selection[157][158] introduction of methanotrophic bacteria into the rumen,[159][160] diet modification and grazing management.[161][162][163] Other options include just using ruminant-free alternatives instead, such as milk substitutes and meat analogues. Non-ruminant livestock (e.g. poultry) generates far fewer emissions.[164]

Methods that 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.[165][166] Because only 5% of US farmland currently uses no-till and residue mulching, there is a large potential for carbon sequestration.[167]

A 2015 study found that farming can deplete soil carbon and render soil incapable of supporting life; however, the study also showed that conservation farming can protect carbon in soils, and repair damage over time.[168] The farming practice of cover crops has been recognized as climate-smart agriculture.[169] Best management practices for European soils were described to be increase soil organic carbon: conversion of arable land to grassland, straw incorporation, reduced tillage, straw incorporation combined with reduced tillage, ley cropping system and cover crops.[170]

In terms of prevention, vaccines are being developed in Australia to reduce the significant global warming contributions from methane released by livestock via flatulence and eructation.[171][needs update]

A project to mitigate climate change with agriculture was launched in 2019 by the "Global EverGreening Alliance". The target is to sequester carbon from the atmosphere with Agroforestry. By 2050 the restored land should sequestrate 20 billion of carbon annually[172]

Transport

Transportation emissions account for roughly 1/4 of emissions worldwide[173][better source needed] and are even more important in terms of impact in developed nations especially in North America and Australia. Many citizens of countries like the United States and Canada who drive personal cars often, see well over half of their climate change impact stemming from the emissions produced from their cars.[citation needed] Modes of mass transportation such as bus, light rail (metro, subway, etc.), and long-distance rail are far and away the most energy-efficient means of motorized transportation for passengers, able to use in many cases over twenty times less energy per person-distance than a personal automobile. Modern energy-efficient technologies, such as electric vehicles and carbon-neutral synthetic gasoline and jet fuel[citation needed] may also help to reduce the consumption of petroleum, land use changes and emissions of carbon dioxide. Utilizing rail transport, especially electric rail, over the far less efficient air transport and truck transport significantly reduces emissions.[174][175] With the use of electric trains and cars in transportation there is the opportunity to run them with low-carbon power, producing far fewer emissions.

Urban planning

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.[176]

Effective urban planning to reduce sprawl aims to decrease Vehicle Miles Travelled (VMT), lowering emissions from transportation. Personal cars are extremely inefficient at moving passengers, while public transport and bicycles are many times more efficient (as is the simplest form of human transportation, walking). All of these are encouraged by urban/community planning and are an effective way to reduce greenhouse gas emissions. 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,[177] and government-supported energy efficiency programmes can make a difference.[178]

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.[179][180] 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 and so forth.[citation needed]

Population

Various organizations[citation needed] promote human population planning as a means for mitigating global warming.[181] 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.

According to a 2017 study published in Environmental Research Letters, having one less child would have a much more substantial effect on greenhouse gas emissions compared with for example living car free or eating a plant-based diet.[100] However this has been criticised: both as a category mistake for assigning descendants emissions to their ancestors[182] and for the very long timescale of reductions.[183]

Population control efforts are impeded by there being somewhat of a taboo in some countries against considering any such efforts.[184] Also, various religions discourage or prohibit some or all forms of birth control. Population size has a vastly different per capita effect on global warming in different countries, since the per capita production of anthropogenic greenhouse gases varies greatly by country.[185]

Costs and benefits

Globally the benefits of keeping warming under 2 °C exceed the costs.[186] However some consider cost–benefit analysis unsuitable for analysing climate change mitigation as a whole, but still useful for analysing the difference between a 1.5 °C target and 2 °C.[187]

Costs

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.[116] Mitigation costs will vary according to how and when emissions are cut: early, well-planned action will minimise the costs.[116]

Many economists estimate the cost of climate change mitigation at between 1% and 2% of GDP.[187] In 2019, scientists from Australia, and Germany presented the "One Earth Climate Model" showing how temperature increase can be limited to 1.5 °C for 1.7 trillion dollars a year.[188][189] According to this study, a global investment of approximately $1.7 trillion per year would be needed to keep global warming below 1.5°C. The method used by the One Earth Climate Model does not resort to dangerous geo-engineering methods. Whereas this is a large sum, it is still far less than the subsidies governments currently provided to the ailing fossil fuel industry, estimated at more than $5 trillion per year by the International Monetary Fund.[190][191]

Benefits

By addressing climate change, we can avoid the costs associated with the effects of climate change. According to the Stern Review, inaction can be as high as the equivalent of losing at least 5% of global gross domestic product (GDP) each year, now and forever (up to 20% of the GDP or more when including a wider range of risks and impacts), whereas mitigating climate change will only cost about 2% of the GDP. Also, delaying to take significant reductions in greenhouse gas emissions may not be a good idea, when seen from a financial perspective.[192][193]

The research organization Project Drawdown identified global climate solutions and ranked them according to their benefits.[194] Early deaths due to fossil fuel air pollution with a temperature rise to 2 °C cost more globally than mitigation would: and in India cost 4 to 5 times more.[195]

Sharing

One of the aspects of mitigation is how to share the costs and benefits of mitigation policies. 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).[196]

Rich people tend to emit more GHG than poor people.[197] Activities of the poor that involve emissions of GHGs are often associated with basic needs, such as cooking. For richer people, emissions tend to be associated with things such as eating beef, cars, frequent flying, and home heating.[198] 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):[197]