Climate change mitigation

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.

• 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]
• Flaring: In crude oil production vast amounts of associated gas are commonly flared as waste or unusable gas.

• 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. [33]

• Most emissions by agriculture, especially meat production: cattle (droppings on pasture), fertilizers, animal manure[33]

• Switchgear in the power sector, semi-conducture manufacture, aluminium production and a large unknown source of SF₆[35]

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]

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]

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.

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]
• 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 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.

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.

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.

In most 1.5 °C pathways nuclear power increases its share.[60] The main 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]

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, 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.

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]

• Li-on batteries are widely used in battery storage power stations and, as of 2020, are starting to be used in vehicle-to-grid storage.[76] They provide a sufficient round-trip efficiency of 75-90 %.[77] However their production can cause environmental problems.[78]

• Hydrogen may be useful for seasonal energy storage.[79] The low efficiency of 30% must improve dramatically before hydrogen storage can offer the same overall energy efficiency as batteries.[77] For the electricity grid a German study estimated high costs of 0.176 €/KWh for reconversion concluding that substituting the electricity grid expansion entirely with hydrogen reconversion systems does not make sense from an economic standpoint.[80] The concept of solar hydrogen is discussed for remote desert projects where grid connections to demand centers are not available.[81] Because it has more energy per unit volume sometimes it may be better to use hydrogen in ammonia.[82]

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]

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.

Electric bus in Montreal

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]

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.

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]

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.

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

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]

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]

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

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.[116][117][118][119]This vision is being executed by the Trillion Tree Campaign. 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).[120][121] other research has found large-sacle tree planting can do more harm than good.[122]

Almost 20 percent (8 GtCO₂/year) of total greenhouse-gas emissions were from deforestation in 2007. 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 could save at least another 1 GtCO₂/year, at an estimated cost of $5–15/tCO₂.[123] Afforestation is where there was previously no forest – such plantations are estimated to have to be prohibitively massive to reduce emissions by itself.[124]

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] Excluding humans and even evicting 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 half a hectare of original old-growth forest cut down, more than 20 hectares 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]

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. A method proposed to prevent such a scenario is to bring back large herbivores such as seen in Pleistocene Park, where their trampling naturally keep the ground cooler by eliminating shrubs and keeping the ground exposed to the cold air.[140]

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.[141]

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.[142] The International Energy Agency says CCS is "the most important single new technology for CO₂ savings" in power generation and industry.[143] 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.[144][143] 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.[145]

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.[142]

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.[149]

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.

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.[150]

Transportation emissions account for roughly 1/4 of emissions worldwide[172] 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. 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 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.[173][174] 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.

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.[175]

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.

Emissions from housing are substantial,[176] and government-supported energy efficiency programmes can make a difference.[177]

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.[178][179] This saves energy because it cools buildings and reduces the urban heat island effect thus reducing the use of air conditioning.

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.

Various organizations promote human population planning as a means for mitigating global warming.[180] 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[181] and for the very long timescale of reductions.[182]

Population control efforts are impeded by there being somewhat of a taboo in some countries against considering any such efforts.[183] 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.[184]

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.[123] Mitigation costs will vary according to how and when emissions are cut: early, well-planned action will minimise the costs.[123]

Many economists estimate the cost of climate change mitigation at between 1% and 2% of GDP.[186] 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.[187][188] 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.[189][190]

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.[191][192]

The research organization Project Drawdown identified global climate solutions and ranked them according to their benefits.[193] 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.[194]

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).[195]

Rich people tend to emit more GHG than poor people.[196] 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.[197] The impacts of cutting emissions could therefore have different impacts on human welfare according to wealth.

There have been different proposals on how to allocate responsibility for cutting emissions (Banuri et al., 1996, pp. 103–105):[196]

• 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: 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. 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).[198]
  • 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.

• Ad hoc: Lashof (1992) and Cline (1992) (referred to by Banuri et al., 1996, p. 106),[196] 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).[198] 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.

The graph shows multiple pathways to limit climate change to 1.5 °C or 2 °C. All pathways include negative emission technologies such as afforestation and bio-energy with carbon capture and storage.

The main current international agreement on combating climate change is the Paris agreement. The Paris Agreement's long-term temperature goal is to keep the increase in global average temperature to well below 2°C above pre-industrial levels; and to pursue efforts to limit the increase to 1.5°C. Each country must determine, plan, and regularly report on the contribution that it undertakes to mitigate global warming.[206] Climate change mitigation measures can be written down in national environmental policy documents like the nationally determined contributions (NDC).

The Paris agreement succeeds the 1997 Kyoto Protocol which expires in 2020, and is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). Countries that ratified the Kyoto protocol 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.

How well each individual country is on track to achieving its Paris agreement commitments can be followed on-line.[207]

Except the main agreements there are many additional pledges made by international coalitions, countries, cities, regions and businesses. According to a report published in september 2019 before the 2019 UN Climate Action Summit, full implementation of all pledges, including those in the Paris Agreement, will be sufficient to limit temperature rise to 2 degrees but not to 1.5 degrees.[208] After the report was published, additional pledges were made in the september climate summit[209] and in december of that year.[210]

All the information about the pledges is collected and analyzed in the Global Climate Action (portal), what is facilitating to the scientific community to check their fulfillment[211]

Human activities are estimated to have caused approximately 1.0 °C of global warming above pre-industrial levels, with a likely range of 0.8 °C to 1.2 °C. [212] There is disagreement among experts over whether or not the 2 °C target can be met.[213]

Official long-term target of 1.5 °C

In 2015, two official UNFCCC scientific expert bodies came to the conclusion that, "in some regions and vulnerable ecosystems, high risks are projected even for warming above 1.5 °C".[214] This expert position was, together with the strong diplomatic voice of the poorest countries and the island nations in the Pacific, the driving force leading to the decision of the Paris Conference 2015, to lay down this 1.5 °C long-term target on top of the existing 2 °C goal.[215]

An emissions tax on greenhouse gas emissions requires emitters to pay a fee, charge or tax for every tonne of greenhouse gas released into the atmosphere.[216] 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.[216] As such, non-transport sectors as the agricultural sector which produces large amounts of methane are typically left untaxed by current policies. Also, revenue of the emissions taxes are not always used to offset the emissions directly.[217][218]

Emission taxes can be both cost-effective and environmentally effective.[216] Difficulties with emission taxes include their potential unpopularity, and the fact that they cannot guarantee a particular level of emissions reduction.[216] 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.[216]

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.[219]

Although state policies tackling climate change are seen as a threat to investors, so is global warming itself. As well as a policy risk, Ernst and Young identify physical, secondary, liability, transitional and reputation-based risks.[220] Therefore, it is increasingly seen to be in the interest of investors to accept climate change as a real threat which they must proactively and independently address.

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.[221]

However, emissions trading may delay the phase-out of fossil fuels.[222]

In the north-east United States, a successful cap and trade program has shown potential for this solution.[223]

The European Union Emission Trading Scheme (EU ETS)[224] is the largest multi-national, greenhouse gas emissions trading scheme in the world. It commenced operation on 1 January 2005, and all 28 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.[225] 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).[226]

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[227] 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.

Since 2000, rising CO
2 emissions in China and the rest of world have eclipsed the output of the United States and Europe.[228]

Per person, the United States generates carbon dioxide at a far faster rate than other primary regions.[228]

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.[229]

Funding, such as the Green Climate Fund, 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.[229] 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.[230]

It has been estimated that only 0.12% of all funding for climate-related research is spent on the social science of climate change mitigation.[231] Vastly more funding is spent on natural science studies of climate change and considerable sums are also spent on studies of impact of and adaptation to climate change.[231] It has been argued that this is a misallocation of resources, as the most urgent puzzle at the current juncture is to work out how to change human behavior to mitigate climate change, whereas the natural science of climate change is already well established and there will be decades and centuries to handle adaptation.[231]

There are numerous issues which result in a current perceived lack of implementation.[229] 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.[232] 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 opposed to one overall enforcement agency.[233] Concerns about metal requirement and/or availability for essential decarbonization technoloqies such as photovoltaics, nuclear power, and (plug-in hybrid) electric vehicles have also been expressed as obstacles.[234]

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.[235] The implementation of many of these is supported by GEF agencies.[236] Many developed countries are implementing 'first generation' institutional adaption plans particularly at the state and local government scale.[235] There has also been a push towards joint implementation between countries by the UNFCCC as this has been suggested as a cost-effective way for objectives to be achieved.[237]

Although not designed for this purpose, the Montreal Protocol has benefited climate change mitigation efforts.[238] The Montreal Protocol is an international treaty that has successfully reduced emissions of ozone-depleting substances (for example, CFCs), which are also greenhouse gases.

Many countries are aiming for net zero emissions, and many have either carbon taxes or carbon emission trading.

Emission trading and carbon taxes around the world (2019)[239]

  Carbon emission trading implemented or scheduled

  Carbon tax implemented or scheduled

  Carbon emission trading or carbon tax under consideration

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.[240]

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.[241] In 2019 a new climate change bill was introduced in Minnesota. One of the targets, is making all the energy of the state carbon free, by 2030.[242]

As to 2019, China implements more than 100 policies to fight climate change. China said in the Paris Agreement that its emission will begin to fall by 2030, but it will possibly occur by 2026. This can position China as a leader on the issue because it is the biggest emitter of GHG emissions, so if it really reduces them, the significance will be large.[243]

The climate commitments of the European Union are divided into 3 main categories: targets for the year 2020, 2030 and 2050. The European Union claim that their policies are in line with the goal of the Paris Agreement.[244][245]

Targets for the year 2020[246]:

• Reduce GHG emissions by 20% from the level in 1990.

• Produce 20% of energy from renewable sources.

• Increase Energy Efficiency by 20%.

Targets for the year 2030[247]:

• Reduce GHG emission by 40% from the level of 1990. In 2019 The European Parliament adopted a resolution upgrading the target to 55%[248]

• Produce 32% of energy from renewables.

• Increase energy efficiency by 32.5%.

Targets for the year 2050[249]:

• Become climate neutral.

Implementation:

The European Union claims that he has already achieved the 2020 target for emission reduction and have the legislation needed to achieve the 2030 targets. Already in 2018, its GHG emissions were 23% lower that in 1990.[250]

New Zealand made significant pledges on climate change mitigation in the year 2019: reduce emissions to zero by 2050, plant 1 billion trees by 2028, and made high taxes on farmers who will not reduce emissions in 2025. Already in 2019 New Zealand banned new offshore oil and gas drilling and decided the climate change issues will be examined before every important decision.[251]

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[252] 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.[253] 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.[253] 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 four main understandings:[253]

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.[253]

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".[254]

In 2019 week of climate action in Latin America and the Caribbean result in a declaration in which leaders says that they will act to reduce emissions in the sectors of transportation, energy, urbanism, industry, forest conservation and land use and "sent a message of solidarity with all the people of Brazil suffering the consequences of the rainforest fires in the Amazon region, underscoring that protecting the world's forests is a collective responsibility, that forests are vital for life and that they are a critical part of the solution to climate change".[255][256]

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.[257]

In 2008, climate scientist Kevin Anderson raised concern about the growing effect of rapidly increasing global air transport on the climate in a paper,[258] and a presentation,[259] 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.[260]

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. 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.[261] Proxy firms can be used to draft guidelines for investment managers that take these concerns into account.[262]

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[263] and the Inuit,[264] as well as non-governmental organizations such as the Sierra Club.[265] Although proving that particular weather events are due specifically to global warming may never be possible,[266] methodologies have been developed to show the increased risk of such events caused by global warming.[267]

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."[268] 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.[269][270]

Besides countries suing one another, there are also cases where people in a country have taken legal steps against their own government. Legal action for instance has been taken to try to force the US Environmental Protection Agency to regulate greenhouse gas emissions under the Clean Air Act,[271] and against the Export-Import Bank and OPIC for failing to assess environmental impacts (including global warming impacts) under NEPA.

In the Netherlands and Belgium, organisations such as the foundation Urgenda[272][273][274] and the vzw Klimaatzaak in Belgium[275][276] have also sued their governments as they believe their governments aren't meeting the emission reductions they agreed to. Urgenda have already won their case against the Dutch government.

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.[277]

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 misled the public and investors about the risks of climate change.[278] In October 2019, the trial began.[279] Massachusetts sued Exxon also, for hiding the impact of climate change.[280]

In 2019, 22 states, six cities and Washington DC in United States, sued the Trump administration for repealing the Clean Power Plan.[281]

Protesters at the Helsinki People's Climate March, 29 November 2015

Environmental organizations organize different actions such as Peoples Climate Marches and Divestment from fossil fuels. 1,000 organizations with a worth of 8 trillion dollars, made commitments to divest from fossil fuel to 2018.[282] Another form of action is climate strike.[283] In January 2019 12,500 students marched in Brussels demanding Climate action.[284] In 2019 The organization Extinction Rebellion organized massive protests demanding "tell the truth about climate change, reduce carbon emissions to zero by 2025, and create a citizens' assembly to oversee progress", including blocking roads. Many were arrested.[285] In many cases, activism brings positive results.[286]

A major event was the global climate strike in September 2019 organized by Fridays For Future and Earth Strike.[287] The target was to influence the climate action summit organized by the UN on September 23.[288] According to the organizers four million people participated in the strike on September 20.[289]

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

• EU Climate Action
• U.S. Mayors Climate Protection Agreement signed by 178 mayors representing nearly 40 million Americans
• UK Committee on Climate Change

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