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Climate change mitigation

Actions to limit climate change in order to reduce the risks of global warming

Top 10 Climate change mitigation related articles

Coal, oil, and natural gas remain the primary global energy sources even as renewables have begun rapidly increasing.[1]

Climate change mitigation consists of actions to limit global warming and its related effects. This involves reductions in human emissions of greenhouse gases (GHGs) as well as activities that reduce their concentrations in the atmosphere.[2]

Fossil fuel combustion accounts for 89% of all CO
2
emissions and 68% of all GHG emissions.[3] The most important challenge is to eliminate the use of coal, oil and gas and substitute them with clean energy sources. Due to massive price drops, wind power and solar photovoltaics (PV) are increasingly out-competing oil, gas and coal[4] though these require energy storage and improved electrical grids. Once that low-emission energy is deployed at large scale, transport and heating can shift to these mostly electric sources.[5]

Mitigation of climate change may also be achieved by changes in agriculture, reforestation and forest preservation and improved waste management.[6] Methane emissions, which have a high short-term impact, can be targeted by reductions in cattle and more generally by reducing meat consumption.

Political and economical responses include carbon taxes and other emission pricing models, abolishing fossil fuel subsidies, simplified regulations for the integration of low-carbon energy and divestment from fossil fuel finance.

Almost all countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC).[7] 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.[8] 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.[9] 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.[10] 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.[11] Pathways that aim for limiting warming to 1.5°C by 2100 after a temporary temperature overshoot rely on large-scale deployment of carbon dioxide removal (CDR) measures, which are uncertain and entail clear risks.[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 despite the limit being economically beneficial globally and to many top GHG emitters such as China and India.[13][14]

Climate change mitigation Intro articles: 17

Greenhouse gas concentrations and stabilization

Stabilizing CO2 emissions at their present level would not stabilize its concentration in the atmosphere.[15]
Stabilizing the atmospheric concentration of CO2 at a constant level would require emissions to be effectively eliminated.[15]

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.[16] Currently human activities are adding CO2 to the atmosphere faster than natural processes can remove it.[15]

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.[17] The rise in temperature to that level would occur with some delay between 2030 and 2052.[18] 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.[19]

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.[20][21][22][23]

If emissions will be reduced to zero, the warming might stop in 10 - 20 years.[24] Potential feedback effects lead to a high degree of uncertainty in any projection. Climate change mitigation scenarios from the IPCC Fifth Assessment Report cover a range from 1.5 °C (2.7 °F) of warming by the end of the 21st century if emissions immediately decline and go to net zero by 2050, or 4.8 °C (8.6 °F) if emissions continue upwards until they are triple current levels.

Climate change mitigation Greenhouse gas concentrations and stabilization articles: 6

Drivers of global warming

GHG emissions 2019 by gas type
without land-use change
using 100 year GWP
Total: 51.8 GtCO
2
e[25]

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

CO
2
emissions by fuel type[26]

  coal (39%)
  oil (34%)
  gas (21%)
  cement (4%)
  others (1.5%)

Carbon dioxide (CO
2
) is the dominant emitted greenhouse gas, while Methane (CH
4
) emissions almost have the same short-term impact.[27] Nitrous oxide (N2O) and fluorinated gases (F-Gases) play a minor role. With the Kyoto Protocol, the reduction of almost all anthropogenic greenhouse gases has been addressed.[28]

GHG emissions are measured in CO
2
equivalents
determined by their global warming potential (GWP), which depends on their lifetime in the atmosphere. Estimations largely depend on the ability of oceans and land sinks to absorb these gases. Short-lived climate pollutants (SLCPs) including methane, hydrofluorocarbons (HFCs), tropospheric ozone and black carbon persist in the atmosphere for a period ranging from days to 15 years as compared to carbon dioxide which can remain in the atmosphere for millennia.[29] Reducing SLCP emissions can cut the ongoing rate of global warming by almost half and reduce the projected Arctic warming by two-thirds.[30]

GHG emissions in 2019 were estimated at 57.4 GtCO
2
e, while CO
2
emissions alone made up 42.5 Gt including land-use change (LUC).[31]

Carbon dioxide (CO
2
)

  • Fossil fuel: oil, gas and coal (89%) are the major driver of anthropogenic global warming with annual emissions of 35.6 GtCO
    2
    in 2019.[26][32]
  • Cement production (4%) is estimated at 1.42 GtCO
    2
    [26]
  • Land-use change (LUC) is the imbalance of deforestation and reforestation. Estimations are very uncertain at 4.5 GtCO
    2
    .[25] Wildfires alone cause annual emissions of about 7 GtCO
    2
    [33][34]
  • Non-energy use of fuels, carbon losses in coke ovens, and flaring in crude oil production.[32]

Methane (CH4)

Methane has a high immediate impact with a 5-year global warming potential of up to 100.[27] Given this, the current 389 Mt of methane emissions[35] has about the same short-term global warming effect as CO
2
emissions, with a risk to trigger off irreversible changes in climate and ecosystems. For methane, a reduction of about 30% below current emission levels would lead to a stabilization in its atmospheric concentration.[15]

  • Fossil fuels (33%), again, account for most of the methane emissions including coal mining, gas distribution, leakages, and gas venting.[3]
  • Cattle (21%) account for two-thirds of the methane emitted by livestock, followed by buffalo (3% of methane total), sheep (2%), and goats (1.5%).[36]
  • Human waste and wastewater (21%): When biomass waste in landfills and organic substances in domestic and industrial wastewater is decomposed by bacteria in anaerobic conditions, substantial amounts of methane are generated.[3]
  • Rice cultivation (10%) on flooded rice fields is another agricultural source, where anaerobic decomposition of organic material produces methane.[3]


Nitrous oxide (N
2
O
)

N2O has a high GWP and significant Ozone Depleting Potential (ODP). It is estimated that the global warming potential of N2O over 100 years is 265 times greater than CO2.[37] For N2O, a reduction of more than 50% would be required for a stabilization.[15]

  • Most emissions (56%) by agriculture, especially meat production: cattle (droppings on pasture), fertilizers, animal manure.[3]
  • Combustion of fossil fuels (18%) and bio fuels.[38]
  • Industrial production of adipic acid and nitric acid.

F-Gases

Fluorinated gases include hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur hexafluoride (SF6). They are used by switchgear in the power sector, semi-conducture manufacture, aluminium production and a large unknown source of SF6.[39] Continued phase down of manufacture and use of hydrofluorocarbons (HFCs) under the Montreal Protocol will help reduce HFC emissions and concurrently improve the energy efficiency of appliances that use HFCs like air conditioners, freezers and refrigerators.

Black carbon

Black carbon is formed through the incomplete combustion of fossil fuels, biofuel, and biomass. It is not a greenhouse gas but a climate forcing agent. Black carbon can absorb sunlight and reduce albedo when deposited on snow and ice. Indirect heating can be caused by the interaction with clouds.[40] Black carbon stays in the atmosphere for only several days to weeks.[41] Emissions may be mitigated by upgrading coke ovens, installing particulate filters on diesel-based engines and minimizing open burning of biomass.

Climate change mitigation Drivers of global warming articles: 23

Fossil fuel substitution

As most greenhouse gas emissions are due to fossil fuels, rapidly phasing out oil, gas and coal is critical.[42] In a system based on fossil fuels, demand is expected to double until 2050. Switching to renewable energy combined with the electrification of transport and heating can lower the primary energy demand significantly.[5] Currently, less than 20% of energy is used as electricity.[43]

A global transition to 100% renewable energy across all sectors is feasible well before 2050. With dropping prices for wind and solar energy as well as storage, the transition no longer depends on economic viability but is considered as a question of political will. The sustainable energy system is more efficient and cost effective than the existing system.[5]

Low-carbon energy sources

Wind and sun can be sources for large amounts of low-carbon energy at competitive production costs. 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. 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. Both can be driven by variable energy prices.

The deployment of renewable energy would have to be accelerated six-fold though to stay under the 2 °C target.[44]

The global primary energy demand exceeded 161,000 TWh in 2018.[45] This refers to electricity, transport and heating including all losses. In transport and electricity production, fossil fuel usage has a low efficiency of less than 50%. Large amounts of heat in power plants and in motors of vehicles are wasted. The actual amount of energy consumed is significantly lower at 116,000 TWh.[43]

The competitiveness of renewable energy is a key to a rapid deployment. In 2020, onshore wind and solar photovoltaics were the cheapest source for new bulk electricity generation in many regions.[46] Storage requirements cause additional costs. On the other hand, a price on carbon emissions can increase the competitiveness of renewable energy.

installed[47]
TWp
growth
TW/yr[47]
production
per installed
capacity*[48]
energy
TWh/yr*[48]
growth
TWh/yr*[48]
costs
US$/KWh[49]
Av. auction prices
US$/KWh[50]
Cost development
2010-2019[51]
Solar PV 0.580 0.098 13% 549 123 6.8 3.9 -82%
Solar CSP 0.006 0.0006 13% 6.3 0.5 18.2 7.5 -47%
Wind Offshore 0.028 0.0045 33% 68 11.5 11.5 8.2 -30%
Wind Onshore 0.594 0.05 25% 1194 118 5.3 4.3 -38%
Hydro 1.310 0.013 38% 4267 90 4.7 +27%
Bioenergy 0.12 0.006 51% 522 27 6.6 -13%
Geothermal 0.014 0.00007 74% 13.9 0.7 7.3 +49%

* = 2018. All other values for 2019.


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.[52]
  • 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$/KWh in desert regions.[53] The growth of photovoltaics is exponential and has doubled every three years since the 1990s.
  • A different technology is concentrated solar power (CSP) using mirrors or lenses to concentrate a large area of sunlight onto a receiver. With CSP, the energy can be saved up for a few hours. Prices in Chile are expected to fall below 0.05 US$/KWh in 2020.[54]
  • 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 GWh). Worldwide, total installed solar water heating systems meet a portion of the water heating needs of over 70 million households.[55]

Wind power

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.[56] Installed capacity has reached 650 GW in 2019. Offshore wind power currently has a share of about 10% of new installations.[57] 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.[58] but there are geographical limits and environmental issues.[59] Tidal power can be used in coastal regions.

Bioenergy

Biogas plants can provide dispatchable electricity generation, and heat when needed.[60] 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.[61] Renewable biofuels are starting to be used in aviation.

Nuclear power

In most 1.5 °C pathways nuclear power increases its share.[62] The main advantage is the ability to deliver large amounts of base load when renewable energy is not available. It has been repeatedly classified as a climate change mitigation technology.[63]

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.[64][65] Public opinion about nuclear power varies widely between countries.[66][67]

As of 2019 the cost of extending nuclear power plant lifetimes is competitive with other electricity generation technologies, including new solar and wind projects.[68] New projects are reported to be highly dependent on public subsidies.[69]

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.[70]

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.[71][72][73]

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.[74] Gas-fired power plants can provide the required flexibility in electricity production in combination wind and solar energy.[75] 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.[76] 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.[77]

  • Hydrogen may be useful for seasonal energy storage.[81] The low efficiency of 30% must improve dramatically before hydrogen storage can offer the same overall energy efficiency as batteries.[79] 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.[82] The concept of solar hydrogen is discussed for remote desert projects where grid connections to demand centers are not available.[83] Because it has more energy per unit volume sometimes it may be better to use hydrogen in ammonia.[84]

Super grids

Long-distance power lines help to minimize storage requirements. A continental transmission network can smoothen local 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[85] with a clear advantage compared to AC. HVDC is currently only used for point-to-point connections. Meshed HVDC grids are reported to be ready-to-use in Europe[86] and to be in operation in China by 2022.[87]

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.[88] A super grid in the US in combination with renewable energy could reduce GHG emissions by 80%.[89]

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.[90]

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

In a system completely based on renewable energy, the transport sector can be emission free. Electric vehicles and ferries are the most effective way to use renewable energy. Between a quarter and three-quarters of cars on the road by 2050 are forecast to be electric vehicles.[91] Hydrogen can be a solution for long-distance transport by trucks and hydrogen-powered ships where batteries alone are too heavy.[92][93] 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.[94] 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.[95]

GHG emissions depend on the amount of green energy being used for battery or fuel cell production and charging. In a system mainly based on electricity from fossil fuels, emissions of electric vehicles can even exceed those of diesel combustion.[96]

In aviation, current 180 Mt of CO
2
emissions (11% of emissions in transport) are expected to rise in most projections, at least until 2040. Aviation biofuel and hydrogen can only cover a small proportion of flights in the coming years. The market entry for hybrid-driven aircraft on regional scheduled flights is projected after 2030, for battery-powered aircraft after 2035.[97]

Decarbonization of heating

The buildings sector accounts for 23% of global energy-related CO2 emissions[98] About half of the energy is used for space and water heating.[99] 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 pumps

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.[100] 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.[101] Using surplus renewable energy in heat pumps is regarded as the most effective household means to reduce global warming and fossil fuel depletion.[102]

Electric resistant heating

Radiant heaters in households are cheap and widespread 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.

Zero heating building

With advances in ultra low U-value glazing a Passive House-based (nearly) zero heating building is proposed to supersede nearly-zero energy buildings. The zero-heating building reduces on the passive solar design and makes the building more opened to conventional architectural design. The annual specific space heating demand for the zero-heating house should not exceed 3 kWh/m2a. Removing the need for heating is the best approach to de-carbonize heating.

Climate change mitigation Fossil fuel substitution articles: 78

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.[103]

Energy efficiency

Energy efficiency means using the least amount of energy to perform a task or the ability of a piece of equipment to use the least amount of energy to perform a task. To conserve energy or reduce electricity costs, individual consumers or businesses may deliberately purchase energy efficient products that use refrigerants with low global warming potential (GWP) or products that are ENERGY STAR certified.[104] In general, the more the number of ENERGY STARS, the more efficient the product is. A procurement toolkit[105] to assist individuals and businesses buy energy efficient products that use low GWP refrigerants was developed by the Sustainable Purchasing Leadership Council[106] and is available for use. Products with refrigerants include household refrigerators and freezers, commercial stand-alone refrigerators and freezers, lab-grade refrigerators and freezers, commercial ice makers, vending machines, water dispensers, water coolers, room air conditioners and vehicles. 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.[107] Examples would be heating a room less or driving less. In general, higher consumption lifestyles have a greater environmental impact. The sources of emissions have also been shown to be highly unevenly distributed, with 45% of emissions coming from the lifestyles of just 10% of the global population.[108] 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).[109]

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.[109][110][111]

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.[112]

Dietary change

The widespread adoption of a vegetarian diet could cut food-related greenhouse gas emissions by 63% by 2050.[113] Addressing the high methane emissions by cattle, a 2016 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.[114][115] 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.[116] Overall, food accounts for the largest share of consumption-based GHG emissions with nearly 20% of the global carbon footprint.

Modal shift

Heavyweight, large personal vehicles (such as cars) require a lot of energy to move and take up much urban space.[117][118] Several alternatives modes of transport are available to replace these. The European Union has made smart mobility part of its European Green Deal[119] and in smart cities, smart mobility is also important.[120]

Climate change mitigation Energy conservation articles: 17

Carbon sinks and removal

About 58% of CO
2
emissions have been absorbed by carbon sinks, including plant growth, soil uptake, and ocean uptake (2020 Global Carbon Budget).
World protected area map with total percentage of each country under protection, where countries in lighter colors have more protected land.

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,[121][122] and sometimes as geoengineering.[123] In combination with other mitigation measures, carbon sinks and removal are crucial for meeting the 2 degree target.[124]

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.[125] However, this also leads to ocean acidification, which may harm marine life.[126] 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.[127]

Conserving areas by protecting areas can boost the carbon sequestration capacity.[128][129][130] The European Union, through the EU Biodiversity Strategy for 2030 targets to protect 30% of the sea territory and 30% of the land territory by 2030. Also, Campaign for Nature's 30x30 for Nature Petition tries to let governments agree to the same goal during the Convention on Biodiversity COP15 Summit.[131] has the same target. The One Earth Climate Model advises a protection of 50% of our lands and oceans. It also stresses the importance of rewilding,[132] like other reports.[133][134] The reason being that predators (which are often keystone species) keep the population of herbivores in check (which reduce the biomass of vegetation), and also impact their feeding behavior.[135]

Proforestation, avoided deforestation, reforestation 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. 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.[136] 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).[137][138] 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.[139][140][141][142] This vision is being executed by the Trillion Tree Campaign. Other studies[143][144] 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. Proforestation, which is maintaining or growing existing forests to their ecological potential, maintains and optimizes carbon sequestration or carbon dioxide removal from the atmosphere while limiting climate change.

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.[145] This includes the protection of such rights entitled in existing laws, such as India's Forest Rights Act.[145] The transferring of such rights in China, perhaps the largest land reform in modern times, has been argued to have increased forest cover.[146][147] 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.[148][149] 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.[146]

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.[150][151] According to a study in Science, promoting regrowth on abandoned farmland could offset years of carbon emissions.[152] Research by the university ETH Zurich estimates that Russia, the United States and Canada have the most land suitable for reforestation.[153][154]

According to a big survey of the United Nations Development Programme of public opinion on climate change, forests and land conservation policies were the most popular solutions of climate change mitigation, followed by renewable energy, and climate-friendly farming techniques.[155]

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.[156] 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.[156][157][158] 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.[156] 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.[156] 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.[159]

Additionally, the global warming induced thawing of the permafrost, which stores about two times the amount of the carbon currently released in the atmosphere,[160] 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.[161] 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.[162]

Protecting healthy soils and recovering damaged soils could remove 5.5 billion tons of carbon dioxide from the atmosphere annually, which is approximately equal to the annual emissions of the USA.[163]

Blue carbon

Estimates of the economic value of blue carbon ecosystems per hectare. Based on 2009 data from UNEP/GRID-Arendal.[164][165]

Blue carbon refers to carbon dioxide removed from the atmosphere by the world's ocean ecosystems, mostly algae, mangroves, salt marshes, seagrasses and macroalgae, through plant growth and the accumulation and burial of organic matter in the soil.[164][166][167]

Historically the ocean, atmosphere, soil, and terrestrial forest ecosystems have been the largest natural carbon (C) sinks. "Blue carbon" designates carbon that is fixed via the largest ocean ecosystems, rather than traditional land ecosystems, like forests. Oceans cover 70% of the planet, consequently ocean ecosystem restoration has the greatest blue carbon development potential. Mangroves, salt marshes and seagrasses make up the majority of the ocean's vegetated habitats but only equal 0.05% of the plant biomass on land. Despite their small footprint, they can store a comparable amount of carbon per year and are highly efficient carbon sinks. Seagrasses, mangroves and salt marshes can capture carbon dioxide (CO
2
) from the atmosphere by sequestering the C in their underlying sediments, in underground and below-ground biomass, and in dead biomass.[168][169]

In plant biomass such as leaves, stems, branches or roots, blue carbon can be sequestered for years to decades, and for thousands to millions of years in underlying plant sediments. Current estimates of long-term blue carbon C burial capacity are variable, and research is ongoing.[169] Although vegetated coastal ecosystems cover less area and have less aboveground biomass than terrestrial plants they have the potential to impact longterm C sequestration, particularly in sediment sinks.[170] One of the main concerns with Blue Carbon is the rate of loss of these important marine ecosystems is much higher than any other ecosystem on the planet, even compared to rainforests. Current estimates suggest a loss of 2-7% per year, which is not only lost carbon sequestration, but also lost habitat that is important for managing climate, coastal protection, and health.[170]

Peatlands

Peat extraction in East Frisia, Germany. Peat extraction degrades peatland and is possible as many peatlands are currently not protected.

Peatland globally stores up to 550 gigatonnes of carbon, representing 42% of all soil carbon and exceeds the carbon stored in all other vegetation types, including the world's forests.[171] Across the world, peat covers just 3% of the land's surface, but stores one-third of the Earth's soil carbon.[172] Restoration of degraded peatlands can be done by blocking drainage channels in the peatland, and allowing natural vegetation to recover.[173]

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.[174] The International Energy Agency says CCS is "the most important single new technology for CO2 savings" in power generation and industry.[175] 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.[176][175] 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.[177]

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.[178]

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.

Enhanced weathering

Enhanced weathering or accelerated weathering refers to geoengineering approaches intended to remove carbon dioxide from the atmosphere by using specific natural or artificially created minerals which absorb carbon dioxide and transform it in other substances through chemical reactions occurring in the presence of water (for example in the form of rain, groundwater or seawater).

Enhanced weathering research considers how natural processes of rocks' and minerals' weathering (in particular chemical weathering) may be enhanced to sequester CO2 from the atmosphere to be stored in form of another substance in solid carbonate minerals or ocean alkalinity. Since the carbon dioxide is usually first removed from ocean water, these approaches would attack the problem by first reducing ocean acidification.

This technique requires the extraction or production of large quantities of materials, crushing them and spreading them over large areas (for example fields or beaches); Besides extracting minerals for the purpose of enhanced weathering, also alkaline industrial silicate minerals (such as steel slags, construction & demolition waste, ash from biomass incineration) can be utilized.[179] In a 2020 techno-economical analysis, the cost of utilizing this method on cropland was estimated at US$80–180 per tonne of CO2. This is comparable with other methods of removing carbon dioxide from the atmosphere currently available (BECCS (US$100–200 per tonne of CO2)- Bio-Energy with Carbon Capture and Storage) and direct air capture and storage (US$100–300 per tonne of CO2). In contrast, the cost of reforestation was estimated lower than (US$100 per tonne of CO2).[180]

It has the side effect of altering the natural salinity of the seas.

Climate change mitigation Carbon sinks and removal articles: 73