Global Warming Potential (GWP) is an index to measure of how much infrared thermal radiation a greenhouse gas would absorb over a given time frame after it has been added to the atmosphere (or emitted to the atmosphere). The GWP makes different greenhouse gases comparable with regards to their "effectiveness in causing radiative forcing". [1] : 2232 It is expressed as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide (CO2), which is taken as a reference gas. Therefore, the GWP has a value of 1 for CO2. For other gases it depends on how strongly the gas absorbs infrared thermal radiation, how quickly the gas leaves the atmosphere, and the time frame being considered.
For example, methane has a GWP over 20 years (GWP-20) of 81.2 [2] meaning that, for example, a leak of a tonne of methane is equivalent to emitting 81.2 tonnes of carbon dioxide measured over 20 years. As methane has a much shorter atmospheric lifetime than carbon dioxide, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95. [2] : 7SM-24
The carbon dioxide equivalent (CO2e or CO2eq or CO2-e) can be calculated from the GWP. For any gas, it is the mass of CO2 that would warm the earth as much as the mass of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times mass of the other gas.
The global warming potential (GWP) is defined as an "index measuring the radiative forcing following an emission of a unit mass of a given substance, accumulated over a chosen time horizon, relative to that of the reference substance, carbon dioxide (CO2). The GWP thus represents the combined effect of the differing times these substances remain in the atmosphere and their effectiveness in causing radiative forcing." [1] : 2232
In turn, radiative forcing is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance. [3] : 1–4 Radiative forcing is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured in watts per meter squared. [4]
The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO2 and evaluated for a specific timescale. [6] Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO2 its GWP will increase when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.
Methane has an atmospheric lifetime of 12 ± 2 years. [7] : Table 7.15 The 2021 IPCC report lists the GWP as 83 over a time scale of 20 years, 30 over 100 years and 10 over 500 years. [7] : Table 7.15 A 2014 analysis, however, states that although methane's initial impact is about 100 times greater than that of CO2, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane's relative role continues to decline. [8] The decrease in GWP at longer times is because methane decomposes to water and CO2 through chemical reactions in the atmosphere.
Examples of the atmospheric lifetime and GWP relative to CO2 for several greenhouse gases are given in the following table:
Gas name | Chemical formula | Lifetime | Radiative Efficiency | Global warming potential (GWP) for given time horizon | ||
---|---|---|---|---|---|---|
20-yr. [7] : Table 7.15 [9] | 100-yr. [7] : Table 7.15 [9] | 500-yr. [7] : Table 7.15 [10] | ||||
Carbon dioxide | CO2 | (A) | 1.37×10−5 | 1 | 1 | 1 |
Methane (fossil) | CH 4 | 12 | 5.7×10−4 | 83 | 30 | 10 |
Methane (non-fossil) | CH 4 | 12 | 5.7×10−4 | 81 | 27 | 7.3 |
Nitrous oxide | N 2O | 109 | 3×10−3 | 273 | 273 | 130 |
CFC-11 | CCl 3F | 52 | 0.29 | 8 321 | 6 226 | 2 093 |
CFC-12 | CCl 2F 2 | 100 | 0.32 | 10 800 | 10 200 | 5 200 |
HCFC-22 | CHClF 2 | 12 | 0.21 | 5 280 | 1 760 | 549 |
HFC-32 | CH 2F 2 | 5 | 0.11 | 2 693 | 771 | 220 |
HFC-134a | CH 2FCF 3 | 14 | 0.17 | 4 144 | 1 526 | 436 |
Tetrafluoromethane | CF 4 | 50 000 | 0.09 | 5 301 | 7 380 | 10 587 |
Hexafluoroethane | C 2F 6 | 10 000 | 0.25 | 8 210 | 11 100 | 18 200 |
Sulfur hexafluoride | SF 6 | 3 200 | 0.57 | 17 500 | 23 500 | 32 600 |
Nitrogen trifluoride | NF 3 | 500 | 0.20 | 12 800 | 16 100 | 20 700 |
(A) No single lifetime for atmospheric CO2 can be given. |
Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change. The most recent report is the IPCC Sixth Assessment Report (Working Group I) from 2023. [7]
The IPCC lists many other substances not shown here. [11] [7] Some have high GWP but only a low concentration in the atmosphere.
The values given in the table assume the same mass of compound is analyzed; different ratios will result from the conversion of one substance to another. For instance, burning methane to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of methane burned is less than the mass of carbon dioxide released (ratio 1:2.74). [12] For a starting amount of 1 tonne of methane, which has a GWP of 25, after combustion there would be 2.74 tonnes of CO2, each tonne of which has a GWP of 1. This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74 (approximately 9 times).
Greenhouse gas | Lifetime (years) | Global warming potential, GWP | ||
---|---|---|---|---|
20 years | 100 years | 500 years | ||
Hydrogen (H2) | 4–7 [13] | 33 (20-44) [13] | 11 (6–16) [13] | — |
Methane (CH4) | 11.8 [7] | 56 [14] 72 [15] 84 / 86f [11] 96 [16] 80.8 (biogenic) [7] 82.5 (fossil) [7] | 21 [14] 25 [15] 28 / 34f [11] 32 [17] 39 (biogenic) [18] 40 (fossil) [18] | 6.5 [14] 7.6 [15] |
Nitrous oxide (N2O) | 109 [7] | 280 [14] 289 [15] 264 / 268f [11] 273 [7] | 310 [14] 298 [15] 265 / 298f [11] 273 [7] | 170 [14] 153 [15] 130 [7] |
HFC-134a (hydrofluorocarbon) | 14.0 [7] | 3,710 / 3,790f [11] 4,144 [7] | 1,300 / 1,550f [11] 1,526 [7] | 435 [15] 436 [7] |
CFC-11 (chlorofluorocarbon) | 52.0 [7] | 6,900 / 7,020f [11] 8,321 [7] | 4,660 / 5,350f [11] 6,226 [7] | 1,620 [15] 2,093 [7] |
Carbon tetrafluoride (CF4 / PFC-14) | 50,000 [7] | 4,880 / 4,950f [11] 5,301 [7] | 6,630 / 7,350f [11] 7,380 [7] | 11,200 [15] 10,587 [7] |
HFC-23 (hydrofluorocarbon) | 222 [11] | 12,000 [15] 10,800 [11] | 14,800 [15] 12,400 [11] | 12,200 [15] |
Sulfur hexafluoride SF6 | 3,200 [11] | 16,300 [15] 17,500 [11] | 22,800 [15] 23,500 [11] | 32,600 [15] |
The values provided in the table below are from 2007 when they were published in the IPCC Fourth Assessment Report. [19] [15] These values are still used (as of 2020) for some comparisons. [20]
Greenhouse gas | Chemical formula | 100-year Global warming potentials (2007 estimates, for 2013–2020 comparisons) |
---|---|---|
Carbon dioxide | CO2 | 1 |
Methane | CH4 | 25 |
Nitrous oxide | N2O | 298 |
Hydrofluorocarbons (HFCs) | ||
HFC-23 | CHF3 | 14,800 |
Difluoromethane (HFC-32) | CH2F2 | 675 |
Fluoromethane (HFC-41) | CH3F | 92 |
HFC-43-10mee | CF3CHFCHFCF2CF3 | 1,640 |
Pentafluoroethane (HFC-125) | C2HF5 | 3,500 |
HFC-134 | C2H2F4 (CHF2CHF2) | 1,100 |
1,1,1,2-Tetrafluoroethane (HFC-134a) | C2H2F4 (CH2FCF3) | 1,430 |
HFC-143 | C2H3F3 (CHF2CH2F) | 353 |
1,1,1-Trifluoroethane (HFC-143a) | C2H3F3 (CF3CH3) | 4,470 |
HFC-152 | CH2FCH2F | 53 |
HFC-152a | C2H4F2 (CH3CHF2) | 124 |
HFC-161 | CH3CH2F | 12 |
1,1,1,2,3,3,3-Heptafluoropropane (HFC-227ea) | C3HF7 | 3,220 |
HFC-236cb | CH2FCF2CF3 | 1,340 |
HFC-236ea | CHF2CHFCF3 | 1,370 |
HFC-236fa | C3H2F6 | 9,810 |
HFC-245ca | C3H3F5 | 693 |
HFC-245fa | CHF2CH2CF3 | 1,030 |
HFC-365mfc | CH3CF2CH2CF3 | 794 |
Perfluorocarbons | ||
Carbon tetrafluoride – PFC-14 | CF4 | 7,390 |
Hexafluoroethane – PFC-116 | C2F6 | 12,200 |
Octafluoropropane – PFC-218 | C3F8 | 8,830 |
Perfluorobutane – PFC-3-1-10 | C4F10 | 8,860 |
Octafluorocyclobutane – PFC-318 | c-C4F8 | 10,300 |
Perfluouropentane – PFC-4-1-12 | C5F12 | 9,160 |
Perfluorohexane – PFC-5-1-14 | C6F14 | 9,300 |
Perfluorodecalin – PFC-9-1-18b | C10F18 | 7,500 |
Perfluorocyclopropane | c-C3F6 | 17,340 |
Sulfur hexafluoride (SF6) | ||
Sulfur hexafluoride | SF6 | 22,800 |
Nitrogen trifluoride (NF3) | ||
Nitrogen trifluoride | NF3 | 17,200 |
Fluorinated ethers | ||
HFE-125 | CHF2OCF3 | 14,900 |
Bis(difluoromethyl) ether (HFE-134) | CHF2OCHF2 | 6,320 |
HFE-143a | CH3OCF3 | 756 |
HCFE-235da2 | CHF2OCHClCF3 | 350 |
HFE-245cb2 | CH3OCF2CF3 | 708 |
HFE-245fa2 | CHF2OCH2CF3 | 659 |
HFE-254cb2 | CH3OCF2CHF2 | 359 |
HFE-347mcc3 | CH3OCF2CF2CF3 | 575 |
HFE-347pcf2 | CHF2CF2OCH2CF3 | 580 |
HFE-356pcc3 | CH3OCF2CF2CHF2 | 110 |
HFE-449sl (HFE-7100) | C4F9OCH3 | 297 |
HFE-569sf2 (HFE-7200) | C4F9OC2H5 | 59 |
HFE-43-10pccc124 (H-Galden 1040x) | CHF2OCF2OC2F4OCHF2 | 1,870 |
HFE-236ca12 (HG-10) | CHF2OCF2OCHF2 | 2,800 |
HFE-338pcc13 (HG-01) | CHF2OCF2CF2OCHF2 | 1,500 |
(CF3)2CFOCH3 | 343 | |
CF3CF2CH2OH | 42 | |
(CF3)2CHOH | 195 | |
HFE-227ea | CF3CHFOCF3 | 1,540 |
HFE-236ea2 | CHF2OCHFCF3 | 989 |
HFE-236fa | CF3CH2OCF3 | 487 |
HFE-245fa1 | CHF2CH2OCF3 | 286 |
HFE-263fb2 | CF3CH2OCH3 | 11 |
HFE-329mcc2 | CHF2CF2OCF2CF3 | 919 |
HFE-338mcf2 | CF3CH2OCF2CF3 | 552 |
HFE-347mcf2 | CHF2CH2OCF2CF3 | 374 |
HFE-356mec3 | CH3OCF2CHFCF3 | 101 |
HFE-356pcf2 | CHF2CH2OCF2CHF2 | 265 |
HFE-356pcf3 | CHF2OCH2CF2CHF2 | 502 |
HFE-365mcfI’ll t3 | CF3CF2CH2OCH3 | 11 |
HFE-374pc2 | CHF2CF2OCH2CH3 | 557 |
– (CF2)4CH (OH) – | 73 | |
(CF3)2CHOCHF2 | 380 | |
(CF3)2CHOCH3 | 27 | |
Perfluoropolyethers | ||
PFPMIE | CF3OCF(CF3)CF2OCF2OCF3 | 10,300 |
Trifluoromethyl sulfur pentafluoride | SF5CF3 | 17,400 |
A substance's GWP depends on the number of years (denoted by a subscript) over which the potential is calculated. A gas which is quickly removed from the atmosphere may initially have a large effect, but for longer time periods, as it has been removed, it becomes less important. Thus methane has a potential of 25 over 100 years (GWP100 = 25) but 86 over 20 years (GWP20 = 86); conversely sulfur hexafluoride has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoting a GWP it is important to give a reference to the calculation.
The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases. [21]
Commonly, a time horizon of 100 years is used by regulators. [22] [23]
Water vapour does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H2O: an estimate gives a 100-year GWP between -0.001 and 0.0005. [24]
H2O can function as a greenhouse gas because it has a profound infrared absorption spectrum with more and broader absorption bands than CO2. Its concentration in the atmosphere is limited by air temperature, so that radiative forcing by water vapour increases with global warming (positive feedback). But the GWP definition excludes indirect effects. GWP definition is also based on emissions, and anthropogenic emissions of water vapour (cooling towers, irrigation) are removed via precipitation within weeks, so its GWP is negligible.
When calculating the GWP of a greenhouse gas, the value depends on the following factors:
A high GWP correlates with a large infrared absorption and a long atmospheric lifetime. The dependence of GWP on the wavelength of absorption is more complicated. Even if a gas absorbs radiation efficiently at a certain wavelength, this may not affect its GWP much, if the atmosphere already absorbs most radiation at that wavelength. A gas has the most effect if it absorbs in a "window" of wavelengths where the atmosphere is fairly transparent. The dependence of GWP as a function of wavelength has been found empirically and published as a graph. [28]
Because the GWP of a greenhouse gas depends directly on its infrared spectrum, the use of infrared spectroscopy to study greenhouse gases is centrally important in the effort to understand the impact of human activities on global climate change.
Just as radiative forcing provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, global warming potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense. GWP is based on a number of factors, including the radiative efficiency (infrared-absorbing ability) of each gas relative to that of carbon dioxide, as well as the decay rate of each gas (the amount removed from the atmosphere over a given number of years) relative to that of carbon dioxide. [29]
The radiative forcing capacity (RF) is the amount of energy per unit area, per unit time, absorbed by the greenhouse gas, that would otherwise be lost to space. It can be expressed by the formula:
where the subscript i represents a wavenumber interval of 10 inverse centimeters. Absi represents the integrated infrared absorbance of the sample in that interval, and Fi represents the RF for that interval.[ citation needed ]
The Intergovernmental Panel on Climate Change (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001, except for methane, which had its GWP almost doubled. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report. [30] The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas:
where TH is the time horizon over which the calculation is considered; ax is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm−2 kg−1) and [x](t) is the time-dependent decay in abundance of the substance following an instantaneous release of it at time t=0. The denominator contains the corresponding quantities for the reference gas (i.e. CO2). The radiative efficiencies ax and ar are not necessarily constant over time. While the absorption of infrared radiation by many greenhouse gases varies linearly with their abundance, a few important ones display non-linear behaviour for current and likely future abundances (e.g., CO2, CH4, and N2O). For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted.
Since all GWP calculations are a comparison to CO2 which is non-linear, all GWP values are affected. Assuming otherwise as is done above will lead to lower GWPs for other gases than a more detailed approach would. Clarifying this, while increasing CO2 has less and less effect on radiative absorption as ppm concentrations rise, more powerful greenhouse gases like methane and nitrous oxide have different thermal absorption frequencies to CO2 that are not filled up (saturated) as much as CO2, so rising ppms of these gases are far more significant.
Carbon dioxide equivalent (CO2e or CO2eq or CO2-e) of a quantity of gas is calculated from its GWP. For any gas, it is the mass of CO2 which would warm the earth as much as the mass of that gas. [31] Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP multiplied by mass of the other gas. For example, if a gas has GWP of 100, two tonnes of the gas have CO2e of 200 tonnes, and 9 tonnes of the gas has CO2e of 900 tonnes.
On a global scale, the warming effects of one or more greenhouse gases in the atmosphere can also be expressed as an equivalent atmospheric concentration of CO2. CO2e can then be the atmospheric concentration of CO2 which would warm the earth as much as a particular concentration of some other gas or of all gases and aerosols in the atmosphere. For example, CO2e of 500 parts per million would reflect a mix of atmospheric gases which warm the earth as much as 500 parts per million of CO2 would warm it. [32] [33] Calculation of the equivalent atmospheric concentration of CO2 of an atmospheric greenhouse gas or aerosol is more complex and involves the atmospheric concentrations of those gases, their GWPs, and the ratios of their molar masses to the molar mass of CO2.
CO2e calculations depend on the time-scale chosen, typically 100 years or 20 years, [34] [35] since gases decay in the atmosphere or are absorbed naturally, at different rates.
The following units are commonly used:
For example, the table above shows GWP for methane over 20 years at 86 and nitrous oxide at 289, so emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 86 or 289 million tonnes of carbon dioxide, respectively.
Under the Kyoto Protocol, in 1997 the Conference of the Parties standardized international reporting, by deciding (see decision number 2/CP.3) that the values of GWP calculated for the IPCC Second Assessment Report were to be used for converting the various greenhouse gas emissions into comparable CO2 equivalents. [40] [41]
After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the UN Framework Convention on Climate Change (UNFCCC, decision number 24/CP.19) to require using a new set of 100-year GWP values. They published these values in Annex III, and they took them from the IPCC Fourth Assessment Report, which had been published in 2007. [19] Those 2007 estimates are still used for international comparisons through 2020, [20] although the latest research on warming effects has found other values, as shown in the tables above.
Though recent reports reflect more scientific accuracy, countries and companies continue to use the IPCC Second Assessment Report (SAR) [14] and IPCC Fourth Assessment Report values for reasons of comparison in their emission reports. The IPCC Fifth Assessment Report has skipped the 500-year values but introduced GWP estimations including the climate-carbon feedback (f) with a large amount of uncertainty. [11]
The Global Temperature change Potential (GTP) is another way to compare gases. While GWP estimates infrared thermal radiation absorbed, GTP estimates the resulting rise in average surface temperature of the world, over the next 20, 50 or 100 years, caused by a greenhouse gas, relative to the temperature rise which the same mass of CO2 would cause. [11] Calculation of GTP requires modelling how the world, especially the oceans, will absorb heat. [22] GTP is published in the same IPCC tables with GWP. [11]
GWP* has been proposed to take better account of short-lived climate pollutants (SLCP) such as methane, relating a change in the rate of emissions of SLCPs to a fixed quantity of CO2. [42] However GWP* has itself been criticised both for its suitability as a metric and for inherent design features which can perpetuate injustices and inequity. [43] [44] [45]
The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.
The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of Jupiter, or from its host star as in the case of the Earth. In the case of Earth, the Sun emits shortwave radiation (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits longwave radiation (heat) that is mostly absorbed by greenhouse gases. That heat absorption reduces the rate at which the Earth can cool off in response to being warmed by the Sun. Adding to greenhouse gases further reduces the rate a planet emits radiation to space, raising its average surface temperature.
Radiative forcing is a concept used in climate science to quantify the change in energy balance in the Earth's atmosphere caused by various factors, such as concentrations of greenhouse gases, aerosols, and changes in solar radiation. In more technical terms, it is "the change in the net, downward minus upward, radiative flux due to a change in an external driver of climate change." These external drivers are distinguished from feedbacks and variability that are internal to the climate system, and that further influence the direction and magnitude of imbalance.
This glossary of climate change is a list of definitions of terms and concepts relevant to climate change, global warming, and related topics.
An emission intensity is the emission rate of a given pollutant relative to the intensity of a specific activity, or an industrial production process; for example grams of carbon dioxide released per megajoule of energy produced, or the ratio of greenhouse gas emissions produced to gross domestic product (GDP). Emission intensities are used to derive estimates of air pollutant or greenhouse gas emissions based on the amount of fuel combusted, the number of animals in animal husbandry, on industrial production levels, distances traveled or similar activity data. Emission intensities may also be used to compare the environmental impact of different fuels or activities. In some case the related terms emission factor and carbon intensity are used interchangeably. The jargon used can be different, for different fields/industrial sectors; normally the term "carbon" excludes other pollutants, such as particulate emissions. One commonly used figure is carbon intensity per kilowatt-hour (CIPK), which is used to compare emissions from different sources of electrical power.
A carbon footprint (or greenhouse gas footprint) is a calculated value or index that makes it possible to compare the total amount of greenhouse gases that an activity, product, company or country adds to the atmosphere. Carbon footprints are usually reported in tonnes of emissions (CO2-equivalent) per unit of comparison. Such units can be for example tonnes CO2-eq per year, per kilogram of protein for consumption, per kilometer travelled, per piece of clothing and so forth. A product's carbon footprint includes the emissions for the entire life cycle. These run from the production along the supply chain to its final consumption and disposal.
The emission reduction unit (ERU) was an emissions unit issued under a Joint Implementation project in terms of the Kyoto Protocol. An ERU represented a reduction of greenhouse gases under the Joint Implementation mechanism, where it represented one tonne of CO2 equivalent reduced.
Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide, from burning fossil fuels such as coal, oil, and natural gas, is one of the most important factors in causing climate change. The largest emitters are China followed by the United States. The United States has higher emissions per capita. The main producers fueling the emissions globally are large oil and gas companies. Emissions from human activities have increased atmospheric carbon dioxide by about 50% over pre-industrial levels. The growing levels of emissions have varied, but have been consistent among all greenhouse gases. Emissions in the 2010s averaged 56 billion tons a year, higher than any decade before. Total cumulative emissions from 1870 to 2017 were 425±20 GtC from fossil fuels and industry, and 180±60 GtC from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2017, coal 32%, oil 25%, and gas 10%.
In Earth's atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in the atmosphere of Earth. The current global average concentration of carbon dioxide (CO2) in the atmosphere is 421 ppm as of May 2022 (0.04%). This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity. Burning fossil fuels is the main cause of these increased CO2 concentrations and also the main cause of climate change. Other large sources of CO2 from human activities include cement production, deforestation, and biomass burning.
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).
Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing (RF) of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.
The history of the scientific discovery of climate change began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect was first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change Earth's energy balance and climate. The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier. The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.
Climate change feedbacks are effects of global warming that amplify or diminish the effect of forces that initially cause the warming. Positive feedbacks enhance global warming while negative feedbacks weaken it. Feedbacks are important in the understanding of climate change because they play an important part in determining the sensitivity of the climate to warming forces. Climate forcings and feedbacks together determine how much and how fast the climate changes. Large positive feedbacks can lead to tipping points—abrupt or irreversible changes in the climate system—depending upon the rate and magnitude of the climate change.
A Representative Concentration Pathway (RCP) is a greenhouse gas concentration trajectory adopted by the IPCC. Four pathways were used for climate modeling and research for the IPCC Fifth Assessment Report (AR5) in 2014. The pathways describe different climate change scenarios, all of which are considered possible depending on the amount of greenhouse gases (GHG) emitted in the years to come. The RCPs – originally RCP2.6, RCP4.5, RCP6, and RCP8.5 – are labelled after a possible range of radiative forcing values in the year 2100. The higher values mean higher greenhouse gas emissions and therefore higher global temperatures and more pronounced effects of climate change. The lower RCP values, on the other hand, are more desirable for humans but require more stringent climate change mitigation efforts to achieve them.
Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% of methane released globally was from human activities, while natural sources contributed about 40%. Reducing methane emissions by capturing and utilizing the gas can produce simultaneous environmental and economic benefits.
The transient climate response to cumulative emissions of carbon dioxide (TCRE) is the ratio of the globally averaged surface temperature change per unit carbon dioxide (CO2) emitted.
An index used to compare the relative radiative forcing of different gases without directly calculating the changes in atmospheric concentrations. GWPs are calculated as the ratio of the radiative forcing that would result from the emission of one kilogram of a greenhouse gas to that from the emission of one kilogram of carbon dioxide over a fixed period of time, such as 100 years.