Appendix 3: the main greenhouse gases (GSG's) [1]*
* Source for header: https://www3.epa.gov/climatechange/ghgemissions/inventoryexplorer/.
Top: US greenhouse gas emissions in 2018: fluorinated gases 3%; nitrous oxide 6%; methane 10%; CO2 82%
Bottom: Total US greenhouse gas emissions by economic sector in 2016: transportation (28%); electricity generation (28%); industry (22%); agriculture (9%); commercial (6%); residential (5%).
The most important GHGs directly emitted by humans include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others.
Carbon dioxide
Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. CO2 is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO2, causing concentrations in the atmosphere to rise.
Atmospheric CO2 concentrations have increased by more than 40% since pre-industrial times, from approximately 280 parts per million by volume (ppmv) in the 18th century to over 400 ppmv in 2015, due primarily to the burning of coal, oil and natural gas. Each rise of one degree in the atmosphere lifts the capacity to hold moisture by 7% and as a consequence the potential for bigger storms and other catastrophic climatic events increases. The monthly average concentration at Mauna Loa Observatory, Hawaii, now exceeds 400 ppmv for the first time in human history, and the current CO2 level is higher than it has been in at least 800,000 years.
Some volcanic eruptions released large quantities of CO2 in the distant past. However, the US Geological Survey (USGS) reports that human activities now emit more than 135 times as much CO2 as volcanoes each year. Human activities currently release over 30 billion tons of CO2 into the atmosphere every year.[2] “The resultant build-up of CO2 in the atmosphere is like a tub filling with water, where more water flows from the faucet than the drain can take away”.[3]
Top: US greenhouse gas emissions in 2018: fluorinated gases 3%; nitrous oxide 6%; methane 10%; CO2 82%
Bottom: Total US greenhouse gas emissions by economic sector in 2016: transportation (28%); electricity generation (28%); industry (22%); agriculture (9%); commercial (6%); residential (5%).
The most important GHGs directly emitted by humans include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others.
Carbon dioxide
Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. CO2 is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO2, causing concentrations in the atmosphere to rise.
Atmospheric CO2 concentrations have increased by more than 40% since pre-industrial times, from approximately 280 parts per million by volume (ppmv) in the 18th century to over 400 ppmv in 2015, due primarily to the burning of coal, oil and natural gas. Each rise of one degree in the atmosphere lifts the capacity to hold moisture by 7% and as a consequence the potential for bigger storms and other catastrophic climatic events increases. The monthly average concentration at Mauna Loa Observatory, Hawaii, now exceeds 400 ppmv for the first time in human history, and the current CO2 level is higher than it has been in at least 800,000 years.
Some volcanic eruptions released large quantities of CO2 in the distant past. However, the US Geological Survey (USGS) reports that human activities now emit more than 135 times as much CO2 as volcanoes each year. Human activities currently release over 30 billion tons of CO2 into the atmosphere every year.[2] “The resultant build-up of CO2 in the atmosphere is like a tub filling with water, where more water flows from the faucet than the drain can take away”.[3]
This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air. [4]
Atmospheric carbon dioxide concentration has risen from pre-industrial levels of 280 parts per million by volume (ppmv) to over 401 ppmv in 2016. Since 1959 alone, concentrations have risen by more than 85 ppmv.
Methane (CH4)
Methane is produced through both natural and human activities. For example, natural wetlands, agricultural activities[5], and fossil fuel extraction and transport all emit CH4. Methane is more abundant in Earth’s atmosphere now than at any time in at least the past 800,000 years. Due to human activities, CH4 concentrations increased sharply during most of the 20th century and are now more than two-and-a-half times pre-industrial levels. Melting tundra in the arctic is also responsible for releasing methane into the atmosphere. Methane is 21 times more powerful than CO2 and if the melting tundra gives up its methane, a tipping point of irreversible climate change could come about.
In July 2020, a paper published in the Environmental Research Letters Journal showed that fossil fuel developments are rapidly increasing emissions of methane, led by the opening of coal mines and gas fields, some of which are in Australia, whereas missions of the gas from natural sources of methane, such as wetlands, termites or volcanoes, barely budged from the average over 2000-2006 to 2017, at about 367 million tonnes a year. By contrast, fossil-fuel sources had risen about 27 per cent to 135 million tonnes annually, with coal-linked methane emissions up 51.7 per cent and those from oil and gas up 16.7 per cent
Agriculture formerly provided about two-thirds of non-natural methane emissions – with fossil fuels making up the remainder – until the early 2000s. Since then, the gap has begun to close with the growth of emissions of the gas from fossil fuels double those from farming. Methane from farming, such as rice cultivation and cattle, also rose 12.7 per cent during the period to 213 million tonnes, annually.
As a greenhouse gas, methane is second only to carbon dioxide in terms of warming the planet, contributing to about 23 per cent of the heating since 1750, the average methane concentration in the atmosphere having risen about 2.5 times over the period. While it typically remains in the atmosphere for much less time than CO2, methane is more potent – about 86 times more per unit of mass over 20 years, the paper noted.
Methane is offsetting coal in the electricity sector and reducing carbon dioxide emissions, but increasing methane emissions in that sector. The US and Canada are also producing more natural gas, and as a result, emitting more methane from oil and gas wells and leaky pipelines. In other words, moving from one fossil fuel to another is not a pathway to net-zero emissions and certainly not the panacea it is supposed to be.
Other greenhouse gases include:
Meanwhile, recent research, published in the Nature Climate Change journal on 19 November 2019, has found that estimates of nitrous oxides, the third most important greenhouse gas, mostly sourced from fertiliser use and manure, have "increased substantially" since 2009, and at a faster rate than forecast by the IPCC. The study found N20 emissions increased globally by 1.6 million tonnes a year between 2000–2005 and 2010–2015, or about twice the rise reported to the United Nations Framework Convention on Climate Change based on the amount of nitrogen fertiliser and manure used.
Also, the emissions of nitrous oxides together with methane, two of the three big contributing greenhouse gases were unlikely to be reduced to zero, and since the Paris climate goals imply a "carbon neutrality" by the second half of this century, this meant that carbon dioxide emissions would have to become negative - more sequestered than released to the atmosphere - to counter the warming contribution of those two other gases.
What are not greenhouse gases?
For various technical reasons pertaining to their internal structure, oxygen (21%), nitrogen (78%) and argon (1%) which also pervade the earth’s atmosphere are not greenhouse gases. Argon exists in the atmosphere and its energy levels are widely spaced, and the only photons it can absorb are of quite high energy, placing them in the ultraviolet part of the spectrum. Nitrogen and oxygen are homonuclear molecules, meaning that both atoms are the same. This means that the electric field of the light beam (electromagnetic radiation) cannot get hold of the molecule, which is a key part of the process of absorption (or emission), so once again, neither of these species is radiatively active. So finally, it is only the trace gases such as water vapour and carbon dioxide which are left to carry the can.[6]
[1] Source: https://www.epa.gov/climate-change-science/causes-climate-change
[2] https://www.epa.gov/climate-change-science/causes-climate-change#ref2
[3] Ibid.
[4] Source: U.S. National Climate Assessment (2014): https://www.epa.gov/climate-change-science/causes-climate-change
[5] For the impact of agriculture on methane production, see Michael Mosley’s excellent programme at http://www.sbs.com.au/news/video/348450371688/Michael-Mosley-The-Truth-About-Meat concluding that factory farming over open range methods may be best for the planet.
[6] Associate Professor Michael Box, “Our atmospheric environment”’ WEA Course. May-June 2017, 2.3.1
Next
Atmospheric carbon dioxide concentration has risen from pre-industrial levels of 280 parts per million by volume (ppmv) to over 401 ppmv in 2016. Since 1959 alone, concentrations have risen by more than 85 ppmv.
Methane (CH4)
Methane is produced through both natural and human activities. For example, natural wetlands, agricultural activities[5], and fossil fuel extraction and transport all emit CH4. Methane is more abundant in Earth’s atmosphere now than at any time in at least the past 800,000 years. Due to human activities, CH4 concentrations increased sharply during most of the 20th century and are now more than two-and-a-half times pre-industrial levels. Melting tundra in the arctic is also responsible for releasing methane into the atmosphere. Methane is 21 times more powerful than CO2 and if the melting tundra gives up its methane, a tipping point of irreversible climate change could come about.
In July 2020, a paper published in the Environmental Research Letters Journal showed that fossil fuel developments are rapidly increasing emissions of methane, led by the opening of coal mines and gas fields, some of which are in Australia, whereas missions of the gas from natural sources of methane, such as wetlands, termites or volcanoes, barely budged from the average over 2000-2006 to 2017, at about 367 million tonnes a year. By contrast, fossil-fuel sources had risen about 27 per cent to 135 million tonnes annually, with coal-linked methane emissions up 51.7 per cent and those from oil and gas up 16.7 per cent
Agriculture formerly provided about two-thirds of non-natural methane emissions – with fossil fuels making up the remainder – until the early 2000s. Since then, the gap has begun to close with the growth of emissions of the gas from fossil fuels double those from farming. Methane from farming, such as rice cultivation and cattle, also rose 12.7 per cent during the period to 213 million tonnes, annually.
As a greenhouse gas, methane is second only to carbon dioxide in terms of warming the planet, contributing to about 23 per cent of the heating since 1750, the average methane concentration in the atmosphere having risen about 2.5 times over the period. While it typically remains in the atmosphere for much less time than CO2, methane is more potent – about 86 times more per unit of mass over 20 years, the paper noted.
Methane is offsetting coal in the electricity sector and reducing carbon dioxide emissions, but increasing methane emissions in that sector. The US and Canada are also producing more natural gas, and as a result, emitting more methane from oil and gas wells and leaky pipelines. In other words, moving from one fossil fuel to another is not a pathway to net-zero emissions and certainly not the panacea it is supposed to be.
Other greenhouse gases include:
- Nitrous oxide (N2O) produced through natural and human activities, mainly through agricultural activities and natural biological processes. Fuel burning and some other processes also create N2O. Concentrations of N2O have risen approximately 20% since the start of the Industrial Revolution, with a relatively rapid increase toward the end of the 20th century. Overall, N2O concentrations have increased more rapidly during the past century than at any time in the past 22,000 years.
- Water vapour, the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime.
- Tropospheric ozone (O3), which also has a short atmospheric lifetime. Ozone depletion causes uv-radiation to pass through the atmosphere. UV-radiation has high energy density, and may lead to cancer.
- Hydro chlorofluorocarbons (HCFCs) and related gases often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Unlike water vapor and ozone, these gases have a long atmospheric lifetime, and some of these emissions will affect the climate for many decades or centuries.
Meanwhile, recent research, published in the Nature Climate Change journal on 19 November 2019, has found that estimates of nitrous oxides, the third most important greenhouse gas, mostly sourced from fertiliser use and manure, have "increased substantially" since 2009, and at a faster rate than forecast by the IPCC. The study found N20 emissions increased globally by 1.6 million tonnes a year between 2000–2005 and 2010–2015, or about twice the rise reported to the United Nations Framework Convention on Climate Change based on the amount of nitrogen fertiliser and manure used.
Also, the emissions of nitrous oxides together with methane, two of the three big contributing greenhouse gases were unlikely to be reduced to zero, and since the Paris climate goals imply a "carbon neutrality" by the second half of this century, this meant that carbon dioxide emissions would have to become negative - more sequestered than released to the atmosphere - to counter the warming contribution of those two other gases.
What are not greenhouse gases?
For various technical reasons pertaining to their internal structure, oxygen (21%), nitrogen (78%) and argon (1%) which also pervade the earth’s atmosphere are not greenhouse gases. Argon exists in the atmosphere and its energy levels are widely spaced, and the only photons it can absorb are of quite high energy, placing them in the ultraviolet part of the spectrum. Nitrogen and oxygen are homonuclear molecules, meaning that both atoms are the same. This means that the electric field of the light beam (electromagnetic radiation) cannot get hold of the molecule, which is a key part of the process of absorption (or emission), so once again, neither of these species is radiatively active. So finally, it is only the trace gases such as water vapour and carbon dioxide which are left to carry the can.[6]
[1] Source: https://www.epa.gov/climate-change-science/causes-climate-change
[2] https://www.epa.gov/climate-change-science/causes-climate-change#ref2
[3] Ibid.
[4] Source: U.S. National Climate Assessment (2014): https://www.epa.gov/climate-change-science/causes-climate-change
[5] For the impact of agriculture on methane production, see Michael Mosley’s excellent programme at http://www.sbs.com.au/news/video/348450371688/Michael-Mosley-The-Truth-About-Meat concluding that factory farming over open range methods may be best for the planet.
[6] Associate Professor Michael Box, “Our atmospheric environment”’ WEA Course. May-June 2017, 2.3.1
Next