Although CO2 is a major anthropogenic emission associated with global warming, there are other gases which contribute to the problem.
Anthropogenic Greenhouse Gases
Greenhouse gases are so-called because they cause a heat-retention effect in the Earth's atmosphere akin to the warming of a greenhouse.
Most lay people have heard of CO2, but this is not the only greenhouse gas. Some are natural, and are responsible for the temperature of the Earth being higher than it would otherwise be. Human activities, such as electricity generation, agriculture, traffic, construction, heating and industry, cause emissions of a number of gases which contribute to the so-called anthropogenic greenhouse gases (GHG). The main ones are:
- Carbon dioxide (CO2).
GWP = 1 over 20 to 100 years. Atmospheric residual time 30-95 years
Responsible for about 9-26% of the overall natural greenhouse effect, but 76% of the enhanced (anthropogenic) greenhouse effect is due to this gas. The IPCC reports that the percentage of CO2 in the atmosphere has risen since 1980 from 335 ppm to over 400 ppm (+20%) by 2015. Pre-industrial levels are believed to have been around 250-280 ppm, meaning humans have added 40% to the natural level of carbon dioxide. Paleoclimatology reveals that high levels of CO2 are associated with a number of mass extinction events in the past. There is an element of tautology here (is it the mass extinction which caused the high proportion of CO2 or the increase in CO2 which caused the climate change which destroyed the habitats of most living species?), but the association between change in niche conditions and change in CO2 is indisputable. It is known with a high degree of confidence that the current concentration of CO2 is higher than any time in the past 800,000 years, and probably for the past 20 million years. At this time in Earth's history the planet looked significantly different to the world we know, with sea levels, species type and distribution, precipitation and weather patterns all non-ideal for our necessary large mammal niche conditions.
By excavating and burning fossil fuels (coal, oil, gas), humans are short-circuiting a natural cycle. In this cycle, a long-term balance is maintained by atmospheric carbon dioxide being drawn into subterranean reservoirs by means of sinks. These sinks are ocean deposits and anaerobic compaction of terrestrial organic matter. After the carbon has passed from fossil deposits to carbonate rock through metamorphic processes, a small amount will be returned to the atmosphere after a residual time of tens of millions of years or longer, by expulsion during volcanic eruptions.
Mitigation involves preventing deforestation from eroding the carbon sink of living matter, more sustainable land use, and substituting fossil fuels with less polluting energy sources.
Geogenic (naturally occurring) CO2 production is 550 Gt per year. This is used (removed from the atmosphere) to build glucose in plants through photosynthesis, and in the formation of calcium carbonate in shells of marine organisms. Residence time (Verweilzeit) in the atmosphere of a CO2 molecule is on average 120 years. The anthropogenic emissions in 2006 were ca. 32 Gt (6% of total CO2 production).
- Methane (CH4)
GWP = 72 over 20-year period, 25 over 100 years. Atmospheric residual time 12 years
[GWP = 72 over 20-year period, 25 over 100 years. Atmospheric residual time 12 years] Responsible for about 16% of the enhanced greenhouse effect. The level has risen from approximately 1.6 ppm in 1980 to over 1.8 ppm (+12%) in 2015.
Human extraction of oil and natural gas causes methane gas escape into the atmosphere. Agriculture also causes methane releases, as by-products of fertilisation, poor irrigation techniques and livestock intestinal emissions (to put it politely). As the seas warm, methane hydrate structures, which lock methane at the bed of the ocean, begin to release their methane. Other sources are waste management and biomass burning.
- Nitrous oxide (NO2)
GWP = 289 over 20-year period, 298 over 100 years. Atmospheric residual time 114 years
Responsible for about 6% of the enhanced greenhouse effect. Levels have risen from 0.30 ppm in 1980 to nearly 0.33 ppm by 2015 (+10%) and show no signs of slowing their growth.
The nitrous oxide group of compounds (mainly NO2, NO, and N2O), are gases which arise during fossil fuel combustion, due to the involvement of nitrogen (78% of the atmosphere) in the reaction. They are also released through fertiliser use and the burning of biomass. Nitrous oxides are photosynthetic gases, so react with oxygen in the presence of sunlight to produce secondary pollutants such as ozone and nitric acid (acid rain).
- Fluorinated gases (F-gases)
GWP = up to 20,000 or more for the fluoromethane gases.
HCFC-22: Atmospheric residual time
= 12 years, GWP (20yr) = 5160, GWP (100yr) = 1810
CFC-12: = Atmospheric residual time
= 100 years, GWP (20yr) =1100, GWP (100yr) = 10900
Responsible for about 2% of the enhanced greenhouse effect. F-gases are entirely human made, and include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), sulphur hexafluoride (SF6), and nitrogen trifluoride (NF3), used in industrial processes, refrigeration and the manufacture and operation of consumer products. Whereas the Montreal Protocol has more of less curtailed further increases in CFCs, the levels are still higher than they were in 1980. CFCs are responsible for the breakdown of the stratospheric ozone layer, which protects life from UV radiation. HCF(-C) gases on the other hand are still being allowed to increase.
- Water vapour
Not to be confused with aerosol water, which is visible in clouds, water vapour is invisible and is the result of extra evaporation from oceans and terrestrial water as the Earth warms. Water also blocks infrared radiation from escaping the Earth, so contributes to the greenhouse effect. It is an example of the feared feedback mechanisms, in which one greenhouse effect causes changes which lead to further greenhouse effects.
- Black carbon
Although not a gas, it is present in the atmosphere and contributes to the greenhouse effect by blocking the escape of infrared radiation from the Earth's surface.
- VOCs and NMVOCs
VOCs are indirect greenhouse gases, the products of industrial processes, and their presence in the atmosphere are the cause of many direct and indirect pollution problems, including global warming.
NMVOCs are non-methane volatile organic compounds, and are regulated by the Montreal Protocol as ozone-depleting substances.
- Carbon monoxide (CO)
Usually the result of incomplete burning of fossil fuel (insufficient oxygen), CO is toxic as well as a contributor to the greenhouse problem.
Historical GHG Emissions
Greenhouse gas (GHG) emissions are being monitored by various UN and international organisations. Nearly all nations have made firm commitments in principle and practice to reduce their emissions through transitioning to low-carbon economies. Although not the primary GHG (water vapour has an overall greater heat-trapping effect), carbon dioxide is the gas which humans are releasing proportionally to its natural concentration at such a rate that it is synonymous with the generic term 'greenhouse gas emissions'.
Greenhouse gases have been emitted by human activity on a large scale since coal was burnt to power the new steam engines and machinery at the beginning of the Industrial Revolution (c. 1750 in the UK).
Coal is by far the most polluting of fossil fuels, due to its low energy density and impurities. Coal does not burn as cleanly or completely as other fossil fuels, releasing SOx, NOx, CO and CO2, all bad air pollutants. Only wood burns less efficiently, but wood and other organic materials are not considered greenhouse gases, because they are part of the carbon cycle on the surface of the planet. Only the introduction of fossil fuel (coal, natural gas, and oil) from deep beneath the surface is 'adding' carbon to the surface carbon cycle, and therefore causing global warming and other pollution problems.
Parameters which determine how significant a gas is for global warming are summed and expressed as the global warming potential (GWP). How much a unit of the gas will trap the long-wave radiation in the lower atmosphere is called radiative climate forcing, where positive radiative climate forcing indicates warming potential, and negative cooling. For example, water vapour has a positive radiative effect, while the clouds which accumulate when the air is saturated is negative, because white clouds reflect sunlight.