Increasing concentration of greenhouse gases in the atmosphere

The gases carbon dioxide CO₂, methane CH₄ and nitrous oxide N₂O are among the principal causes of human-induced climate change. They are found in the atmosphere in the absence of human activity, but the increases in their concentrations are due to human activities such as burning fossil fuels, deforestation and agriculture (livestock, rice paddies, and the use of nitrogen-based fertilisers). They are all from biological or fossil fuel sources.

There are a number of other gases that are present in trace amounts in the atmosphere and which are known to aggravate the problems of global warming. Only a few are present in sufficient concentration and have sufficiently strong radiation absorption bands in the thermal radiation window that contains up to 30% of the upward infrared radiation from the Earth's surface. However practically all these gases are increasing in the troposphere and their total effect may be comparable to that of the major gases. They include other gaseous oxides of nitrogen, carbon monoxide, volatile organic compounds and tropospheric ozone and its precursors and synthetic greenhouse gases: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), most perfluorocarbons (PFCs), sulfur hexafluoride (SF₆), and others. Several, most famously CFCs, also deplete the ozone layer and are regulated under the Montreal Protocol. Others, such as HFCs, were actually first produced in large quantities to replace the ozone-depleting substances, but unfortunately turned out to be potent greenhouse gases too. Most of these gases are emitted exclusively by humans and have been used variously as aerosol spray propellants, refrigerants, fire-extinguishing agents, and in the production of semiconductors, among other industrial applications. Dichlorodifluoromethane (CFC-12) and trichlorofluoromethane (CFC-11) are the third and fifth most important greenhouse gases respectively, in terms of their overall contributions to global warming.

the synthetic greenhouse gases chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), most perfluorocarbons (PFCs), sulfur hexafluoride (SF₆), and others. Several, most famously CFCs, also deplete the ozone layer and are regulated under the Montreal Protocol. Others, such as HFCs, were actually first produced in large quantities to replace the ozone-depleting substances, but unfortunately turned out to be potent greenhouse gases too. Dichlorodifluoromethane (CFC-12) and trichlorofluoromethane (CFC-11) are the third and fifth most important greenhouse gases respectively in terms of their overall contributions to global warming.

How much a greenhouse gas contributes to warming depends on three factors. The first is how much gas is emitted. Second is how much a kilogram of that gas will warm the planet once it’s in the atmosphere. And third is how long the gas will remain in the atmosphere.

CO₂ is the most important greenhouse gas in warming the planet, despite being the weakest greenhouse gas per unit of mass. Its contribution to warming comes from the sheer scale of emissions (40 billion tonnes emitted each year) and the fact that a large part effectively hangs around in the atmosphere for hundreds or thousands of years after emission. The resulting concentration makes CO₂ responsible for about 65% of all warming due to greenhouse gas emissions from human activities. Methane (CH₄) is the next most important greenhouse gas, with current concentration contributing about 15% of overall human-induced warming.

Most synthetic greenhouse gases have very high global warming potentials. The one with the highest current emissions is the refrigerant HFC-134a, which is 1,300 times more potent than CO₂ (per mass unit emitted). Other synthetic greenhouse gases have even more extraordinary warming potentials, with CF₄ (used in the semiconductor industry) and SF₆ (from industrial electricity transformers) being 6,500 and 23,400 times more potent than CO₂, respectively. CFC-12, a former refrigerant, is both a potent ozone-depleting substance and a powerful greenhouse gas. Although its emissions and atmospheric concentrations are now declining thanks to global compliance with the Montreal Protocol, it is still the third most important greenhouse gas and responsible for 6-7% of all warming since the beginning of the industrial era.

The Global Warming Potential (GWP) of various greenhouse gases is a measure of the strength of their warming effects over time. For a period of 20 years, the GWP of one kg of carbon dioxide is 1, of methane is 11, of nitrous oxide is 280, of CFC-11 is 3,400, of CFC-12 is 7,100, of HCFC-22 is 4,300, and of HFC-134a is 1,200.

The IPPC announced in 1992 that the global rates of emission of methane and CFCs were decreasing, that emissions of carbon dioxide had remained stable for 1990 and 1989, after a 5% increase over 1987, and other rates of emissions of greenhouse gases continued to grow. The US government prediction in 2002 was that that US production of greenhouse gases will rise 43 percent by 2020.

According to the European Environment Agency: The total concentration of all greenhouse gases, including cooling aerosols, reached a value of 449 ppm in CO2 equivalents in 2016 — an increase of more than 4 ppm compared with 2015, and 33 ppm more than 10 years ago.