Greenhouse gases are chemical compounds that contribute to the greenhouse effect. When in the atmosphere a greenhouse gas allow sunlight (solar radiation) to enter the atmosphere where it warms the Earth’s surface and is reradiated back into the atmosphere as longer-wave energy (heat). Greenhouse gases absorb this heat and ‘trap’ it in the lower atmosphere.
The rapid increase in atmospheric concentrations of the three main human-made greenhouse gases – carbon dioxide, methane, and nitrous oxide – is clear from the data sets for these gases over the last 420,000 years.
Since around the time of the Industrial Revolution in Western countries, concentrations of carbon dioxide, methane, and nitrous oxide have all risen dramatically. Fossil fuel combustion, increasingly intensive agriculture, and an expanding global human population have been the primary causes for these rapid changes.
Methane concentrations have seen the biggest relative increase in the last 200 years, with concentrations more than doubling. However, the rate of methane increase appears now to be lessening and it is concentrations of the human-made greenhouse gases carbon dioxide and nitrous oxide that are likely to increase most in the next 100 years.
Fossil fuel-burning is the primary anthropogenic source of carbon dioxide, with cement and lime production also being important. Ruminant livestock and rice cultivation are the leading human activities contributing to methane emissions, while agriculture is the primary source of human-made nitrous oxide emissions.
In climate science, the relative climate-forcing strength of different greenhouse gases is described relative to that of carbon dioxide. Methane is much more effective at absorbing infrared radiation (heat) and is thus a more powerful greenhouse gas. Yet its lifetime in the atmosphere is only about 10 years, compared to about 100 years for a molecule of carbon dioxide. As such, the climate-forcing strength of a kilogram of methane on a 100 year time-horizon – its Global Warming Potential (GWP) – is 23. That is, every kilogram of methane in the atmosphere has the equivalent global warming potential of 23 kilograms of carbon dioxide. The GWP of nitrous oxide is 296.
Our impact on the global climate since the Industrial Revolution has been complex. Though emissions of greenhouse gases, like carbon dioxide and methane have had a net warming effect, emissions of sulphate aerosols have had a net cooling effect.
The overall effect is net global warming, but the complex interaction of these positive and negative influences on global warming make predicting future warming difficult.
The problem is exacerbated by our poor level of understanding of exactly how some factors, like land-use albedo (the reflectance of the land), operate and interact. Another very important greenhouse gas is water vapor and, though human activities are not primarily responsible for its concentration in the atmosphere, an indirect increase through elevated surface temperatures may lead to one of the most important postive feedbacks to global warming in the 21st century.