The strength of the Earth’s greenhouse effect is determined by the concentration in the atmosphere of a handful of greenhouse gases. The one that causes the most warming overall is water vapour – though human activity affects its level in the atmosphere indirectly rather than directly.
The greenhouse gases that humans do emit directly in significant quantities are:
• Carbon dioxide (CO2). Accounts for around three-quarters of the warming impact of current human greenhouse-gas emissions. The key source of CO2 is the burning of fossil fuels such as coal, oil and gas, though deforestation is also a very significant contributor.
• Methane (CH4). Accounts for around 14% of the impact of current human greenhouse-gas emissions. Key sources include agriculture (especially livestock and rice fields), fossil fuel extraction and the decay of organic waste in landfill sites. Methane doesn’t persist in the atmosphere as long as CO2, though its warming effect is much more potent for each gram of gas released.
• Nitrous oxide (N2O). Accounts for around 8% of the warming impact of current human greenhouse-gas emissions. Key sources include agriculture (especially nitrogen-fertilised soils and livestock waste) and industrial processes. Nitrous oxide is even more potent per gram than methane.
• Fluorinated gases (“F gases”). Account for around 1% of the warming impact of current human greenhouse-gas emissions. Key sources are industrial processes. F-gases are even more potent per gram than nitrous oxide.
Human activity also changes the planet’s temperature in other ways. For example, vapour trails from planes, soot from fires and and tropospheric ozone created indirectly by local pollution all tend to increase warming. On the other hand, aerosol particles produced by some vehicles and industrial processes tend to bounce sunlight away from the earth, temporarily counteracting some of the warming caused by man-made greenhouse gases.
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