Climate change - Introduction
What it means for us - The impact of climate change
What we can do about it - Suggestions for action
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Greenhouse gases and global warming

What are the greenhouse gases?

Carbon dioxide (CO₂) is the most important gas. By examining ice cores in the polar ice caps, scientists have established that the natural variation of CO₂ in the atmosphere over the past 650,000 years is in the range 180-300 parts per million (ppm). Today, the atmospheric concentration of CO₂ is over 380 ppm, and rising by 2 ppm every year. Even if CO₂ emissions are stabilised at current levels, a rise to about 450 ppm by the end of this century is inevitable. And scientific models suggest that a level of about 550 ppm could trigger irreversible climate change leading to the extinction of all life on Earth. The increase of CO₂ in the atmosphere contributes as much to global warming as all other factors put together, including the increased solar activity of the 20th century.

Methane is the second most important of the greenhouse gases, but it contributes only a third as much as carbon dioxide to global warming. However, the atmospheric concentration of methane is more than double the natural maximum of the last 650,000 years. The rate of growth of methane in the atmosphere is now slowing.

Nitrous oxide (dentist's laughing gas) in the atmosphere continues to increase. Its contribution to global warming is a third that of methane, but it is no laughing matter.

What is causing the rise in greenhouse gas emissions?

Overwhelmingly, human activity. The main source of the rise in CO₂ is the burning of fossil fuels - oil, gas and coal. Industry, transport and domestic energy use are all responsible for this, through the generation of electricity and the use of oil, gas and coal to light and heat our homes, fuel our cars, lorries, buses, trains and aircraft, and provide energy for industrial and manufacturing processes. Some of the rise in methane concentrations is also due to fossil fuel use, with much of the rest coming from agriculture, which is also a major contributor to the increase of atmospheric nitrous oxide.

Global warming: other factors

Changes in the amount of ozone in the atmosphere, caused by chemicals released by human activities, have added as much to global warming as the increase in methane concentrations.

On the other hand, atmospheric pollution has had an overall cooling effect, mainly because it encourages the formation of clouds which block and reflect the sun's rays. The overall cooling effect is roughly equivalent to three-quarters of the warming caused by carbon dioxide. Without atmospheric pollution, undesirable as it is, global warming would already be much worse than it is.

Climate change - Introduction
What it means for us - The impact of climate change
What we can do about it - Suggestions for action
Join the debate - Read what others have said, and have your say
Links - Find out more