How global warming works
The composition of gases in the Earth’s atmosphere has been changed by human activity. Billions of tonnes of greenhouse gases are emitted into the Earth’s atmosphere every year by the burning of fossil fuels, such as coal, oil and gas.
Greenhouse gases are the elements of the atmosphere, both natural and human-made, that absorb and emit radiation. The natural role of these gases is to trap the sun’s heat in the Earth’s atmosphere (after this heat has been reflected off the Earth’s surface), warming the Earth’s surface by over 30 ˚C to a level at which humans and other living things can survive. This is known as the ‘natural greenhouse effect’. Natural global warming by the greenhouse effect means that the planet is on average 15 ˚C, rather than the -18 ˚C that it would be without the Earth’s atmosphere.
Human activity, such as the burning of fossil fuels, is increasing the concentrations of greenhouse gases in the atmosphere above natural levels. These additional greenhouse gases act like a blanket around the planet; they allow the sun’s energy to reach the Earth’s surface but trap more of the heat that is reflected back. This is called the ‘enhanced greenhouse effect’.
'There is broad agreement within the scientific community that amplification of the Earth's natural greenhouse effect by the build-up of various gases introduced by human activity has the potential to produce dramatic changes in climate. Only by taking action now can we ensure that future generations will not be put at risk.'
Statement by 49 Nobel Prize winners and 700 members of the National Academy of Sciences, 1990
There is a wide consensus among climate scientists that the enhanced greenhouse effect is increasing the world's average temperature, changing our climate. Since 1961 the average temperature in Scotland has increased in all seasons. It is thought that annual temperatures averaged across Scotland will rise by up to 3.5˚C in the summer and 2.5˚C in the winter in the 21st century.
Around the world the average global temperature has increased by about 0.2 to 0.3°C over the last 40 years and by about 0.7°C since 1860. Levels of the greenhouse gas carbon dioxide (CO2) remained around 280 parts per million (ppm) until the industrial revolution. This level has now risen to over 380ppm, and continues to rise every year.
Greenhouse gases
Governments around the world are seeking ways to reduce emissions of greenhouse gases.
The Kyoto Protocol makes reference to a basket of six greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the F-gases: HFC, PFC and SF6
Carbon Dioxide (CO2) is a natural part of the atmosphere and also the principal gas emitted by human activity. It is produced when fossil fuels are burned and during respiration and decomposition. Forests, plants and the oceans absorb huge amounts of CO2, reducing the levels of CO2, but the exchange is small compared to the total amount of CO2 in the atmosphere. A molecule of CO2 can stay in the atmosphere for as long as 100 years.
Methane (CH4) can come from cows and sheep and paddy fields as well as from landfills or the breakdown of organic materials. Although methane stays in the air for about 10 years, in that time it absorbs about 20 times more heat energy than a CO2 molecule does in 100 years. 1 kg of methane has the equivalent global warming potential of 25 kg of CO2. There are huge amounts of CH4 trapped in frozen tundra which may be released if the tundra thaws.
Nitrous oxide (N2O) is only found in small amounts in the atmosphere but has a powerful effect because of its ability to absorb infrared energy. The global warming effect of 1kg of N2O is the equivalent of 298 kg of CO2.
F-gases (HFC, PFC and SF6) are man-made gases which have only been widely used since the 1990's. They can last for thousands of years in the atmosphere so have very high global warming potentials.
Photo credit: _gee_
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