Causes of climate change
The changes that our planet has undergone throughout its history are a result of natural factors like tiny changes in the Earth's path around the sun, volcanic activity and fluctuations within the climate system. However, humans are having an increasing influence on our climate by burning fossil fuels, cutting down rainforests and farming livestock.
The Greenhouse Effect
As the sun's energy warms up the Earth, our planet radiates some of this heat back out towards space. Certain gases in the atmosphere act like the glass in a greenhouse, allowing the sun's energy in but preventing heat from escaping.
Some greenhouse gases, such as water vapour - the most abundant greenhouse gas - are naturally present in the atmosphere; without them, the Earth's average temperature would be an unbearably cold -18ºC instead of the 15ºC it is today.
However, human activities are releasing enormous amounts of greenhouse gases into the atmosphere and this is enhancing the greenhouse effect.
Feeling the heat
The consensus among the world's leading climate scientists is that there is no doubt the climate system is warming, and that it is extremely likely that emissions of greenhouse gases from human activities are the dominant cause.
The global average temperature has risen by 0.85ºC since the late 19th century and each of the past three decades has been warmer than any preceding decade since records began in 1850. Since the middle of the 20th century concentrations of greenhouse gases have increased, the atmosphere and ocean have warmed, the amounts of snow and ice have diminished and sea level has risen.
Need to keep warming below 2°C
The latest scientific evidence suggests that, if little or no action is taken to reduce global greenhouse gas emissions, by the end of this century global warming is likely to exceed 2°C above the average temperature in 1850-1900 and could be as much as 5°C.
An increase of 2°C compared to the temperature in pre-industrial times is seen by scientists as the threshold beyond which there is a much higher risk that dangerous and possibly catastrophic changes in the global environment will occur. For this reason the international community has recognised the need to keep warming below 2°C.
CO2 emissions rising
The greenhouse gas most commonly produced by human activities is carbon dioxide (CO2). It is responsible for some 63% of man-made global warming. One of the main sources of CO2 in the atmosphere is the combustion of fossil fuels - coal, oil and gas.
Over the past two and a half centuries, our societies have burnt increasing amounts of fossil fuels to power machines, generate electricity, heat buildings and transport people and goods. Since the Industrial Revolution in 1750 the concentration of CO2 in the atmosphere has increased by around 40%, and it continues to rise.
Deforestation: a double blow for the climate
Trees help to regulate the climate by taking up CO2 from the atmosphere, and immense amounts of carbon are stored in the world's forests. When forests are cut down, the carbon stored in the trees is released into the atmosphere as CO2, adding to the greenhouse effect. On top of that, when a forest is destroyed, it can no longer absorb CO2 from the atmosphere.
Other greenhouse gases are emitted in smaller quantities than CO2. However, these all trap heat far more effectively than CO2, and are in some cases thousands of times stronger, making them powerful contributors to global warming.
In addition to CO2, six other gases are controlled by the Kyoto Protocol, the international treaty which sets limitations on greenhouse gas emissions from developed countries. These gases are:
- Methane: The most common greenhouse gas after CO2, methane is responsible for around one-fifth of global warming from human activities. One reason for rising methane emissions is the expansion of livestock farming due to the growing consumption of meat and dairy products. The bacteria that help cattle and sheep digest their food produces methane gas, which the animals belch back into the atmosphere.
- Nitrous oxide: Emission sources include nitrogen fertilisers, the combustion of fossil fuels, and some industrial processes, including nylon production. Nitrous oxide is responsible for around 6% of man-made global warming.
- Fluorinated gases: Certain industrial gases which have been found to deplete the Earth's protective ozone layer account for around 12% of global warming. They are being phased out, but in some cases they are being substituted by so-called fluorinated gases which can be even more powerful greenhouse gases. Four types of fluorinated gases are controlled by the Kyoto Protocol: hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and (for 2013-2020 only) nitrogen trifluoride. They are used in a variety of industrial applications.