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.
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 immense additional amounts of greenhouse gases into the atmosphere, enhancing the greenhouse effect and warming the climate.
The greenhouse gas most commonly produced by our 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 couple of 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 the concentration of CO2 in the atmosphere has increased by around 41%, and it continues to rise.
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, they all trap heat far more effectively than CO2 does, in some cases by a factor of thousands of times, making them also powerful contributors to global warming.
In addition to CO2, five other gases are controlled by the Kyoto Protocol, the international treaty which sets limitations on greenhouse gas emissions from developed countries. These gases are: