There are many factors that could cause a change in our climate.
Anything that affects the amount of energy being absorbed from the Sun, or the amount being radiated by the Earth - the planet's energy balance - may produce long- or short-term cooling or warming.
Natural or 'forcing'
An imbalance in the planet's 'energy account' can be caused by changes in the energy radiated by the Sun, changes in greenhouse gases, particles or clouds, or changes in the reflectivity of the Earth's surface. Imbalances caused by these changes are often called 'forcings'. A positive climate forcing will tend to cause a warming, and a negative forcing a cooling.
Changes in climate can also arise from variations within the climate system. For example, the El Niño/La Niña system, in which interactions between the oceans and the atmosphere cause global temperature changes lasting a number of years. This is natural variability.
Relatively small changes in the Earth's energy account can lead to further changes, and these can further modify, for example, the reflectivity of the Earth or the amount of water vapour. The climate system is therefore highly sensitive to small changes, as these often 'feedback', and have large, long-term effects on the climate.
Increased solar energy
Scientific research into the energy we receive from the Sun has found that it is not the main cause of the current warming trend. However, solar radiation is thought to have been responsible for increased warming early in the 20th century.
Greenhouse gas increase
There's overwhelming and growing evidence that the warming is due to vastly increased - and still increasing - quantities of greenhouse gas in the atmosphere.
The most important greenhouse gas, in that it has the strongest greenhouse effect, is water vapour. It increases in concentration as the atmosphere warms. The amount of water vapour in the atmosphere has increased, but there's no reason for this scale of change other than the increase in temperature.
Carbon dioxide (CO2) and methane are both important greenhouse gases, which have a 'forcing' effect (they increase the effect of warming). Their increase in concentration is mainly caused by emissions from human activity. However, there are also potentially large secondary effects, for example decreased carbon storage due to reduced forest growth or the potential release of large amounts of methane from permafrost, caused by raised temperatures.
The amount of CO2 in the atmosphere has increased dramatically - by about 38% (as of 2012) - since the Industrial Revolution. As we continue burning fossil fuels and other activities, the amount of CO2 will continue to rise. This means the extra CO2 will absorb and emit more and more of the Earth's outgoing radiation, and this will further warm our climate. As the atmosphere warms, the amount of water vapour it holds also increases - which further adds to the warming effect.
Methane has a strong greenhouse effect, but it doesn't stay in the atmosphere for more than about a decade. CO2 lasts for about 100 years or more, meaning it has a very long time to build up and affect our climate. Some of the CO2 in our atmosphere was emitted before World War I.
Cutting down forests, one of the major natural storage 'sinks' for carbon, is further increasing the imbalance between the CO2 we emit and the planet's capacity to re-absorb it.
Is climate change really caused by human activity?
Last updated: 30 September 2013