There are only a few plausible things that could cause this sudden warming of our climate — more energy from the sun, big natural variations in our climate, or an increased greenhouse effect.
Scientists have done lots of research on the energy we get from the Sun and have been able to rule that out as the main cause. Lots of natural cycles have been identified in the climate, such as El Niño, but none of the one's we know about could cause the relatively big, long-term changes we've seen.
Therefore, there's overwhelming and growing evidence that the warming we've seen is due to increasing amounts of greenhouse gases in the atmosphere. It's very likely this warming has been caused by human activity, such as burning fossil fuels (like petrol and coal) and changing land use (such as chopping down forests for cattle grazing).
There are several different greenhouse gases in the atmosphere. There are also two distinct types recognised by scientists:
'Forcing gases' are those that are in the atmosphere on a long-term basis and don't react to changes in temperature.
'Feedback gases' are those which do respond to changes in temperature, and the amount of them in the atmosphere is dependent on other factors.
The most important greenhouse gas is water vapour, but this is a feedback - 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.
CO2 and methane are both important greenhouse gases which have a forcing effect. Methane has the strongest greenhouse effect, but it doesn't stay in the atmosphere for as long - lasting only about a decade. CO2 on the other hand lasts for about 100 years or more, meaning it has a very long time to build up in the atmosphere and affect our climate.
The amount of CO2 in the atmosphere has dramatically increased since the industrial revolution, going up by about 38%. Because of its long lifespan, as we emit more CO2 from burning fossil fuels and other activities, the amount of CO2 will continue to rise. This means the extra CO2 will trap more and more of the sun's heat, and this will warm our climate. As the atmosphere warms, the amount of water vapour it holds increases - which further adds to the warming effect. This is how human activity has had an impact on our climate.
Climate researchers are always evaluating new evidence to see if there's any other possible cause for climate change, but so far they have all been ruled out. These are:
It's true that there's a natural CO2 cycle. This involves CO2 going into the atmosphere from a variety of sources, from animals breathing through to volcanoes, then being absorbed by things like trees, plants, rocks and the oceans. This cycle has been delicately balanced for thousands of years, but human influence has offset that balance to send the levels of CO2 soaring.
Erupting volcanoes can have a significant effect on our climate but even they don't put out anywhere near as much CO2 as humans.
There is natural variability in Earth's climate but the current climate change is very unusual as it is not exclusively part of a natural cycle.
Natural factors include aerosols and phenomena such as El Niño and La Niña (which cause warming and cooling of the Pacific Ocean surface). Natural climate variations can lead to periods with little or no warming, both globally and regionally, and other periods with very rapid warming. However, there is an underlying trend of warming that is almost certainly caused by man's activities.