Is it just a natural cycle?
Some sceptics have suggested that the recent warming is just part of a natural cycle. But previous changes in the earth's climate - such as ice ages - have been driven by changing output from the sun, or changes in the planet's orbit. None of these are sufficient to explain current warming, whilst greenhouse gases provide a clear match. But uncertainties do remain. No-one knows for sure exactly how sensitive the earth's climate system is to an increase in greenhouse gas levels.
Most computer models suggest that a doubling of CO2 (from a pre-industrial level of around 270 parts per million (ppm) to 550 ppm) would lead to a temperature rise of about 3°C. So why is the IPCC's figure – of 1.4°C to 5.8°C by 2100 – so broad?
The reason is that the biggest uncertainty of all is future human use of fossil fuels. If we carry on using these fuels as we currently are, it would lead to atmospheric carbon dioxide concentrations nearing 1000 ppm by the end of this century, pushing the planet closer to the latter figure of extreme warming.
But if we chose alternatives – such as renewables to oil, coal and gas, it would keep global warming at a much lower level.