It's true that the Earth's climate has altered over time, but the changes we are seeing now are different to the natural variations that have happened in the past.

We know about the earlier changes because of ice.  In places like Antarctica the ice hasn't thawed for centuries, so when scientists drill down they can take out ice that is thousands of years old.  Bubbles in the ice contain samples of prehistoric air which tell us about the climate when the ice was formed. 

 For example, if we look at ice cores from deep below the surface of Antarctica and Greenland, we see that the world has gone in and out of very cold periods pretty regularly about once every 120,000 years. We know that this is because of changes in the Earth's orbit.  These cycles are regular and predictable. They're known as the Milankovitch cycles after the scientist who discovered them.  Natural warming and cooling has also been noted on other planets, without any help from man - this is oftem used by climate change sceptics as "proof" that human impact on climate change is not of importance.

The difference between these natural cycles and what's happening now is the speed of the changes.  At the moment the changes are about 10 times faster than any changes recorded in ice cores.

We know what effect greenhouse gases have in our atmosphere.  We know how much gas we have produced since the start of the Industrial Revolution.  We know that this is having an effect on our long-term weather.

To explain climate change, a computer model was used to simulate global climate from 1860 to 2000 taking into account natural factors, human factors and then both sets of factors combined. Only when both natural and human factors were included could the model accurately reflect the course of global-average temperature over the entire 140 year period, and especially the warming since the 1970s.

This is one of several important pieces of evidence that suggest increasing human influence on global climate.

The Intergovernmental Panel on Climate Change (IPCC) concluded that, " .. most of the warming observed over the last 50 years is likely to have been due to increasing concentrations of greenhouse gases."

Myth three - one record summer doesn't mean we're warming up