Just because something happened before without humans doesn't mean that humans are not causing it today. For instance, the dinosaurs became extinct before there were people, but animals are dying out today because of hunting or loss of habitat from human activity.

Sure, over time our climate has varied. This has been for different reasons. The changes we are seeing at the moment are different.

We know about changes earlier in earth's history by looking at ice. In really cold places like Antarctica this ice hasn't melted for thousands of years, so when scientists drill down they can take something called an ice core. This is a block of ice that can be studied. Bubbles in the ice contain samples of air from thousands of years ago, which tell us about the climate in which the ice 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. These cycles are regular and predictable. If you want to be really cool you can give them their scientific name: Milankovitch cycles. 

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 core samples.

Our atmosphere is warmed by the presence of greenhouse gases, which can be created when coal, oil and gas are burned to make energy in homes, power stations and cars.  We know how much gas we have produced over the last couple of hundred years and we know that this is having an effect on our long-term weather, over and above the natural changes that we expect to see.

Not convinced ? This video might help.

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