The US space agency, which uses satellites orbiting the earth to monitor the environment and study climate change, looked at how much the the huge ice sheet was still attached to bed rock underneath.

For the first time, the agency obtained a series of temperatures from the base of the sheet - the second biggest in the world after that in the Antarctic - and found it was up to tens of degrees warmer at the base than the surface.

The Greenland ice sheet is around 1,500 miles north to south and up to 680 miles across.

Scientists found alarmingly high areas of ice at the base of the sheet had melted and come free from the bedrock below.

But they also said it was heat coming out of the bedrock itself which was causing the melting.

This means the ice sheet would melt from below anyway even if global warming was not taking place.