The Earth has natural, long cycles of climate lasting hundreds of thousands of years.
But it also has short and dramatic shifts where the climate can change significantly within a decade.
We know this, because the glaciers here in Greenland give us an accurate weather report stretching back to the stone age.
Ice is drilled in long cylindrical cores, which reveal distinct layers representing different periods in time. The further down you drill, the older the ice is.
Prof Richard Alley, Penn State University – "This ice core is beautifully layered, and we can ask of it – what happened? What was coming through the atmosphere at that time? Is there ash and acid from a big volcano? Is there lead from Roman lead refining or what have you? So there's this history of what was blowing through the air and piling up on top of the Greenland ice sheets, sitting here in these beautiful layers."
Scientists measure historical temperature by looking at the chemical make up of the water inside the each ice layer.
Hydrogen has three different states.
One of these states needs more energy to evaporate it from the water.
If more of this type of hydrogen is found in an ice layer, we know it was warmer at that time.
Using the information, the ice cores reveal, we can build a picture of a climate over Earth's recent history.
This graph shows average temperature of the Greenland ice sheet over the past 50,000 years.
For 40,000 of those, we were in an ice age and temperatures only varied from -33 to -39°C.
Then over just a few thousand years, temperature rose to give us a much warmer climate.
But if we look at the climate on much smaller time scales, the ice cores throw up a big surprise.
In just a decade, the temperature could change by several degrees.
The Greenland ice sheet gives us accurate information about climate changes in the past.
But what is it that caused these changes in the first place?