Who are you?
My name is David Karoly. I'm a professor here in the School of Earth Sciences at the University of Melbourne. I do a lot of work on climate change research and was heavily involved in the fourth assessment report of the Intergovernmental Panel on Climate Change that was released in 2007.
What has been observed in the climate in the 20th century?
There is ample evidence that temperatures have increased substantially globally and regionally in the twentieth century and the magnitude of that – about seven or eight tenths of a degree over the last hundred years – is very large compared with any other century-time scale change.
How do we get evidence for past temperatures?
When we compare that to temperature changes over the last 1,000 years or 2,000 years we can't get direct thermometer evidence. We have to look at other evidence like changes in temperatures that we can infer from tree rings, from ice cores, from coral records or other evidence of temperature changes – even documentary records from people living in different areas around the world.
What do we learn from this evidence?
What we find is that the magnitude of temperature variations in the 20th century – seven or eight tenths of a degree – is more than double the temperature changes in any period over the last 1,000 or 2,000 years and the rate of warming is much greater.
There is evidence of a cool period called the ‘Little Ice Age’ primarily in Europe around the 18th century and also of a mediaeval warm period, but the temperature variations in those periods were much smaller than we've seen in the 20th century – smaller by at least a factor of two.