The strongest reasons for doubting the man-made global warming hypothesis come from pre-industrial climate change. During the past 10,000 years, since soon after the end of the last ice age, there have been bigger ups and downs in global temperatures than during the 20th Century. Yet according to the IPCC, citing measurements of gas trapped in polar ice, the concentration of CO2 varied by only about 20 ppm throughout those 10,000 years – less than a quarter of the change during the 20th Century.When I look at the graph, it is pretty clear to me that the recent rise in temperatures is not unusual. The idea that this can only be explained by the rise of CO2 in modern times just doesn't pass the smell test.
So whatever caused the pre-industrial climate changes, it was not CO2. The record is well accounted for, in fact, by (1) a change in the Earth’s attitude in orbit, which brought a gradual cooling to the Northern Hemisphere, and (2) solar variations that superimposed on that trend a succession of warmings and coolings. Yet to offer such natural explanations for past events does not logically falsify the man-made global warming hypothesis, because its supporters say it represents an important new factor in climate change.
If increasing CO2 had no effect whatever on the climate, that might be surprising, but the claim of the hypothesis and its computer models is that the new contribution from man-made greenhouse gases has become the main driver of recent climate change. Implied here is a falsifiable statement, namely that there is something very unusual about the recent warming of the world. That is certainly the sense of many scientific and political pronouncements about global warming.
Here are four ways in which the recent warming might be judged unusual:
Data on climate changes of the past 10,000 years are available from various parts of the world, but to make the test as favourable as possible for the man-made warming hypothesis, let’s go to the Arctic. That is where the computer models predict the strongest and most rapid warming, and where the media have publicized repeated claims that Greenland is melting away and the polar bears are drowning.
- by reaching unprecedented temperatures
- by an especially large recent rise
- by an especially rapid recent rise
- by an accelerating rate of rise
Below is a graph using a particularly careful reconstruction of Greenland temperatures during the past 10,000 years, by Bo Vinther of the Niels Bohr Institute in Copenhagen and his colleagues (2009). Ice drilled at the sites called Agassiz and Renland, 1500 km apart on either side of the Greenland ice sheet, provided the data. Anyone who suspects I may misrepresent the results is welcome to compare my graph with Fig. 2c in the 2009 Vinther paper.
Go read the original post. He has a lot more details.