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A climate skeptic's guest post: Why David Evans bet against Brian Schmidt over global warming
...This evidence was good enough: not conclusive, but why wait until we are absolutely certain when we apparently need to act now? So the idea that carbon emissions were causing global warming passed from the scientific community into the political realm, and actions started to happen. Research increased, bureaucracies were formed, international committees met, and eventually the Kyoto protocol was signed in 1997 -- with the aim of curbing carbon emissions.

And the political realm in turn fed money back into the scientific community. By the late 1990's, lots of jobs depended on the idea that carbon emissions caused global warming. Many of them were bureaucratic, but there were a lot of science jobs created too. I was on that gravy train, making a high wage in a science job that would not have existed if we didn't believe carbon emissions caused global warming. And so were lots of people around me; and there were international conferences full of such people. And we had political support, the ear of government, big budgets, and we felt fairly important and useful (well, I did anyway). It was great. We were working to save the planet!

But starting in about 2000, the last three of the four pieces of evidence outlined above fell away or reversed....

...A credible alternative suspect now exists. Clouds both reflect incoming radiation (albedo) and prevent heat from escaping (greenhouse), but with low clouds the albedo effect is stronger than the greenhouse effect. Thus low clouds cause net cooling (high clouds are less common and do the opposite). In October 2006 a team led by Henrik Svensmark showed experimentally that cosmic rays affect cloud formation, and thus that

Stronger sun's magnetic field

=> Less cosmic rays hit Earth

=> Fewer low clouds are formed

=> Earth heats up.

And indeed, the sun's magnetic field has been stronger than usual for the last three decades. So maybe cosmic rays cause global warming. But investigation of this cause is still in its infancy, and it's far too early to judge how much of the global warming is caused by cosmic rays.

So three of the four arguments that convinced me in 1999 that carbon emissions caused global warming are now questionable....

...By 2000 the political system had responded to the strong scientific case that carbon emissions caused global warming by creating thousands of bureaucratic and science jobs aimed at more research and at curbing carbon emissions. This was a good and sensible response by big government to what science was telling them.

But after 2000 the evidence for carbon emissions gradually got weaker -- better temperature data for the last century, more detailed ice core data, then laboratory evidence that cosmic rays precipitate low clouds. Future evidence might strengthen or further weaken the carbon emissions hypothesis. At what stage of the weakening should the science community alert the political system that carbon emissions might not be the main cause of global warming? None of the new evidence actually says that carbon emissions are definitely not the cause of global warming, there are lots of good science jobs potentially at stake, and if the scientific message wavers then it might be difficult to recapture the attention of the political system later on. What has happened is that most research effort since 2000 has assumed that carbon emissions were the cause, and the alternatives get much less research or political attention....