Why I Bet against Global Warming

Dr David Evans

I devoted six years to carbon accounting, building models for the Australian government to estimate carbon emissions from land use change and forestry (Google on "FullCAM"). When I started that job, in 1999, the evidence that carbon emissions caused global warming seemed pretty conclusive, but since then new evidence has weakened the case that carbon emissions are the main cause. I am now skeptical. As Lord Keynes famously said, "When the facts change, I change my mind. What do you do, sir?"

In the late 1990s the evidence suggesting that carbon emissions caused global warming was basically:

1. Carbon dioxide is a greenhouse gas. Proved in a laboratory a century ago.

2. Global warming has been occurring for a century, especially since 1975, and concentrations of atmospheric carbon have been rising for a century, especially since 1975. Correlation is not causation, but in a rough sense it looked like a fit.

3. Ice core data, starting with the first cores from Vostok in 1985, allowed us to measure temperature and atmospheric carbon going back hundreds of thousands of years, through several dramatic global warming and cooling events. To the temporal resolution then available (data points were generally more than a thousand years apart), atmospheric carbon and temperature moved in lock-step: there was an extremely high correlation, they rose and fell together. Talk about a smoking gun!

4. There weren't any other credible suspects for causing global warming. So presumably it had to be carbon emissions.

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 1990s, 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. Using the same point numbers as above:

2. Closer examination of the last century using better data shows that from 1940 to 1975 the earth cooled at about 0.1ºC/decade while atmospheric carbon increased. But any warming effect of atmospheric carbon is immediate. By 2003 or so, we had discovered global dimming, which might be adequate to explain this 35-year non-correlation. But what had seemed like a good fit between recent atmospheric carbon and global warming now looks shaky, in need of the recently-discovered unquantified global dimming factor to explain 35 years of substantial cooling. I reckon the last century of correlation evidence now neither supports carbon emissions as the cause nor eliminates it. Further quantitative research on global dimming might rescue this bit of evidence, or it might weaken it further.

3. As more ice core data was collected, the temporal resolution was improved. By 2004 or so, we knew from the ice core data that in the warming events of the last million years, the temperature increases generally started about 800 years before the rises in atmospheric carbon started. Causality does not run in the direction I had assumed in 1999---it runs the opposite way. Presumably temperature rises cause a delayed rise in atmospheric carbon because it takes several hundred years to warm the oceans enough for the oceans to give off more of their carbon.

It is possible that rising atmospheric carbon in these past warmings then went on to cause more warming ("amplification" of the initial warming), but the ice core data does not prove that. It could just be that the temperature rose for some other reason, that this caused the oceans to raise the atmospheric carbon levels, and that the increased atmospheric carbon had an insignificant effect on the temperature.

The pre-2000 ice core data was the central evidence for believing that atmospheric carbon caused temperature increases. The new ice core data show that past warmings were not initially caused by rises in atmospheric carbon, and say nothing about the strength of any amplification. This piece of evidence casts reasonable doubt that atmospheric carbon had any role in past warmings, while still allowing the possibility that it had a supporting role.

4. 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
=> Fewer 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.

The case for carbon emissions as the cause of global warming now just boils down to the fact that we know that it works in the laboratory, and that there is no strong evidence that global warming is definitely not caused by carbon emissions. Much the same can be said of cosmic rays---we have laboratory evidence that it works, and no definitely contradictory evidence.

So why did I bet against global warming continuing at the current rate? Let's return to the interaction between science and politics.

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.

(By the way, I quit my job in carbon accounting in 2005 for personal reasons. It had nothing to do with my weakening belief that carbon emissions caused global warming. I felt that the main value of our plant models was in land management and plant simulation, and that carbon accounting was just a by-product.)

Unfortunately, politics and science have become even more entangled. The science of global warming has become a partisan political issue, so positions become more entrenched. Politicians and the public prefer simple and less-nuanced messages. At the moment, the political climate strongly supports carbon emissions as the cause of global warming, to the point of sometimes rubbishing or silencing critics.

The integrity of the scientific community will win out in the end, following the evidence wherever it leads. But in the meantime, the effects of the political climate is that most people are overestimating the evidence in favour of carbon emissions as the cause of global warming. Which makes it a good time to bet the other way.

I would like to bet against carbon emissions being the main cause of the current global warming. But I can't bet on that directly, because all betting requires an unambiguous and measurable criterion. About the only related measure we can bet on is global temperature. So I accepted Brian's[1] bets about trends in global temperatures over the next 10 to 20 years. Basically, if the current warming trend continues or accelerates, then Brian will win; if the rate of warming slows, then I will win. Even if carbon emissions are not the main cause of this global warming, I can still lose:

• Global warming might be due to a side-effect of industrialization other than carbon emissions. Possible causes include atmospheric reactions of industrial chemicals that hinder the rate of low cloud formation.
• Global warming might be primarily due to a non-human cause, such as something related to the sun or to underground nuclear reactions. If this cause persists over the next 20 years, as it has for the last 30 years, then I will lose, but if it fades in the next decade, then I win.

I emphasize that we are making a bet involving odds and judgment. The evidence is not currently conclusive either for or against any particular cause of global warming. I think that it is possible that carbon emissions are the dominant cause of global warming, but in light of the weakening evidence I judge that probability to be about 20% rather than the almost 90% as estimated by the IPCC.

I worry that politics could seriously distort the science. Suppose that carbon taxes are widely enacted, but that the rate of global warming increase starts to decline by 2015. The political system might be under pressure to repay the taxes, so it might in turn put a lot of pressure on scientists to provide justifications for the taxes. Or the political system might reject the taxes and blame science for misinforming it, which could be a terrible outcome for science because the political system is powerful and not constrained by truth.

Some people take strong rhetorical positions on global warming. But the cause of global warming is not just another political issue that is subject to endless debate and distortions. The cause of global warming is an issue that falls into the realm of science, because it is falsifiable. No amount of human posturing will affect what the cause is. The cause just physically is there, and after sufficient research and time we will know what it is. Looking back in another 40 years, we will almost certainly know the answer, and Brian and I will be in agreement on the issue.

Given that betting is thus possible on this issue, it seems strange that some people who take strong positions and profit by those positions are not prepared to bet even a small amount of their own money. Betting something of one's own money adds, shall we say, credibility. And people whose own money is at stake try a little harder---a well known advantage of private business over public. A good side-effect of widespread betting would be a market in betting that would represent a community-wide best guess. Such markets exists in sports betting, and are the best predictors of game outcomes.

Let's hope for the planet's sake that I win the bets. Meanwhile let's do more research, and take cheap measures to curb carbon emissions!

[1] The reference is to Brian Schmidt, with whom I made the bet. Details of the bet itself can be found on Brian's blog, Backseat driving, at
http://backseatdriving.blogspot.com/2007/04/new-global-warming-bet-for-7-10.html