Thursday, May 3, 2007

Reply to James Annan

James Annan posted this. Here is my reply:

C02 has almost no effect? Implied belief that C02 has a negligible effect? Hardly. Straw man, James.

My guest post on Backseat Driving explains why I bet:
  • The evidence in favor of carbon emissions as the main cause of global warming has weakened substantially, especially now that higher-resolution ice core data shows that past temperatures rises occurred before atmospheric carbon increases. This weakening is not widely recognized.
  • The political climate is running strongly in favor of carbon emissions as the main cause.
So now is a good time to bet against carbon emissions, at the best odds available.

Here is part of what I said: "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 almost 90% as estimated by the IPCC."

From which, given that global temperatures are currently increasing at about 0.2C/decade, one would presumably conclude that I believe that atmospheric carbon is responsible for at least a 20% * 0.2C/decade = 0.04C/decade temperature rise, at least in a probabilistic sense. Hardly negligible. In any case, what I believe (this year!) is: atmospheric carbon has a non-negligible effect, but is probably not the main cause of global warming.

James if I read your post correctly, we would probably agree that:
  • There is a warming effect due to the extra carbon we humans have put into the atmosphere.
  • There are other causes for the global temperature to change, including human aerosols and maybe cosmic rays.
  • The current warming, about 0.2C/decade, is due to the extra carbon and the other effects.
  • If warming due to extra carbon exceeds 0.15C/decade, carbon is the main cause of global warming and I will lose my bets.
  • If warming due to extra carbon is less than 0.05/C decade, carbon is not the main cause of global warming and I will win my bets (provided the other causes reverse themselves in the next decade).

And where we disagree is in what the contribution of the extra carbon is:
  • I think it likely that the current global warming is mainly due some other cause, which may subside in time, but that the warming due to the extra carbon will continue (since we keep increasing the extra carbon). The increase in extra carbon would put an upward bias on the natural temperature variability, an upward-sloping channel on a graph of global temperature. We might want to curb carbon emissions some day.
  • You presumably think it likely that current global warming is mainly due to the increasing extra carbon, and that global warming will therefore continue at much the same rate, with some relatively minor fluctuations due to natural variability, until we curb carbon emissions substantially.

So let's focus on quantifying the warming effect of the extra carbon. I agree with you, this is crucial.

You quote a ballpark warming figure due to a doubling of atmospheric carbon, ignoring feedback and clouds but including extra water vapor, of 2C. Can you explain this figure, at a level of physics in say Resnick and Halliday or Feynman's Lectures, so that it is convincing to an interested person with a modest scientific background? Can you then add in all the complications of a real atmosphere and feedbacks? What are the lowest and highest values of warming due to extra carbon that come out of that process? I'd be delighted if someone did that. If the minimum possible warming was not small, the assumptions were watertight, and the data inputs all incontrovertible, then you would persuade a lot of skeptics like me over to your side.

Alas, the only figures offered to me are from opaque models. I find that unsatisfactory of course, because I do not know the assumptions built into those models, the input data, or how the calculations were made. How confident can we be that nothing was omitted (after all, we only discovered global dimming a few years ago)? And the intrinsic difficulty of convincing me by calculation and model is massive because they are so complex -- I haven't got the time to look over every detail and check every assumption.

The warming effect of the extra carbon, to date, is presented as an argument by authority. So naturally we wonder about the political situation of those authorities. All a bit unsatisfactory and unconvincing.

What we need is to measure the warming due to extra carbon, to tie it in with some observations. How are we going? Presumably the parts of the atmosphere with the extra carbon should be showing a warming trend. There has been a big effort trying to measure these changes, with balloons since 1958 and with satellites since 1979.

As we all know, science works by falsifying hypotheses by experiment or observation. (Warning: The following point is relevant for the process of science, as I will note below. It is not offered as an argument against carbon-causes-warming.) As pointed out in the first substantial point in The Great Global Warming Swindle, the observations collected on atmospheric warming contradicted the greenhouse hypothesis. As noted here however, due to better data and corrections to old data, by 2006 those observations no longer rule out the possibility of greenhouse warming in the atmosphere (except that in the tropics the observed results still contradict current climate models in some important respects). That is, until 2006 the results of direct measurements on warming in the atmosphere were falsifying the hypothesis that the extra carbon was causing warming! And now the data does not contradict the hypothesis. But (to get back to the main point), the data doesn't really allow us to directly measure the amount of warming due to the extra carbon.

So, in the absence of good observational data, we simply don't know how much the extra carbon is warming the world.

Now let's turn to cosmic rays. Svensmark's experiments in 2006 put the cosmic ray theory on roughly the same basis as the atmospheric carbon theory: we know they work in a laboratory, but the jump to quantifying that in our atmosphere is a bit hard. (Well ok, we will know a lot more about how cosmic rays perform in test situations after the CERN experiments, scheduled for 2010.)

Research into the cosmic-ray possibility should be given a fair go. Will the proponents of carbon as the main cause resist looking at alternative causes? Will they give other possibilities a thorough examination? Bear in mind that until last year the atmospheric observations noted above appeared to falsify atmospheric carbon as the cause, yet we persisted with blaming global warming on carbon emissions. So even if we find some data that falsifies cosmic rays, we should continue researching them in a well-funded and enthusiastic fashion for a good few years -- if only to be fair.

Well James, in the absence of good observational data about how much warming is caused by the extra carbon, we can each demand that the burden of proof is on the other -- either because of the precautionary principle, or because of the competing principle of not meddling unless you're sure of the cause.

But, having seen the way we got here and seeing the political climate, I bet against warming intensifying. And, in the groundbreaking first major bet on global warming, you bet the other way. Bring on more evidence!

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I recently bet Brian Schmidt of Backseat Driving $US6,000 that the rate of global warming would slow down over the next 10 - 20 years.

I put a guest post on his blog, explaining why I bet that way. However the ensuing discussion is overflowing from the Comments section of that post.

So I started this blog.