How do we know what caused climate to change – or even if anything did?

This is a central question with respect to recent temperature trends, but of course it is much more general and applies to a whole range of climate changes over all time scales. Judging from comments we receive here and discussions elsewhere on the web, there is a fair amount of confusion about how this process works and what can (and cannot) be said with confidence. For instance, many people appear to (incorrectly) think that attribution is just based on a naive correlation of the global mean temperature, or that it is impossible to do unless a change is ‘unprecedented’ or that the answers are based on our lack of imagination about other causes.

In fact the process is more sophisticated than these misconceptions imply and I’ll go over the main issues below. But the executive summary is this:

• You can’t do attribution based only on statistics
• Attribution has nothing to do with something being “unprecedented”
• You always need a model of some sort
• The more distinct the fingerprint of a particular cause is, the easier it is to detect

Note that it helps enormously to think about attribution in contexts that don’t have anything to do with anthropogenic causes. For some reason that allows people to think a little bit more clearly about the problem.
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The first published response to Lindzen and Choi (2009) (LC09) has just appeared “in press” (subscription) at GRL. LC09 purported to determine climate sensitivity by examining the response of radiative fluxes at the Top-of-the-Atmosphere (TOA) to ocean temperature changes in the tropics. Their conclusion was that sensitivity was very small, in obvious contradiction to the models.

In their commentary, Trenberth, Fasullo, O’Dell and Wong examine some of the assumptions that were used in LC09’s analysis. In their guest commentary, they go over some of the technical details, and conclude, somewhat forcefully, that the LC09 results were not robust and do not provide any insight into the magnitudes of climate feedbacks.

Coincidentally, there is a related paper (Chung, Yeomans and Soden) also in press (sub. req.) at GRL which also compares the feedbacks in the models to the satellite radiative flux measurements and also comes to the conclusion that the models aren’t doing that badly. They conclude that

In spite of well-known biases of tropospheric temperature and humidity in climate models, comparisons indicate that the intermodel range in the rate of clear-sky radiative damping are small despite large intermodel variability in the mean clear-sky OLR. Moreover, the model-simulated rates of radiative damping are consistent with those obtained from satellite observations and are indicative of a strong positive correlation between temperature and water vapor variations over a broad range of spatiotemporal scales.

It will take a little time to assess the issues that have been raised (and these papers are unlikely to be the last word), but it is worth making a couple of points about the process. First off, LC09 was not a nonsense paper – that is, it didn’t have completely obvious flaws that should have been caught by peer review (unlike say, McLean et al, 2009 or Douglass et al, 2008). Even if it now turns out that the analysis was not robust, it was not that the analysis was not worth trying, and the work being done to re-examine these questions is a useful contributions to the literature – even if the conclusion is that this approach to the analysis is flawed.

More generally, this episode underlines the danger in reading too much into single papers. For papers that appear to go against the mainstream (in either direction), the likelihood is that the conclusions will not stand up for long, but sometimes it takes a while for this to be clear. Research at the cutting edge – where you are pushing the limits of the data or the theory – is like that. If the answers were obvious, we wouldn’t need to do research.

Update: More commentary at DotEarth including a response from Lindzen.

Guest Commentary by John Fasullo, Kevin Trenberth and Chris O’Dell

A recent paper by Lindzen and Choi in GRL (2009) (LC09) purported to demonstrate that climate had a strong negative feedback and that climate models are quite wrong in their relationships between changes in surface temperature and corresponding changes in outgoing radiation escaping to space. This publication has been subject to a considerable amount of hype, for instance apparently “[LC09] has absolutely, convincingly, and irrefutably proven the theory of Anthropogenic Global Warming to be completely false.” and “we now know that the effect of CO2 on temperature is small, we know why it is small, and we know that it is having very little effect on the climate”. Not surprisingly, LC09 has also been highly publicized in various contrarian circles.
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