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Is the new IPCC report conservative? Optimistic? Misleading? Inquiring minds…

ipcc.jpgThe fourth assessment report of Intergovernmental Panel on Climate Change is a remarkable document. It contains the consensus views of thousands of climate experts, and is so thoroughly vetted that it represents the true gold standard of our current understanding of global warming.

It’s also an odd sort of document. I can’t think of any other academic discipline that undertakes a similar process of self-analysis. The sausage-making that goes into the report has been well-documented, and has led to a conventional wisdom in certain circles that the IPCC report is a sort of optimistic baseline, the least-bad scenario that could possible come to pass.

Said one senior UK climate expert:

The really chilling thing about the IPCC report is that it is the work of several thousand climate experts who have widely differing views about how greenhouse gases will have their effect. Some think they will have a major impact, others a lesser role. Each paragraph of this report was therefore argued over and scrutinized intensely. Only points that were considered indisputable survived this process. This is a very conservative document — that’s what makes it so scary.

This logic has an intuitive sort of appeal. And yet, it feels off to me, or at least deeply oversimplified. The term “conservative” — elsewhere the report has been labeled “optimistic” — is one that crops up over and over again in discussions of the report’s conclusions.

The term conservative in this context refers to scientists’ natural reticence to draw conclusions that aren’t supported by solid data. But is that the same thing as suggesting the the report consistently underpredicts the effects of global warming? Common sense suggests that there is nothing particularly conservative about being systematically wrong.

Let’s take a look at the headline conclusions from all four reports:

  • 1990: The unequivocal detection of the enhanced greenhouse gas effect from observations is not likely for a decade or more.
  • 1995: The balance of evidence suggestions a discernible human influence on global climate.
  • 2001: There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
  • 2007: Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the increase in anthropogenic greenhouse gas concentrations.

Here the so-called conservatism of the report comes into slightly sharper focus. The conclusions of the report are probabilistic in nature, dealing in likelihoods rather than certainties. Every statement has a confidence interval around it, determined by the quality and amount of supporting data.

This doesn’t strike me as “conservatism” so much as “science.” The term “very likely” in the most recent report means “with a greater than 90% chance of being true.” It doesn’t mean “100% true, but we’re hedging our bets (wink wink).”

However, there seem to be at least a few areas in which the report genuinely does understate the risks of climate change. One of these is sea level. The most recent report actually reduces the projections for sea level changes this century, despite the fact that sea level increases have actually been outpacing the predictions of climate models.

The issue here is not that the authors of the IPCC actually think sea level changes have been overstated in past reports. Rather, the science and data around glacier flow have become murkier due to recent observations that glaciers are melting faster than expected. The report summary acknowledges as much:

Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.

Due to these uncertainties, the ice flow effects have been dropped from the report. Some critics have labeled the change misleading, but this seems off-base to me. If scientists don’t have enough data to model the effects of ice flow, then they don’t have enough data. This isn’t misleading, but it is conservative.

(As an aside, I keep waiting for someone more knowledgeable than myself to chime in on this topic, but so far, no dice.)

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