We’ve spilled a lot of pixels around here discussing areas of climate science that are officially — that is, according to the IPCC — unsettled (e.g., hurricanes). In these areas the IPCC has not ratified a conclusion, but many prominent scientists line up on one side or another. The important questions raised by these areas of science are:

  1. Are advocates and other laypeople — non-scientists — doing something epistemologically improper or politically "extreme" when they take sides in these debates?
  2. Should policymakers operate purely on the basis of IPCC conclusions, and thereby implicitly accept the organization’s conservative standards of evidence, or should they take more dire scenarios into account? Put another way: should policymakers weight their decisions toward the high end of risk when responding to global warming?

There’s a great new Associated Press article that addresses one of the most concerning such areas: the question of what will befall the Greenland and Antarctica ice sheets. If a positive feedback loop spins out of control and one or both substantially melts, sea-level rises would be drastically higher than most current high-end estimates, and would be cataclysmic. (This is where Al Gore shows slides of water inundating Manhattan.)

Ice sheet decay could mean 55 inches or more rise in seal level. But this is a contentious and difficult area of climate science, and climate models just aren’t up to the task of predicting such chaotic state-shifts. Thus, the IPCC simply sets the possibility aside and predicts sea-level rises of 5 to 23 inches (at least in an early draft of the new report).

However, many scientists think the evidence is suggestive enough to warrant inclusion in the IPCC report:

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The prediction being considered this week by the IPCC is "obviously not the full story because ice sheet decay is something we cannot model right now, but we know it’s happening," said Stefan Rahmstorf, a climate panel lead author from Germany who made the larger prediction of up to 55 inches (140 centimeters) of sea level rise. "A document like that tends to underestimate the risk," he said.

“This will dominate their discussion because there’s so much contentiousness about it,” said Bob Corell, chairman of the Arctic Climate Impact Assessment, a multinational research effort. “If the IPCC comes out with significantly less than one meter (about 39 inches), there will be people in the science community saying we don’t think that’s a fair reflection of what we know.”

Note that what’s at issue here is precisely the conservatism of the IPCC:

Those scientists who say sea level will rise even more are battling a consensus-building structure that routinely issues scientifically cautious global warming reports, scientists say. The IPCC reports have to be unanimous, approved by 154 governments — including the United States and oil-rich countries such as Saudi Arabia — and already published peer-reviewed research done before mid-2006.

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It’s difficult to get a sense from the article, and from most stories on this subject, just what the balance of scientific opinion is — just how many scientists think the IPCC is underestimating this risk. Perhaps one of our climate scientist friends could weigh in.

But the real question for us concerned citizens is: what should policymakers do in this situation? We’re often cautioned, wisely, that science does not dictate policy. It’s a value decision just how much risk we’re willing to bear. This area of science points up just how difficult and fraught those value judgments are.

If one of these massive ice sheets melts, it will be a civilization-altering event. There will at minimum be catastrophic financial loss, death, and a refugee problem with no precedent in history. The risk can hardly be overstated.

But we don’t know — and probably won’t, within the time frame in which it will matter — just how probable such an event is. Some smart scientists say it’s a real and looming possibility. Some say the chances are virtually nil.

Should the gravity of the risk drive policy? If so, we need a massive crash program to reduce global GHG emissions in the near-term. Or should the uncertainty of the risk drive policy? If so, perhaps we’re better off slowly phasing in emission reductions and adaptive measures, and waiting for technology to develop.

What do you think?