Mistakes Revealed in IPCC Sea-Level Measurements

With everyone and his mother combing through the last report of the Intergovernmental Panel on Climate Change for more errors like the prediction that Himalayan glaciers could melt away by 2035, it was inevitable that additional mistakes would be uncovered. After all, the report does run 2,927 pages. But given how crucial forecasts of sea-level rise are to our understanding of the impact of global warming (rising seas will not only swamp low-lying coasts, but also worsen flooding), it is doubly unfortunate that the IPCC erred on the sea-level forecasts in its last (2007) report.

By the end of this century, for instance, one IPCC scenario (a "scenario" reflects a set of assumptions about how much the concentration of greenhouse gases in the atmosphere will rise) foresees a sea-level rise of just over nine feet. But look at how that number was derived.

First, although the scenario in question (a worst-case one) assumes that global mean temperatures will have risen 6.4 degrees Celsius (11.5 degrees Fahrenheit) by then, in the calculations of sea-level rise the IPCC authors inexplicably used a temperature rise of 7.6 degrees. Why the exaggeration of more than a full degree of warming? Second, although the forecast is supposed to be for 2100, the IPCC authors ran the calculation through 2105, giving sea-level rise an extra five years. Why did the IPCC tack on another five years, if not to make sea-level rise look that much worse?

Before this turns into my own little Wellesian War of the Worlds, I have to stop this parody, for which I am indebted to physicist and oceanographer Stefan Rahmstorf of Germany's Potsdam Institute for Climate Impact Research. In a post on the RealClimate blog, he makes a crucial point about the ongoing climate-change backlash: by putting all the focus on how mainstream climate science may have exaggerated the threat of global warming, we're in real danger of ignoring how the threat has been lowballed.

So, just to set the record straight on sea-level rise. The IPCC's worst-case scenario forecasts a sea-level rise of 26 to 59 centimeters (10 to 23 inches) by the end of this century. But that is based on a temperature rise of 5.2 degrees Celsius—whereas the IPCC itself said that temperatures might rise 6.4 degrees Celsius. By lowballing the possible temperature increase, the IPCC reduced the estimate of sea-level rise by six inches, Rahmstorf calculates.

Second, the IPCC chose a date of 2095, not 2100. Picking a date five years sooner reduced the projected sea rise by another two inches.

Worst of all, over the last 40 years seas have risen50 percent more than predicted by the models the IPCC uses. Yet the IPCC did nothing to correct for the gap between model and reality. Can you imagine the outcry if, instead, over the last 40 years seas had risen only one half of what models forecast, but the IPCC had stubbornly stuck to the models that overstate the seas' rise by a factor of two? By sticking with models that have underestimated actual sea-level rise so far, there is a real possibility that the IPCC forecasts underestimate how much more the waters will rise in this century.

Some IPCC scientists warned years ago that picking dates, temperatures, and other parameters guaranteed to produce low-end estimates of sea-level rise "could lead to a credibility problem," writes Rahmstorf, "but the IPCC decided to go ahead anyway." Why? Because in the IPCC culture, "being 'alarmist' is bad and being 'conservative' (i.e. underestimating the potential severity of things) is good," he charges.

Let me quote Rahmstorf a little more: lowballing the estimate of sea-level rise is "the opposite of 'erring on the safe side' (assuming it is better to have overestimated the problem and made the transition to a low-carbon society a little earlier than needed, rather than to have underestimated it and sunk coastal cities and entire island nations). Just to avoid any misunderstandings here: I am squarely against exaggerating climate change…I am deeply convinced that scientists must avoid erring on any side, they must always give the most balanced assessment they are capable of…Why do I find this IPCC problem far worse than the Himalaya error? Because it is not a slipup by a Working Group 2 [the scientists who assess the impact of global warming] author who failed to properly follow procedures and cited an unreliable source. Rather, this is the result of intensive deliberations by Working Group 1 [which assesses the physics of climate change] climate experts. Unlike the Himalaya mistake, this is one of the central predictions of IPCC, prominently discussed in the Summary for Policy Makers."

The full analysis of how the IPCC has lowballed its estimate of sea-level rise, from when the IPCC report was released in 2007, is here. Now try looking for outrage over that on the blogosphere. Himalayan glacier mistake? Everywhere. Sea-level rise error? Not so much.