Ice sheets, they could melt faster

July 23, 2011

And our reporters and bloggers, they could do a little more reading and explaining.

Here’s the boingboing article, which references this NASA science brief, which in turn references these two(PDF, melt rate) articles(PDF, paleoclimate), which contain the real meat, which of course is not discussed in the “popular” articles (or even really in the NASA Science Brief).

The fast explanation is that paleoclimate shows, in times with climates similar to our new and predicted one, ice sheets melting significantly (meters of sea level rise) in as little as a century’s time. That doesn’t match our currently observed melt rates, Greenland and Antarctic icecap melt is projected to only contribute 10 centimeters by 2100 (from 2011).

How sure are we of a constant-rate ice melting? With what little data we have, if you choose to fit a quadratic melt (melt rate increases linearly with time) we already get a better fit than if linear. (This is a conservative, simple choice for increase melt rate. Why not cubic, or quartic, or exponential? Quadratic will give the least-alarming answers of the bunch.) A quadratic fit predicts between 58 and 98 centimeters of melt, with an expected value of 78cm. That’s a lot more than 10.

But James Hansen is a bit more alarmed and a bit less conservative, because of the paleoclimate data. Even the high-end quadratic estimate is not quite a meter, even starting the century at 2000. He sees measurements implying multiple meters, and either we need a corresponding multiple of the quadratic acceleration term (because perhaps our temperature estimations are buggy) or we need a melt rate that grows more quickly than just linear in time (perhaps meltwater makes glaciers move faster, smaller sea ice caps reduce albedo, allow more circulation of sea water to the glacier face, who knows?)

So at this stage, we don’t have a physical model that would justify a particular melt rate that is faster, but we do have measurements that suggest it could be quadratic, and we have paleoclimate that suggests it could be faster than that. Big fun.

Even a meter starts to cause a few problems. If nothing else, it backs up the flood plain by 3 feet for serious storms. At 3 meters, part of Cambridge’s fresh water supply (Fresh Pond) goes below sea level. Where I grew up, 3 meters cuts all but one road out of the county, for a county of about 900,000 people.

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