If you are interested in sea level rise news, you have a busy week. First we learn from a Nature Climate Change publication that the Greenland ice sheet is already gone. Then earlier today two studies published in Environmental Research Letters conclude that in 40 years time due to the current sea level rise 4 million Americans will be at risk of storm surge floods. And now there is a study that does not look into the future, but into the past.

In a complex physical world even sea level rise is not a straightforward phenomenon, a new publication in Nature reminds us. Then again, a lot of water is a lot of water.

In a geological sense we don’t have to go far back in time – we stay within the late Pleistocene.

The Pleistocene is the age of ice ages. Typically these are separated by relatively short intervals in which [due to Milankovitch cycles] the climate is milder. We live in one such interglacial ourselves, the Holocene.

The study of preceding interglacials is of particular interest to get a sense of ice sheet sensitivity to climate warming. On the Pleistocene timescale of 2.5 million years the world underwent some changes, but not so many in geological terms. The continents were pretty much where they are today – and so were the oceans basins.

That means the amount of water in these basins versus the amount locked up in ice sheets on land, is largely correlated by temperature. When it’s cold during the ice ages, the ice sheets grow and the sea level drops by about 100 meters [enough to form land bridges between Alaska and Siberia and the Indonesian Isles]. When the Earth’s climate switches to mild mode, the sea comes up again. But the one ‘mild’ is a bit different than the other mild.

Interglacial ice sheet climate sensitivity comparisons

During the last interglacial for instance, the Eemian, temperatures were about 0.7 degrees higher than they are today – and the average sea levels could have been 8-8.5 meters higher than they are today. The Eemian comparison is an important reason why for instance the Head of NASA GISS, climatologist James Hansen, states setting climate goals as high as 2 degrees is a recipe for disaster.

We were a bit sceptical about that, because due to slight Milankovitch differences between the Eemian and the Holocene it could have been the case that Greenland and Antarctica suffered from stronger summer insolation at that time, which creates a melt that winter freezes cannot compensate. However with that new Greenland study in Sunday’s Nature Climate Change we fear reality could be working out in Hansen’s unwanted direction. [Oops, we indeed forgot about that climate feedback, didn’t we?]

Although none of us can of course actually risk that the analogy is right, perhaps for now we can just say at least the Eemian interglacial offers one stark warning about ice sheet climate sensitivity – and leave it there.

Interglacials out of reach of old world’s nomenclature

Wouldn’t it be a clever thing to do to try and dig a paleoclimatic layer deeper? That’s what a duo of researchers of Columbia University and Harvard decided to do. They focussed on an interglacial with a particularly charming name, Marine Isotopic Stage 11, which reached its climatic optimum some 400,000 years ago.

Previous research had suggested that during MIS 11 the world’s oceans could have been 20 meters higher than they are today.

Well, that is probably not the case. The two researchers have better examined the marine sediment layers that suggested that very drastic sea level rise, at sites at the island of Bermuda and the Bahamas – where indeed marine sediments dating back to the interglacial can be found 20 meters above the current sea level, in a tectonically stable environment.

But paleoclimatologists sometimes know a thing or two about tectonics that regular geologists may initially overlook. Uplift is not always caused by plate tectonics – and it doesn’t even require the presence of large fault lines nearby. Isostatic rebound is another phenomenon, and between ice ages and interglacials that is actually an important factor.

The isostatic ice sheet seesaw

During glacials, regions that are covered under kilometres-thick ice sheets subside under all that weight. Meanwhile, like a seesaw, areas that lie a good deal beyond the glacier front, rise.

During interglacials, when the ice melts and all that weight disappears, the opposite happens. Areas that were covered by ice sheets rebound, and the peripheral zone sinks.

The researchers think that happened a bit stronger during MIS 11, the write in their publication in Nature. During the mild period 400.000 years ago the Bahamas must have dipped a little deeper than they do now.

The actual climate sensitivity lessons

Still, the researchers conclude, the average sea levels on Earth must have been 6-13 meters higher than they are today.

“This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11,” the researchers conclude.

We think that is a very interesting conclusion. Not that the East Antarctic Ice Sheet is stable, but that the West Antarctic and Greenland are not.

We are however left with a bit of uncertainty here. In the 90s paleoclimatologists thought that MIS 11 must have been quite a bit warmer than the Holocene. Now however some think the climate was very similar to our preindustrial climate (and so were CO2 levels) – and some even think MIS 11 could have been cooler.

It really makes you wonder why, 10,000 years into the Holocene, we still have that large ice sheet on Greenland. More and more studies seem to suggest it is actually an ice age relict – and perhaps indeed one that has managed all these years to keep itself in place and intact because of its albedo and its high ice surface height – factors, like atmospheric temperature, that are clearly no constants…

© Rolf Schuttenhelm | www.bitsofscience.org