Another Reviewer Weighs in on Hansen Paper July 27, 2015

James Hansen’s release last week of a landmark study on climate change and sea level rise continues to reverberate in the science and journalism community. The study, as yet unpublished in a peer reviewed journal, was deliberately released early, so as to become part of the public discussion prior to the important climate talks scheduled in Paris for the end of the year.

This approach has been criticized, and indeed, for a scientist of lower stature than Dr. Hansen, and his stellar group of co-authors, it might have been a major mistep. But the credibility of this team of authors is so high, that the paper will continue to command attention for some time, and may in fact go down as a landmark.

There’s already been some pushback on the paper’s main points, raised by highly respected scientists, (see Kevin Trenberth’s comments posted here last week) – but now, some of the formal editorial review responses are coming thru as well. The respected Oceanographer David Archer has released one such response, and it is much more supportive of the paper’s conclusions than Trenberth.

We’ve come to a watershed moment in the climate discussion, where the dialogue has turned from endlessly squashing denialist talking points, to a public debate more in line with where the science actually is.

In this case, the scientists are arguing, not, will sea level rise, but will it rise 3 feet by the end of the century? or 10 feet?

Washington Post:

Granted, the new Hansen study is simultaneously advancing a gigantic new synthesis of existing research and also pushing the envelope — it will need to be scientifically digested for some time, and has already drawn some critical comments from experts. However, the Hansen paper also just received its first official peer review by one of several reviewers designated by the journal, Atmospheric Chemistry and Physics Discussions – the University of Chicago geoscientist David Archer. And it is a strong review – Archer says that the paper is a “masterwork of scholarly synthesis, modeling virtuosity, and insight, with profound implications.”

From the review by David Archer:

This is another Hansen masterwork of scholarly synthesis, modeling virtuosity, and insight, with profound implications. The main thrust of the paper, the part getting all the press, arises from the confluence of several recent developments in glaciology. First is the identification of a runaway condition in outflow glaciers of the West Antarctic ice sheet that makes the IPCC prediction for year-2100 sea level rise clearly obsolete. The other is the recognition that warming ocean temperatures at the grounding line for the glaciers is driving a really strong flow and thus melting response. Temperatures at this depth tend to have a paradoxical inverse relationship with surface temperatures, which can cool due to fresh meltwater input, trapping heat in the subsurface. This idea may also explain the mystery of why Heinrich events, collapses of the Laurentide ice sheet,always came at cold times in the D-O cycles. Analysis of sea level changes during Eemian time, the last interglacial, show changes of several meters in time scales of a century. If our ice sheets are going to change our sea level that much, from its current rate of melt, the melt rate would have to increase exponentially in the future. The way that could happen is if there is a positive feedback, such that melting begets faster melting, as opposed to a linear response where the melting rate is driven simply by temperature. The climate modeling results in this paper identify such a feedback. Release of freshwater around the margins of the ice sheets causes freshening at the ocean surface, stratification, and warming of subsurface waters. The melting water has a significant cooling impact on the planet, which I hadn’t expected, but I guess the difference here is the huge rate of freshwater addition; the authors argue that the responsiveness of the model is not much different from other climate models. The melting water actually results in an increase in heat uptake by the planet, with the increase going directly into the ocean, exacerbating the feedback. Antarctic cooling and increase in sea ice causes a warming-induced increase in precipitation in the Antarctic region to fall over the ocean rather than to Antarctica, another amplification of the freshwater forcing mechanism. This seems like a plausible interplay of mechanisms to me, given that it’s observed happening today, and that something like this is required to explain evidence from the past such as Heinrich events. The conclusions of this paper confirm what I had gloomily expected people would figure out, and they provide a mechanism by which the implications of the past can be explained and cast into a forecast for the future.