Another Reviewer Weighs in on Hansen Paper

July 27, 2015

antarctica_nasaJames 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 The study, as yet unpublished in a peer reviewed journal, was deliberately released early, so as to become part of the public discussion prior to the important climate talks scheduled in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.”

archerFrom 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.

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16 Responses to “Another Reviewer Weighs in on Hansen Paper”


  1. One thing I have not understood about Hansen’s paper is if there will be ice/snow accumulation on the poles as well from the rapid cooling those areas experience, and if so, wouldn’t that offset sea level rise from melting? The models with images of both north and south poles cooling show below 21th century averages which would imply that ice would build up again in both areas over time?

    In spite of this there would be a massive temperature gradient though which could create some serious storms.

    As Tamino also have noticed in his latest post there has been a clear sign of cooling south of Greenland that seems to indicate something is going on though. Question is how fast the doubling time of glacier melt will turn out to be.

    • petermogensen Says:

      Isn’t the argument that the cooling south of Greenland is related to a slowdown of the AMOC?

      • Gingerbaker Says:

        I think the argument is that both south of Greenland, and north of the Antarctic are areas of ocean which are cooler than surrounding areas, because of melting ice leading to frigid-cold freshwater on the surface.


      • The idea is that there is sufficient melt, both from the Arctic and Greenland that the resulting fresh water, which is less dense than regular sea water leads to ocean stratification, reducing the strength of the Atlantic Meridional Overturning Circulation. Such circulation is responsible for poleward heat transport from the tropics, deep water formation, and ocean ventilation. Reduced heat transport leads to cooling. I believe it has been estimated that 1/3 of the reduction has been due to Arctic sea ice melt, 2/3 due to Greenland run-off.

        For more, please see:

        If our analysis is correct, then this indicates that climate models underestimate the weakening of the Atlantic circulation in response to global warming – probably because the flow in these models is too stable (see Hofmann and Rahmstorf 2009). Although these models predict a significant weakening for the future, they do not suggest this as early as the observations show it (see Fig. 2 of our paper). That the real flow may be more unstable than previously thought would be bad news for the future.

        What’s going on in the North Atlantic?
        Stephan Rahmstorf, 2015-03-23
        http://www.realclimate.org/index.php/archives/2015/03/whats-going-on-in-the-north-atlantic/


        • Thanks for the Real Climate link. Other key quotes:

          “Therefore, in our study we use data on sea surface temperatures in order to infer the strength of the flow: we use the temperature difference between the region most strongly influenced by the AMOC and the rest of the northern hemisphere.”

          “And even more recently, NOAA last week released the stunning temperature analysis for the past winter shown in Fig. 4. That winter was globally the warmest since records began in 1880. But in the subpolar North Atlantic, it was the coldest on record! That suggests the decline of the circulation has progressed even further now than we documented in the paper.”

          “The ice loss amounts to a freshwater volume which should have made an important contribution to the observed decrease in salinity in the northern Atlantic – probably including the “great salinity anomaly” of the 1970s, famous amongst oceanographers.”

  2. ubrew12 Says:

    “The melting [Antarctic] water actually results in an increase in heat uptake by the planet, with the increase going directly into the ocean, exacerbating the feedback. … increase in sea ice causes a warming-induced increase in precipitation… over the ocean rather than to Antarctica, another amplification… This seems like a plausible interplay of mechanisms… it’s observed happening today, and… something like this is required to explain… the past”

    (I realize most people know this, but): Freshwater has a lower density than saltwater, so can float even if its colder than the saltwater below it. It thus insulates the saltwater below, and that contained heat more rapidly melts the Antarctic land mass at its margin. I found this a good article from last Fall that explains this positive feedback that features so heavily in Hansen’s paper and which Archer is describing:
    http://www.climatecentral.org/news/antarctica-ice-melt-sea-level-18155

  3. Mike Roddy Says:

    Thanks for this, Peter. I’ve looked in vain for a response to Hansen’s paper in our
    media, and Archer explains Hansen’s work better than everything else I’ve read.

  4. redskylite Says:

    A very positive review from Prof David Archer, and he notes that it could explain some of the unknowns (such as Heinrich events). Glad to hear it will be a focus of discussion by Chicago’s graduate students. Like Trenberth, Prof Archer also notes that the paper is very long, but I’m sure he has digested the whole report carefully. Certainly this timely paper has advanced the science, and as Prof Archer notes the IPCC prediction for year-2100 sea level rise clearly obsolete.

    Hope they sort out their chairmanship and get revising soon.

    After all the world’s policy makers need expert guidance to address this urgent threat…

  5. dumboldguy Says:

    I was going to make a bit of a joke and suggest that we can solve this problem with a bit of geoengineering—-simply take all that nasty brine from the desalination plants that CA will need to build if the drought continues, load it into tankers, and use it to “salt” the cold surface fresh water in the Arctic and Antarctic so that it becomes more dense and sinks, thereby rejuvenating the AMOC and causing the sinking cold water to cool the warm waters getting under the edge of the Antarctic glaciers that accelerates their flow.

    On second thought and after reading the comments here, especially ubrew’s, this is far too serious a situation to joke about. ubrew said:

    (I realize most people know this, but): Freshwater has a lower density than saltwater, so can float even if its colder than the saltwater below it. It thus insulates the saltwater below, and that contained heat more rapidly melts the Antarctic land mass at its margin. I found this a good article from last Fall that explains this positive feedback that features so heavily in Hansen’s paper and which Archer is describing:

    http://www.climatecentral.org/news/antarctica-ice-melt-sea-level-18155

    I’m not sure that “insulates the saltwater” is quite the proper term, but ubrew is definitely on the right track. The stratification is hugely worrisome.

    To do some math that may clarify the physics and show how big the problem is:

    The temperature of water affects its density very little. It ranges from ~1,0 g/cm3 at just above freezing to 0.96 g/cm3 at boiling.

    The oceans have a density of ~1.027 g/cm3, and heating up the deeper Antarctic waters even to near boiling is not going to decrease the density enough to allow the needed mixing (and certainly not when the surface water freezes every year and thereby desalinates and continues the density stratification).

    To scale up the numbers, 1.027 g/cm3 = 1027 Kg/m3, with 27 Kg being the weight of the dissolved salts. That’s about 60 pounds of salt dissolved in a cube of sea water 1 meter on a side.

    To scale that up further, 1 cubic kilometer of seawater contains 1 billion cubic meters of water and 60 billion pounds of salt. For English unit devotees, there are just over 4 cubic kilometers in 1 cubic mile.

    If my math is correct, it would take an awful lot of salt to get that cold surface freshwater density up to the point that it will sink, and the “salt shaker” idea is therefore not very funny and really just wishful thinking. The largest supertankers can carry about 320+ million liters of desalination brine, and since the brine is only 2 to 3 times as salty as seawater, it would take an awful lot of it to make a difference. (To say nothing of all the other nasty crap in the brine beside the “salt” that would be poisoning sea life).

    Hansen is right to release this study now. It needs to be well hashed out before the December talks.

    The situation in the Arctic could be bad news for folks in northern Europe, and what’s going on at the other pole is bad news for ALL.


  6. […] positive reactions as well. University of Chicago geophysicist David Archer calls the paper a “masterwork” and praises the group for its clever and convincing use of the Eemian period as a model for the […]


  7. […] positive reactions as well. University of Chicago geophysicist David Archer calls the paper a “masterwork” and praises the team for its clever and convincing use of the Eemian period as a model for the […]


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