Hansen, melting ice and linear thinking

OK. I've read Hansen's new paper, which has been submitted to Environmental Research Letters, but not published. It's basically a review of existing, well-established science followed some personal opinion on the responsibility of scientists to express themselves, so I doubt it will be edited much before publication. And published it should be. The basic thrust is

Reticence is fine for [the] IPCC. And individual scientists can choose to stay within a comfort zone, not needing to worry that they say something that proves to be slightly wrong. But perhaps we should also consider our legacy from a broader perspective. Do we not know enough to say more?

Hard to disagree there, as the Intergovernmental Panel on Climate Change has politicians vetting its report. For Hansen, the answer to the question is yes.

...the paleoclimate record contains numerous examples of ice sheets yielding sea level rise of several meters per century, with forcings smaller than that of the BAU (fossil-fuel-based business as usual) scenario. The problem with the paleoclimate ice sheet models is that they do not generally contain the physics of ice streams, effects of surface melt descending through crevasses and lubricating basal flow, or realistic interactions with the ocean.

What he's doing here is responding to the fuss over the failure of the IPCC to include the latest data on how fast the Greenland and West Antarctic ice sheets are melting and slipping into the ocean. When the panel released its policy summary for its Fourth Assessment last month, notably absent was anything relying on science published in the last nine months or so, as this was deemed too soon to allow for adequate review.

This subject is also one of the most contentious to be raised by non-scientists like Al Gore, who uses animations of the effects of a six-meter rise in sea level in his climate change slide show (the one I just presented to 60 high school students). The IPCC, by contrast, says they can only be confidence of a rise of a mere 18-59 cm this century. (This isn't quite what the IPCC says, as Stefan Real Climate details rather well, but it's what everyone's talking about when they say the IPCC lowered its sea level rise estimates.)

Rahmstorf (2007) has noted that if one uses observed sea level rise of the past century to calibrate a linear projection of future sea level, BAU warming will lead to sea level rise of the order of one meter in the present century. This is a useful observation, as it indicates that sea level change would be substantial even without non-linear collapse of an ice sheet. However, this approach cannot be taken as a realistic way of projecting likely sea level rise under BAU forcing. The linear approximation fits the past sea level change well for the past century only because the two terms contributing significantly to sea level rise were (1) thermal expansion of ocean water and (2) melting of alpine glaciers.

Under BAU forcing in the 21st century, sea level rise undoubtedly will be dominated by a third term (3) ice sheet disintegration. This third term was small until the past few years, but it is has at least doubled in the past decade and is now close to 1 mm/year, based on gravity satellite measurements discussed above.

As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005-2015 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields sea level rise of the order of 5 m this century. Of course I can not prove that my choice of a 10 year doubling time for non-linear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise.

An important point is that the non-linear response could easily run out of control, because of positive feedbacks and system inertias. The nonlinearity of the ice sheet problem makes it impossible to accurately predict sea level change on a specific date.

And here comes the money quote:

However, as a physicist, I find it almost inconceivable that BAU climate change would not yield a sea level change measured in meters on the century time scale.

Meters. Plural.

Hansen, who's been trying to get our attention on climate chance since 1981, is probably getting frustrated with playing it safe, which is what most scientists are trained to do. Not that that's a bad thing, most of the time. But what if we don't have time to let science proceed through the early, conservative phases that new ideas usually entail? What Hansen is saying -- and rather eloquently, I would say -- is that the we can't afford such luxury.

Reticence is fine for IPCC. And individual scientists can choose to stay within a comfort zone, not needing to worry that they say something that proves to be slightly wrong. But perhaps we should also consider our legacy from a broader perspective. Do we not know enough to say more? There is enough information now, in my opinion, to make it a near certainty that IPCC BAU climate forcing scenarios would lead to disastrous multi-meter sea level rise on the century time scale.

Linear thinking is so 2005. Time to get with the non-linear program. At least, that's Hansen's take. Is he right when he hints that most climatologists agree with him, but are afraid to say so publicly? Or is he walking out on a fragile limb?