07 April 2010

Emerging Consensus on Sea Level Rise?

Last January, the Sunday Times published an article detailing an apparent split among researchers about projections of future sea level rise, focusing on a difference of opinion between Jason Lowe of the UK Met Office and Stefan Rahmstorf of PIK in Germany. Here is an excerpt (emphasis added):

Climate science faces a new controversy after the Met Office denounced research from the Copenhagen summit which suggested that global warming could raise sea levels by 6ft by 2100.

The research, published by the Potsdam Institute for Climate Impact Research in Germany, created headline news during the United Nations summit on climate change in Denmark last month.

It predicted an apocalyptic century in which rising seas could threaten coastal communities from England to Bangladesh and was the latest in a series of studies from Potsdam that has gained wide acceptance among governments and environmental campaigners. . .

Jason Lowe, a leading Met Office climate researcher, said: "These predictions of a rise in sea level potentially exceeding 6ft have got a huge amount of attention, but we think such a big rise by 2100 is actually incredibly unlikely. The mathematical approach used to calculate the rise is simplistic and unsatisfactory." . . .

Rahmstorf calculated that such a spike in temperature would raise sea levels by up to 74in — a jump that stunned other experts. . .

Rahmstorf said the last decade had, however, seen preliminary evidence suggesting that the ice sheets of Greenland and West Antarctica were becoming unstable. He said: "In my heart I hope my critics are right because a rise of the kind my work predicts would be catastrophic," he said. "But as a scientist I have to look at the evidence . . . my figures for sea-level rise are likely to be an underestimate of what the world will face by 2100."

Apparently picking up on the debate, Nature Reports: Climate Change has published essays by Lowe and Rahmstorf, and it turns out, they seem to pretty much agree.

Here is what Rahmstorf now says (emphasis added):
A number of recent studies taking the semi-empirical approach have predicted much higher sea level rise for the twenty-first century than the IPCC, exceeding one metre if greenhouse gas emissions continue to escalate (Fig. 1 [above]). These new results have found wide recognition in the scientific community, as recent broad-based assessments show5, 6, 7. The question is: how plausible are the new estimates?

Although the popular media tend to focus on the upper limits of these projections, reaching the upper limits is, by definition, extremely unlikely. And at the high temperatures that produce extreme rises in sea level, predicting the response of the climate system is difficult. Upper limits also depend on how uncertainties are treated. Comparing the central estimates of sea level rise projections is therefore more informative. For a moderately pessimistic emissions scenario, named A1B, which results in about 3 °C global warming above the 1990 level by the 2090s, the IPCC projects 35 centimetres of sea level rise. This, rather implausibly, assumes no acceleration beyond the rate of sea level rise observed during the past 15 years, despite temperatures increasing by four times as much as in the twentieth century. A recent study by Martin Vermeer and me8, in contrast, yields a central estimate of 124 centimetres by 2100 and 114 centimetres by 2095.
And here are Lowe and Jonathan M. Gregory (emphasis added):

New research suggests that the possibility of sea level rise of up to two metres by 2100 should be given serious consideration. One key study5 examined the ice flow rates that would be required to produce substantial sea level rise by 2100 and concluded that a rise of much more than two metres would be “physically untenable”. Although increases of up to two metres could not be excluded, a sea level rise of less than one metre by 2100 was judged more likely on grounds of physical plausibility. Proxy evidence from oxygen isotope ratios in Red Sea sediment cores6 suggests that sea level rose by as much as 1.6 metres per century at a time in the past when the large ice sheets covered an area similar to their present-day extent.

Although increases of up to two metres this century can't be ruled out, this does not mean that they are inevitable or even likely. For climate change to produce much more than one metre of sea level rise, ice sheets would probably have to contribute considerably more to the rise than they do now; one 2009 study put their current contribution at 0.15 metres per century2. The recent acceleration of Greenland outlet glaciers and Antarctic ice streams may be due in part to natural variability, and it might not continue. Some observations indicate that a number of the outlet glaciers and ice streams that accelerated in the 1990s have since started to slow down7, and a recent study based on detailed modelling of the Helheim glacier on Greenland suggested that “recent rates of mass loss in Greenland's outlet glaciers are transient and should not be extrapolated into the future”8.

Lowe and Gregory hit the nail on the head with their conclusion:
For society, ignoring the need for adaptation could prove costly, but so could overcommitting to adaptation, a potential outcome of placing too much confidence on upper estimates of sea level rise. The climate science community needs to communicate effectively that sea level rise is likely to continue, but that the rise by the year 2100 is almost certain to be below two metres and that there is currently very little evidence to suggest that increases at the top of this range are likely. It is vital to continue to monitor sea level and its components and to develop a capability to make reliable projections. Meanwhile, as we cannot provide certainties, we must become better at explaining the uncertainties to decision-makers. These uncertainties imply a need to keep open a range of adaptation options and to be able to change the approach as the predictions become more robust17.
In an editorial on the subject, Olive Heffernan calibrates the significance of the current state of science for adaptation decision making in the face of sea level rise:
Ultimately, better projections are needed to determine the likely extent of future sea level rise. In the absence of such information, the best option is neither a strict policy of defence nor all-out retreat, but incremental adaptation.
Kudos to Nature Reports: Climate Change for getting these scientists on the record and for the healthy dose of common sense.