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.

8 comments:

  1. The Vermeer & Rahmstorf paper (V/R), referred to above, has an interesting history, currently covered in a guest blog by Martin Vermeer, at RealClimate.

    I believe it's a paradigm-changing contribution, not only to estimation of sea level increase, but by providing a better 'surrogate' for global mean surface temperature (GMST) trend .

    The GMST for the last decade has been approximately 'flat'. This has contributed a lot of loose discussion about the increasing uncertainty associated with the putative ~2ºC/century trend in the GMST.

    The V/R analysis now strongly suggests that the 'flat' is almost certainly the result of the accidental confluence of various sources of 'heat-in-the-pipeline noise' (e.g., ENSO, PDO, AO, AMDO etc.).

    The energy absorbed by the ocean, from incident radiation, is almost instantly converted into heat. A lot of that is generated below the surface and is distributed within the mixed layer of the ocean, before it has a chance to equilibrate with the atmosphere and land surfaces. This is what Hansen et al. have referred to as "heat in the pipeline". It takes decades for it to exert its effect on the GMST. The rate at which it reaches the atmosphere is perturbed by such periodic oceanic overturning as ENSO, etc. And those perturbations are very substantial compared to the GMST. But the same perturbations have a much smaller impact on the enormous heat content of the oceans. Therefore, the global temperature trend, as recovered by CAREFUL estimation of sea level increase (as in V/R), is much less noisy, and much more robust than what we see in GMST.

    And that signal, in the V/R figure for Sea Level vs Time, is clearly rising throughout the last decade, even while it cleanly preserves the details of 'heat-in-the-pipeline-noise-events'!

    So although the caveat that all projections into the future must be uncertain stands, the general 'uncertainty' that global warming has continued rather steadily through the last decade HAS BEEN SUBSTANTIALLY REDUCED.

    The scales have been tipped significantly towards the "common sense" of trying to carefully plan Adaptation/Mitigation, NOW, rather than later.

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  2. Given that Rahmstorf has been demonstrated to be an incompetent in stats, why has he any credibility at all? I don't raise this to impugn Rahmstorf. After all, he's done that for himself. Ask any stats geek who laughed himself silly after looking at his "it's even worse than we thought" travesty. see e.g. http://rankexploits.com/musings/2009/source-of-fishy-odor-confirmed-rahmstorf-did-change-smoothing/

    I'm directing the question to other climate scientists. Why would someone who screwed up this badly and this prominently retain any credibility? At a time when climate science is reeling from massive blows to credibility, why aren't the competent scientists doing more to police the incompetents?

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  3. Will this be as robust as the consensus on climate catastrophe that is failing currently?

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  4. I don’t know who Olive Heffernan is, but this statement "Ultimately, better projections are needed to determine the likely extent of future ***. In the absence of such information, the best option is neither a strict policy of defence nor all-out retreat, but incremental adaptation." can be generalized to just about every climate adaptation issue IMHO.

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  5. #2:

    "The energy absorbed by the ocean, from incident radiation, is almost instantly converted into heat. A lot of that is generated below the surface and is distributed within the mixed layer of the ocean, before it has a chance to equilibrate with the atmosphere and land surfaces. This is what Hansen et al. have referred to as "heat in the pipeline"."

    I don't get it. That is true only for visible radiation, which has not changed much. IR is stopped VERY close to the surface.

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  6. Rhamstorf's "semi-empirical" method has zero credibility. Tom Moriarty has shown that his math produces impossible results-see his last three posts:

    http://climatesanity.wordpress.com/

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  7. Dr Robert E Stevenson, an eminent oceanographer who died in 2001, commented on the first Levitus et al paper in 2000:
    http://www.21stcenturysciencetech.com/articles/ocean.html

    "Contrary to recent press reports (in 2000), that the oceans hold the still-undetected global atmospheric warming predicted by climate models, ocean warming occurs in 100-year cycles, independent of both radiative and human influences."

    For 15 years, (in 2000) modellers have tried to explain their lack of success in predicting global warming. The climate models had predicted a global temperature increase of 1.5°C by the year 2000, six times more than that which has taken place.

    Not discouraged, the modellers argue that the heat generated by their claimed "greenhouse warming effect" is being stored in the deep oceans, and that it will eventually come back to haunt us. They've needed such a boost to prop up the man-induced greenhouse warming theory, but have had no observational evidence to support it. The Levitus, et al. article is now cited as the needed support.

    How the Oceans Get Warm:
    Warming the ocean is not a simple matter, not like heating a small glass of water. The first thing to remember is that the ocean is not warmed by the overlying air.

    Let's begin with radiant energy from two sources: sunlight, and infrared radiation, the latter emitted from the "greenhouse" gases (water vapor, carbon dioxide, methane, and various others) in the lower atmosphere. Sunlight penetrates the water surface readily, and directly heats the ocean up to a certain depth. Around 3 percent of the radiation from the Sun reaches a depth of about 100 meters.

    The top layer of the ocean to that depth warms up easily under sunlight. Below 100 meters, however, little radiant energy remains. The ocean becomes progressively darker and colder as the depth increases.

    The infrared radiation penetrates but a few millimeters into the ocean. This means that the greenhouse radiation from the atmosphere affects only the top few millimeters of the ocean. Water just a few centimeters deep receives none of the direct effect of the infrared thermal energy from the atmosphere! Further, it is in those top few millimeters in which evaporation takes places. So whatever infrared energy may reach the ocean as a result of the greenhouse effect is soon dissipated.

    ....anomalous heat associated with changing solar irradiance is stored in the upper 100 meters. The heat balance is maintained by heat loss to the atmosphere, not to the deep ocean.

    Worth reading for the full story, from someone who was a career oceanographer, Secretary General of the International Association for the Physical Sciences of the Oceans, and a consultant to NASA instructing astronauts on earth observation from space.

    He was a hands on scientist:
    "The archived data used by Levitus, and a plethora of other oceanographers, were taken by me, and a whole cadre of students, post-docs, and seagoing technicians around the world."

    "Surface water samples were taken routinely, however, with buckets from the deck and the ship's engine-water intake valve. Most of the thermometers were calibrated into 1/4-degrees Fahrenheit. They came from the U.S. Navy. Galvanized iron buckets were preferred, mainly because they lasted longer than the wood and canvas. But, they had the disadvantage of cooling quickly in the winds, so that the temperature readings needed to be taken quickly. I would guess that any bucket-temperature measurement that was closer to the actual temperature by better than 0.5° was an accident, or a good guess. But then, no one ever knew whether or not it was good or bad."

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  8. jae and DennisA:

    Yes, it is only the direct 'visible' solar radiation that is 'instantaneously' absorbed by below-surface-volumes of the ocean.

    But its part of THAT heat that's returned to the surface with variable delays, as "heat in the pipeline".

    From Vermeer and Rahmstorf, it appears that the particular variability, in THIS transfer, has most probably been responsible for the 'flat' in the the GMST trend of the last decade.

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