On ‘scientific reticence’ and sea-level rise
Sea level rise of 5 meters in one century? Even if most scientists will not say so publicly, that catastrophe is a real possibility, according to the director of NASA’s Goddard Institute Of Space Studies.
It may seem like I single Hansen out for recommended reading. But that’s only because he:
- is the nation’s top climatologist
- writes prolifically
- speaks with unusually bluntness for a scientist
- has been more right than just about any climate scientist
He has written a terrific piece for the open-access Environmental Research Letters on "Scientific Reticence and Sea Level Rise":
I suggest that a "scientific reticence" is inhibiting the communication of a threat of a potentially large sea level rise. Delay is dangerous because of system inertias that could create a situation with future sea level changes out of our control. I argue for calling together a panel of scientific leaders to hear evidence and issue a prompt plain-written report on current understanding of the sea level change issue.
I could not agree more. In researching my book Hell and High Water, many leading climate scientists spoke to me candidly off the record that they share Hansen’s fear. Fortunately, more and more are speaking out.
Hansen is especially concerned that sea level rise is nonlinear:
Rahmstorf (2007) has noted that if one uses the observed sea level rise of the past century to calibrate a linear projection of future sea level, BAU warming will lead to a sea level rise of the order of one meter in the present century. This is a useful observation, as it indicates that the sea level change would be substantial even without the nonlinear collapse of an ice sheet. However, this approach cannot be taken as a realistic way of projecting the 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 [business as usual] forcing in the 21st century, the sea level rise surely 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 the gravity satellite measurements discussed above. As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005–15 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields a sea level rise of the order of 5 m this century. Of course I cannot prove that my choice of a ten-year doubling time for nonlinear 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 under BAU forcing.
An important point is that the nonlinear response could easily run out of control, because of positive feedbacks and system inertias. Ocean warming and thus melting of ice shelves will continue after growth of the forcing stops, because the ocean response time is long and the temperature at depth is far from equilibrium for current forcing. Ice sheets also have inertia and are far from equilibrium: and as ice sheets disintegrate their surface moves lower, where it is warmer, subjecting the ice to additional melt. There is also inertia in energy systems: even if it is decided that changes must be made, it may require decades to replace infrastructure.
Feedbacks and inertia and the very real threat of catastrophic sea level rise mean the time to act is now!
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