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Estimating 21st-Century Sea Level Rise
Reference
Pfeffer, W.T., Harper, J.T. and O'Neel, S. 2008. Kinematic constraints on glacier contributions to 21st-Century sea-level rise. Science 321: 1340-1343.

Background
In his testimony of 26 April 2007 before the U.S. House of Representatives' Select Committee on Energy Independence and Global Warming, NASA's James Hansen stated "there is increasing realization that sea level rise this century may be measured in meters if we follow business-as-usual fossil fuel emissions," as Hansen also contended that year in two separate scientific publications (Hansen, 2007; Hansen et al., 2007). But is this really so?

The answer to this question is of considerable consequence, because as Pfeffer et al. write in the introduction to their new analysis of the subject, "underestimates will prompt inadequate preparation for change, [but] overestimates will exhaust and redirect resources inappropriately." Hence, as with all things, it is important to determine where the truth lies.

What was done
The three U.S. researchers gave particular emphasis to Greenland in their analysis, in light of its supposed "vulnerability to ongoing Arctic warming and meltwater-related feedbacks, recent accelerations of ice motion, and its large volume reductions during the last interglacial," as well as to Antarctica, employing "a simple kinematic approach" that determined the velocities of their outlet glaciers that would be "required to achieve various magnitudes of sea level rise by 2100."

What was learned
Pfeffer et al. determined -- and concluded with readily apparent confidence -- that "increases in excess of 2 meters are physically untenable [our italics]," noting that "a total sea-level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits [our italics and bold]," after which they state that "more plausible but still accelerated [our italics] conditions lead to total sea-level rise by 2100 of about 0.8 meter," while for comparison they indicate that the consensus estimate of the IPCC's most recent Fourth Assessment Report was a sea-level rise of only 0.18 to 0.60 meter by 2100.

What it means
Both James Hansen and Al Gore -- who also likes to talk of sea levels rapidly rising to great heights -- appear to be on the far, far fringes of the twilight zone when they warn of mean global sea level rising by multiple meters over the next nine decades.

References
Hansen, J.E. 2007. Scientific reticence and sea level. Environmental Research Letters 2: 10.1088/1748-9326/2/2/024002.

Hansen, J., Sato, M., Kharecha, P., Russell, G., Lea, D.W and Siddall, M. 2007. Climate change and trace gases. Philosophical Transactions of the Royal Society Series A 365: 1925-1954.

Reviewed 17 December 2008