June 29, 2012

NRC Sea Level Rise Scare: Losing Sight of the Science

Last week, the National Academies of Sciences’ National Research Council (NRC) released a report Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future. The apparent intent of the report was to raise global warming alarm by projecting rapidly rising seas—some 2-3 times higher than recent IPCC estimates—along the California coast and elsewhere. Based on the news coverage, the NRC was successful.

Successfully handling the media does not equate to successfully handling the science, if scientific success is judged by scientific accuracy.

The NRC was quite adept at sidestepping the inconvenient scientific literature which would have tempered their conclusions and which would have replaced alarm with prudent vigilance. Sure, global sea level will continue to rise, but the rate of future rise will likely be closer to the rise observed during the 20th century, about 8-12 inches—a rate to which coastal residents have easily adapted—than to the NRC’s upper bound which approaches some 4-5 feet by the year 2100.

How the NRC came up with a global sea level rise by the year 2100 of some 50 to 140 cm (20 to 55 inches)—is an example of using only a careful selection of available data and turning a deaf ear to warnings (made by the scientists themselves) of its unreliability for long-term projections.

Let’s review how the NRC came to make its future projections of sea level rise.

They independently extrapolated the two major components of sea level rise—ocean thermal expansion from increasing ocean temperatures and water input from the melting of ice—and then added them together.

For each, their high-end projections are very likely way too high. In combination, they represent something that is virtually impossible.

Let’s look at oceanic thermal expansion. To make their projection of this component of future sea level rise, the NRC simply used the 21st century temperature change projections from the IPCC AR4 collection of climate models to derive the resulting ocean thermal expansion (actually the NRC didn’t do any of this themselves, they just interpolated values as derived from previously published research).

Anyone see the problem here? Well, here is a hint… since the beginning of the IPCC AR4 climate model projections (which started in ~2001), the global temperature has warmed at rate that is less than half of that projected by the climate models. So, if the current global temperature behavior under rapidly rising greenhouse gas concentrations is any indication of the future behavior under rapidly rising greenhouse gas concentrations then the future projections of the sea level rise due to the thermal expansion of ocean water are grossly too high. But the NRC did not even consider such a possibility, instead preferring to base their projections on the assumption that the future climate behavior will depart radically from the current climate behavior, basically, throwing out observations in favor of what appear to be failing climate models. This model-based method produces a lot more sea level rise than extrapolating observations would produce.

Which brings us to the NRC projections for the other element of sea level rise—melting of land-based ice, primarily from Greenland and Antarctica. For this projection, the NRC turned their previous methodology on its head, and opted to extrapolate current trends (to the absurd) rather than rely on ice models. In this case, the models produce a heck of a lot less sea level rise than the NRC extrapolation of current trends.

This is how the NRC describes their method:

The base-rate extrapolation assumes that present-day observed trends in loss rates continue in the future.

And to get even more sea level rise than current trends allow, the NRC additionally does this:

Increased ice discharge beyond presently observed rates was simulated by extrapolating a multiple of present-day observed discharge forward in time to 2100.

For example, here is how they handled ice loss from Greenland

For Greenland, the average speed of all outlet glaciers was increased by 2 km yr-1, equivalent to a net discharge of 375.1 GT yr-1. These values are consistent with the observed doubling of Greenland’s mass balance deficit between ca. 1996 and 2000.

For what it’s worth, the current ice discharge rate from Greenland, as determined from an improved satellite algorithm (Wu et al., 2010) is 104 GT/yr—three and a half times less than the NRC’s high end case.

The result is that the NRC projects that by 2100, Greenland will have contributed between 14.8 cm and 33.8 cm (5.8 in. to 13.3 in.) to global sea level rise, with their best projection of 20.1 ± 2.7 cm (7.9 ± 1.1 in.).

Is this at all realistic? In a word, no.

There has been a whole lot of research published in recent years that was the result of investigations of ice loss from Greenland’s glaciers. In virtually all cases, previously hypothesized methods of prolonged rapid speed-up of Greenland’s glaciers have been debunked. See here for our most recent coverage of this research).

And as to the reliability of extrapolating future trends into the future, here are a few relevant quotes:

From Van der Wahl et al., Science, 2008 :

“Longer observational records with high temporal resolution in other ablation areas of the [Greenland] ice sheet are necessary to test the importance of the positive-feedback mechanism between melt rates and ice velocities. At present, we cannot conclude that this feedback is important.”

From Nick et al., Nature Geoscience, 2009:

“Our results imply that the recent rates of mass loss in Greenland’s outlet glaciers are transient and should not be extrapolated into the future.”

And most recently, from Moon et al., Science, 2012:

“Finally, our observations have implications for recent work on sea level rise. Earlier research used a kinematic approach to estimate upper bounds of 0.8 to 2.0 m for 21st-century sea level rise. In Greenland, this work assumed ice-sheet–wide doubling of glacier speeds (low-end scenario) or an order of magnitude increase in speeds (high-end scenario) from 2000 to 2010. Our wide sampling of actual 2000 to 2010 changes shows that glacier acceleration across the ice sheet remains far below these estimates, suggesting that sea level rise associated with Greenland glacier dynamics remains well below the low-end scenario (9.3 cm by 2100) at present. Continued acceleration, however, may cause sea level rise to approach the low-end limit by this century’s end.”

So, even though the authors of papers reporting on the recent glacier flow rates in Greenland say that current rates should not be extrapolated into the future and that even the current glacial acceleration of flow rate produces only very minimal contributions of sea level rise from Greenland, the NRC blunders ahead, extrapolating current rates into the future, but then, for good measure, increasing the flow even more on top of that. This assures an extreme value of sea level rise from the contribution of melting ice.

Combine a too large estimate from ocean thermal expansion with a too large estimate from ice melt, and you get an alarming amount of sea level rise. Crank it up even higher with more unrealistic assumptions about ice behavior in the future and you get a high end sea level rise projection (55 inches) that is some two times larger than the high end IPCC AR4 estimate. Talk about making the news!

There is much more to find fault with in the NRC sea level rise report, but we think from the glaring examples above, that it should be plenty obvious that the report authors’ main task was to produce projections of alarming sea level rise. In that task they surely succeeded. But in doing so, they failed to in remain true to the science.

Such a tale is far too often told in these pages, but it is sadder still that it comes from the “prestigious” NRC.

References:

Moon, T., I. Joughin, B. Smith, and I. Howat, 2012. 21st-century evolution of Greenland outlet glacier velocities. Science, 336, 576-578, doi:10.1126/science.1219985

Nick, F. M., et al., 2009. Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus. Nature Geoscience, DOI:10.1038, published on-line January 11, 2009.

van de Wal, R. S. W., et al., 2008. Large and rapid melt-induced velocity changes in the ablation zone of the Greenland ice sheet. Science, 321, 111-113.

Wu, X., M.B. Heflin, H. Schotman, B.L.A. Vermeersen, D. Dong, R.S. Gross, E.R. Ivins, A. W. Moore, and S.E. Owen. 2010. Nature Geoscience, doi:101038/NGEO938




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