The Current Wisdom is a series of monthly articles in which Senior Fellow Patrick J. Michaels reviews interesting items on global warming in the scientific literature that may not have received the media attention that they deserved, or have been misinterpreted in the popular press.
The Current Wisdom only comments on science appearing in the refereed, peer-reviewed literature, or that has been peer-screened prior to presentation at a scientific congress.
Three weeks ago, 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. You can find it at: (http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=13389).
The apparent intent of the report was to raise global warming alarm by projecting rapidly rising seas (some two to three times higher than estimates from the United Nations' Intergovernmental Panel on Climate Change (IPCC)) — along the California coast and elsewhere.
Unfortunately, to produce such a large and rapid sea level rise, the NRC had to throw overboard a lot of good science and do a lot of cherry-picking of the rest.
Of course global sea level will continue to rise, with or without human-induced climate change, as readers will see below. The NRC projects a wildly high upper bound of some 4-5 feet this century, while, in reality, the rate of future rise will likely be closer to the rise observed during the 20th century, about 8-12 inches. Coastal residents adapted to this with little or no overt effort.
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.
How did the NRC come up with such an enormous rise in sea level?
They independently projected the sea level rise from its two major components — ocean thermal expansion (from increasing ocean temperatures) and water input from the melting of ice — and then added them together.
But for each component, their high end projections are very likely much too high. And in combination, they represent something that is virtually impossible.
Consider thermal expansion. The NRC simply used the 21st century temperature change projections from the IPCC's 2007 collection of climate models. Is there a problem here?
Since they started in 2001, the IPCC model projections have warmed at a rate over twice what is being observed in reality. 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 overestimated. 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, i.e. 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 yield much less sea level rise than the NRC extrapolation of current trends.
This is how the NRC describes its 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 — less than a third of 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 ± 1.1in.).
Is this at all realistic? In a word, no.
There has been a whole lot of research published in recent years (and covered by The Current Wisdom) on ice loss from Greenland's glaciers. In virtually all cases, previously hypothesized mechanisms for a prolonged rapid speed-up of Greenland's glaciers have been debunked.
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 Geosciences, 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 authors decide not only to go ahead and extrapolate current rates into the future, but then, for good measure, to increase 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 highest IPCC estimate. This kind of alarmism may get you on MSNBC, but that doesn't mean it's right.
And there is much more to find fault with in the NRC sea level rise report.
Another topic that we have covered in The Current Wisdom on several occasions has been the recent research examining the contribution to sea level rise from non-climate related human activities. These activities can both act to lower sea level (by impounding water behind dams) and to raise it (by extracting water from aquifers that eventually finds a way into the oceans). Several researchers have been working to compile a temporal history of these effects on a global scale across much of the past 100 years. What they have found is that early-on, a lot of dams were being built, and more water was being impounded than was being pumped out of aquifers. The net result was a negative contribution to sea level rise. But in recent years, the rate of dam building has declined and the rate of groundwater extraction increased to the point where, starting in about the early 1980s, the net effect is a positive contribution to sea level rise. Over past decade or so, that contribution has been responsible for some 15-25% of the observed sea level rise.
But the NRC largely sets aside this topic, asserting:
The contributions of groundwater withdrawal and reservoir storage to sea-level change remain poorly constrained, largely due to sparse data and inadequate models. Each process likely has a significant but opposite effect on sea-level change, on the order of 0.5 mm per year.
In our last Current Wisdom article, we showed that the acceleration in the rate of sea level rise apparent during the past 50 years or so in the historical sea level record from tide gauges is completely explained by the history of anthropogenic alterations to the flow of water into the oceans (i.e., impoundment and dewatering).
This has an impact on the apportionment of observed sea level rise into its various components, which of course has an impact on our understanding of the various influences and controls.
For instance, the NRC determines that the current rate of contribution to sea level rise from ice loss from Greenland is 0.56 ± 0.13 mm/yr (0.02 inches/year) and from Antarctica is 0.37 ± 0.14mm/yr (0.01 inch/year). The NRC arrives at these numbers by using a weighted average of several recently published estimates made by various methodologies and from various research groups. But one of the estimates is much lower than the others — the one from Wu et al. (2010). Long-time Wisdom fans may recognize this citation as we discussed it in the very first Wisdom that we wrote back in the fall of 2010 (https://www.cato.org/the-current-wisdom/).
Wu et al. (2010) determined that all the other estimates of ice loss from Greenland and Antarctica that were being derived from the satellite measured gravity fields (using the GRACE satellite) were in error as they were arrived at by using an improperly calibrated glacial isostatic adjustment (GIA) model — that is, a model of how the land surface has been slowly responding since the removal of the large ice burden of the Last Ice Age. When Wu et al. (2010) supplied a better model, the ice loss estimated dropped considerably. Wu et al. calculated that ice loss from Greenland was occurring at a rate of 0.29 ± 0.06 mm/yr and 0.23 ± 0.12mm/yr from Antarctica. According to Wu et al. (2010):
"We conclude that a significant revision of the present estimates of glacial isostatic adjustments and land–ocean water exchange is required."
But the NRC buries this result in a multi-estimate average. This certainly produces the wrong answer. Either the other studies, using the old GIA model are correct and Wu et al. is wrong, or else, as concluded by Wu et al., the other studies are wrong and the Wu et al. numbers are closer to being correct. And if Wu et al. are correct, then the NRC is wrong.
Is there good evidence that the ice loss rates calculated by Wu et al. (2010) are correct?
In the past two years, the central estimate of the average amount of water contributed to the global oceans from non-climate activities is about 0.4 mm/yr (from Wada et al., 2012), and the amount contributed from ice loss from Greenland and Antarctica is 0.52mm/yr (from Wu et al., 2010). Together, this sums to 0.92 mm/yr. But according to the NRC, the net human non-climate contribution is near zero, and the net Greenland and Antarctic contribution is 0.93 mm/yr. So the sea level budget continues to close, no matter which of these sets of results is used. But if the Wada/Wu set is correct, then it means that ice loss is contributing just a tad but more than half of what the NRC cooked up, which must mean that ice loss from Greenland and Antarctic is much less sensitive to rising temperatures than determined by the NRC.
At the very, very least, the NRC should have at least separately included the Wada/Wu possibility, rather than largely tossing it out.
Doomsaying from the NRC is unfortunately the expected result when science careers are enhanced by the presence massive amounts of taxpayer funds, which tend to get doled out to those who cry "wolf" the loudest. This phenomenon, which ultimately results in very bad public policy, is something we are now studying intently here at Cato.
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.
Wada, Y., et al., 2012. Past and future contribution of global groundwater depletion to sea-level rise. Geophysical Research Letters, 39, L09402, doi:10.1029/2012GL051230
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