The Current Wisdom is a series of monthly articles in which Patrick J. Michaels, director of the Center for the Study of Science, 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.
In its 2007 Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) projected that as a result of warming primarily caused by anthropogenic greenhouse gas emissions, global mean sea level would rise from 1990 through 2100 from 0.18 to 0.59 meters (7 to 23 in.) with perhaps another 0.1 to 0.2 meters (4 to 8 in.) on top of that if the rates of ice loss from Greenland and Antarctica continued to grow linearly (from their 1993-2002 rates).
Some of our colleagues who are of a particularly “concerned” persuasion accused the IPCC of being far too conservative in these estimates; it’s true that the IPCC’s central estimate of 15 inches is hardly alarming. NASA’s Jim Hansen went as far as to accuse the IPCC and others of “scientific reticence” (Hansen claims that scientists are generally reluctant to announce bad news. Judge that one for yourself.) with regard to sea level rise. For his part, Hansen shares no such reticence, and doesn’t mind telling anyone who asks (and many who don’t) that we should be expecting upwards of 6 meters (236 in.) of sea level rise in a hundred years. He has even stated that the majority of this could occur by 2100. Clearly, there is a world of difference (and a different world) between the IPCC central estimate of 15 inches and Hansen’s 20 feet.
As if on cue, a bunch of apparently non-“reticent” scientists suddenly emerged, trying to show that the IPCC was wrong and of course, that things are “worse than we thought”.
They developed a technique that, to the non-perseverators in the crowd, seems quite reasonable, a “semi-empirical” method which ties historical sea level rise to the rate of global mean temperature change. They used this relationship rather than the computer-generated rises in sea level that come out of climate models, which is what backs the IPCC projections. Instead they only used the global temperature change projections from the same climate models, and then coupled those with their semi-empirical relationship between temperature and sea level to make their projections. Such a technique almost invariably yields greater rates of sea level rise, with the upper end of the range of projected values often exceeding 1 meter (39 in.) by the year 2100.
One potential problem is that the empirical relationship between sea level and global temperature may not be valid—even though it seems simple and straightforward. A new multi-authored study argues cogently that indeed the semi-empiricists have fallen into this simple trap.
The new paper’s lead author is Jonathan Gregory of the U.K.’s University of Reading, and the other authors are a who’s who of sea level researchers (repeating my professions nauseating belief that putting a large number of authors (most of whom have—at best—just read the manuscript) somehow makes it more persuasive). The paper concludes that the causes of sea level rise, and its temporal variations, across the 20th century were many, and that a link to anthropogenic global climate changes has been weak or absent over this period. Basing future sea level rise projections on a presumed historical relationship between anthropogenic global warming and corresponding sea level rise turns out to be a bad idea.
Here is how Gregory et al., 2012 put it:
The implication of our closure of the [global mean sea level rise, GMSLR] budget is that a relationship between global climate change and the rate of GMSLR is weak or absent in the past. The lack of a strong relationship is consistent with the evidence from the tide-gauge datasets, whose authors find acceleration of GMSLR during the 20th century to be either insignificant or small. It also calls into question the basis of the semi-empirical methods for projecting GMSLR, which depend on calibrating a relationship between global climate change or radiative forcing and the rate of GMSLR from observational data (Rahmstorf, 2007; Vermeer and Rahmstorf, 2009; Jevrejeva et al., 2010).
And here are the main conclusions, now seriously questioned, from the semi-empiricical citations included in the above quote:
When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level [by 2100].
Vermeer and Rahmstorf (2009):
For future global temperature scenarios of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990–2100.
Jevrejeva et al. (2010):
With six IPCC radiative forcing scenarios we estimate sea level rise of 0.6–1.6 m, with confidence limits of 0.59 m and 1.8 m.
Seems like three strikes against projecting those high rates of sea level rise.
For a little reality check, the current rate of rise is somewhere in the range of 1.8 to 3.5 mm/yr (0.07 to 0 .14 in/yr) depending on the time period over which you calculate the trend.
Further, as we have previously written, it doesn’t look as if the recent increased rates of ice loss from Greenland and Antarctica are sustainable—much less going to linearly increase to the end of the century. All of this strongly argues that the 21st century sea level rise is not a problem that we can’t keep up with.
Gregory, J., et al., 2012. Twentieth-century global-mean sea-level rise: is the whole greater than the sum of the parts? Journal of Climate, doi:10.1175/JCLI-D-12-00319.1, in press.
Hansen, J.E., 2007. Scientific reticence and sea level rise. Environmental Research Letters, 2, doi:10.1088/1748-9326/2/2/024002
Jevrejeva, S., et al., 2010. How will sea level respond to 1019 changes in natural and anthropogenic forcings by 2100? Geophysical Research Letters, 37, L07703, doi:10.1029/2010GL042947.
Rahmstorf, S., 2007. A semi-empirical approach to projecting future sea-level rise. Science, 315, 368–370, doi:10.1126/science.1135456.
Vermeer, M. and S. Rahmstorf, 2009. Global sea level linked to global temperature. Proceedings of the National Academy of Sciences, 106, 51, 21527–21532, doi:10.1073/pnas.0907765106.