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.