In a December 17th Fox News story (See full report here) Steven Milloy comments on a lecture by Lonnie Thompson at the Annual meeting of the American Geophysical Union in San Francisco. He uses a common ploy of truncating what Thompson said, to ensure that a quotation fits with his message. According to Milloy, Thompson said, “Any prudent person would agree that we don’t yet understand the complexities with the climate system.” But what he actually said was “Any prudent person would agree that we don’t yet understand the complexities with the climate system and, since we don’t, we should be extremely cautious in how much we ‘tweak’ the system.” (see full press release here). Such manipulations are designed so that Milloy can’t be accused of misquoting, but clearly, he completely contorts Thompson’s point. Milloy also misunderstands the science.
In his talk, Thompson described two samples of moss that are 5,000 and 50,000 years old, respectively (based on radiocarbon dating). These samples have been revealed as the Quelccaya ice cap in Peru receded over the last few years. Milloy argues that “the plant find is a strong indication that, thousands of years ago, the high Andean climate must have been warm enough to cause the glacier to be recessed and to allow for the plants to grow in the first place…”. That is correct. But he goes on to say, “So if natural forces caused those climate changes, isn’t it reasonable to conclude that perhaps natural forces might also be largely responsible for whatever climate changes may be occurring now?” Unfortunately, that isn’t reasonable.
Milloy makes the common mistake of confusing (1) different factors that cause climates to change (see forcings) and (2) the rates of climate change. Warming in the early to mid-Holocene (the post-glacial period that covers the last 12,500 years) resulted from changes in the earth’s orbit (as described by Milankovitch). In the western United States, many glaciers disappeared altogether at this time, only to re-form around 4500 years ago. The temperatures slowly changed as the earth’s position altered, in relation to the sun, causing the distribution of energy received on earth to change geographically and seasonally. The changes observed by Thompson (since he started studying the Quelccaya ice cap in the late 1970s) have been extremely large and rapid; in fact, the rate of ice recession has increased over time. Thompson noted in a 2003 peer-reviewed article, that “The rate of retreat from 1983 to 1991 was three times that measured from 1963 to 1983.” (Climatic Change, vol 59, p.137-159). Evidence of glacier retreat has been observed in almost all parts of the world in the 20th century, and the rate of retreat has also increased in the latter half of the 20th century. This has nothing to do with the slow changes that result from orbital forcing. It is a consequence of rapid worldwide global warming, the rate of which has increased in the last 20 years. As discussed elsewhere in these pages, there is strong evidence that anthropogenic effects are largely responsible for this warming.
On a more general point, uninformed commentators often refer to periods in the past when it was warmer, then claim that this somehow “proves” that contemporary changes are “normal”. But there were countless warm periods in the past that resulted from quite different conditions than those prevailing today (see this link on the Medieval period, or this link on the “mid-Holocene” period). In some cases, these were due to a different orbital configuration, or different levels of greenhouse gases, or even different world geography (lower mountain ranges, ocean seaways altered, no polar ice sheets etc). What makes the recent changes stand out is that they are extremely rapid and global in extent. Another error commonly made is to pick one spot on earth where it may have been warm, and claim that this demonstrates that the earth as a whole was warm at that time. This is also incorrect. If it was warmer in southern Greenland when the Vikings arrived, this tells us nothing about conditions in the Pacific, or Eurasia or South America. To get a true picture of whether there was “global warming” at that time requires, not surprisingly, a set of data from many places around the globe (see this discussion on one of the popular “myths” regarding past climate history). Thus, Thompson’s observation about the retreat of the Quelccaya ice cap would be interesting, but not that important, if it was the only data point we had. But it isn’t — we observe similar things happening in virtually all mountainous regions of the world.