Saturday, September 15, 2007

Yes, NASA revised those numbers, but...

The global warming deniers have recently seized on a story regarding NASA's revisions of its annual temperature numbers as evidence that global warming is baloney. The story appeared in an August 16 column by Cal Thomas and has been blog and letter-to-the-editor fodder since then.

There are parts of the story that Mr. Thomas and others have reported correctly, parts that they have omitted, and parts that they have flat out gotten wrong.

The part of the story that is true is that NASA did revise its numbers. The new and old numbers, as well as NASA's explanation can be found at and by following links on that page.

Mr. Thomas and others claim that the revisions have led to changes in the rankings of the warmest years on record. The implication is that rankings of global averages have changed, but this isn't the case.

NASA's changes in rankings apply ONLY to temperatures for the U.S. and not for the world as a whole. In NASA's revised figures, 1934 is the hottest year on record for the U.S. but not for the world. NASA still reports that the 5 hottest years on record for the world happened in the last decade (in order 2005, 1998, 2002, 2003 and 2006).

Having garbled the facts, Mr. Thomas goes on to ask, "Has any of this new information changed the minds of the global warming fundamentalists?" and helpfully answers his own question, "Nope." He skips the more logical question--should the new information have changed anyone's mind--to which the answer is again, "Nope."

Why is this so? Well, the revisions to NASA's numbers for the U.S. were very slight (the largest change was .15 degrees) as were the revisions to the rankings. Prior to the revisions, 1934 and 1998 had been in a virtual dead "heat" (no pun intended). In both the earlier and later figures, 4 of the 10 hottest years on record in the U.S. occurred in the 1930s and 3 of the 10 hottest years occurred in the last decade.

There were also revisions to the global numbers but these were an order of magnitude smaller (in the thousandths). The revisions did not affect the global rankings at all.

As the scientist who put out the revised numbers explained, "Contrary to some of the statements flying around the internet, there is no effect on the rankings of global temperature. Also our prior analysis had 1934 as the warmest year in the U.S. (see the 2001 paper above), and it continues to be the warmest year, both before and after thecorrection to post 2000 temperatures. However, as we note in that paper, the 1934 and 1998 temperature are practically the same, the difference being much smaller than the uncertainty."

While the deniers have focused on a handful of numbers (which again hardly changed), it useful to remember that the figures in question come from a 127 year record. Over that time, the U.S. data are more variable and flatter than the global data. Nevertheless, the trend in the U.S. has been upwards. If you look at the average over the last decade, you will see that it is higher than the average for the decade of the 1930s (U.S. temperatures in the 1930s were quite variable and there were a few "low" years along with the "high" years). NASA also provides 5-year averages. It turns out that even in the revised data, the first, second, third, fourth, and sixth highest 5-year averages for the U.S. occurred in the last decade.

The global trends are even more starkly upward.

A cogent critique of the global warming argument needs to address all of the evidence, not just a figure or two. NASA's revisions in this particular series were minor, and the corrected figures continue to show increasing temperatures over the past century and a quarter. NASA's figures are also not the only evidence. As NOAA reports, changes in sea ice, rises in sea levels, the general retreat of glaciers, ice core data, tree rings, and sediment samples provide near- and long-term evidence that global warming is occuring and that human activity is a contributor.