Correcting Global Warming Data

Watts Up With That? reports 1998 no longer the hottest year on record in USA. It seems blogger Steve McIntyre of Climate Audit found a mistake in NASA's data of US temperatures.



Notice the jump that happens to be at January 1, 2000? It's not a Y2K bug as has been reported, but represents a change in the data was collected. McIntyre contacted Jim Hansen earlier this month and they changed their data to correct the error.

With the correction, 1998 is no longer the US's hottest year, 1934 is, by just a 1/10 of a degree Celsius. "Four of the top 10 years of US CONUS high temperature deviations are now from the 1930s: 1934, 1931, 1938 and 1939, while only 3 of the top 10 are from the last 10 years (1998, 2006, 1999). Several years (2000, 2002, 2003, 2004) fell well down the leaderboard, behind even 1900." The new data is here.

Note that 2005 and 2006 both look to be particularly warm (2006 is 3rd) but there has been enough time to compute 5 year averages so they aren't included. If you look at 5 year means the top 11 years (all those 0.5 degrees hotter than average) are 2000, 1999, 2004, 2001, 1932, 1933, 2003, 2002, 1998, 1988, 1989.

It seems McIntyre was also involved in correcting other climate data, the so-called Hockey stick controversy. The wikipedia article seems to be quite a good summary. In short, in 1998 several scientists published a graph of the mean temperature changes of the northern hemisphere over the last 1,000 years. It looked like this:



It was prominently featured in the 2001 IPCC report and was in An Inconvenient Truth in a slightly altered form. The thing to note is that it shows temperatures being in a narrow range for 1,000 years and now they are warmer than they've ever been.

However it seems there's a medieval warming period centered around 1450. Climate scientists knew of this but the 1998 graph was the first that smoothed out that data. Now computing all this stuff is difficult. There are various data sets used from different parts of the world in different forms (tree rings, ice cores, etc.) There is a lot of math applied to the data to normalize it and make it useful. Stephen McIntyre and Ross McKitrick went through the data, needing to reconstruct it because the graph's authors wouldn't provide it. They found numerous errors and believe the hockey stick isn't accurate because it understates the medieval warm period which was warmer than it is now. That casts some doubt that the current temperatures are caused principally by man-made greenhouse gases. The wikipedia article describes various investigations and further papers. The matter isn't settled but more scientists are questioning the data. "The 4th Assessment Report of the IPCC now presents a whole range of historical reconstructions instead of favoring prematurely just one hypothesis as reliable."