Parts of the northern hemisphere may have had an extremely cold December, but nevertheless, last year tied for the second-warmest in 130 years of global instrumental temperature records, according to the latest surface temperature analysis of the NASA Goddard Institute for Space Studies (GISS). The analysis finds that global temperatures were so similar in 1998, 2002, 2003, 2006, 2007, and 2009, that they are all tied for second place. In the Southern Hemisphere, 2009 set the record as the warmest year, according to this report.
James Hansen, head of NASA’s GISS, and his team have released their end-of-year summary for 2009, initially posted on the Real Climate blog. It’s pretty dense, but here are some additional highlights:
– The scientists offer an explanation for an apparent data discrepancy over whether 1998 or 2005 was the warmest year. In short, it comes down to the difference in the way GISS and HadCRUT (Hadley Centre/University of East Anglia Climatic Research Unit) assign or do not assign temperature data for areas without observing stations. (HadCRUT leaves them out of the analysis, while GISS assigns values based on various factors outlined in the summary.) GISS maintains that 2005 was the warmest year.
– According to the report:
“There were strong negative temperature anomalies at middle latitudes in the Northern Hemisphere, as great as ‐8°C in Siberia, averaged over the month. But the temperature anomaly in the Arctic was as great as +7°C.”
In other words, 2009’s cold December in certain areas of the planet, as well as an unusually cold 2009 summer in the United States and Canada, do not reflect overall global temperatures nor signal a cooling trend:
“It is obvious that in December 2009 there was an unusual exchange of polar and mid‐latitude air in the Northern Hemisphere. Arctic air rushed into both North America and Eurasia, and, of course, it was replaced in the polar region by air from middle latitudes. The degree to which Arctic air penetrates into middle latitudes is related to the Arctic Oscillation (AO) index, which is defined by surface atmospheric pressure patterns…”
According to GISS data, December 2009 was the most extreme negative Arctic Oscillation since the 1970s.
– The report underscores that monthly temperature anomalies tend to be greater than seasonal anomalies and that the the mean temperature of a particular month might not be the best way to identify global warming. Instead, one needs to look at measurements over the long-term, which, according to GISS data, indicate general warming over at least the last 50 years, just about everywhere on the planet.
The summary concludes with a sort of admonishment:
“The bottom line is this: there is no global cooling trend. For the time being, until humanity brings its greenhouse gas emissions under control, we can expect each decade to be warmer than the preceding one. Weather fluctuations certainly exceed local temperature changes over the past half century. But the perceptive person should be able to see that climate is warming on decadal time scales.”