December 19, 2010

Global Temperature Shifts

This graph from the New York Times suggests that global warming has stalled:

However, a more recent (Jan. 13, 2011) graph suggests that it keeps on truckin'

This graph from NASA's Earth Observatory site also shows the same warming trend from four different scientific organizations:

NASA explains:
Each year, scientists from several major institutions—NASA’s Goddard Institute for Space Studies (GISS), NOAA’s National Climatic Data Center (NCDC), the Japanese Meteorological Agency, and the Met Office Hadley Centre in the United Kingdom—tally the temperature data collected at stations around the world and make independent judgments about whether the year was warm or cool compared to previous years.

On January 12, 2011, the NASA group announced that 2010 had tied 2005 as the warmest year in their 131-year instrumental record. NOAA also declared 2010 to be tied with 2005. The Japanese Meteorological Agency noted in a preliminary analysis that 2010 was the second warmest. The Met Office Hadley Centre has yet to make its announcement.

But how much does the ranking of a single year matter? Not all that much, said James Hansen, the director of NASA GISS. In his group’s analysis, 2010 differed from 2005 by less than 0.01°C (0.018 °F), a difference so small that the temperatures of the two years are almost indistinguishable, given the uncertainty of the calculation. Meanwhile, the third warmest year, 2009, is so close to 1998, 2002, 2003, 2006, and 2007 (the maximum difference between years is 0.03°C), that all six years are virtually tied.

What matters more than a yearly record from a single group is the longer trend, as shown in the plot at the top of this page. The four records are unequivocal: the world has warmed since 1880, and the last decade has been the warmest on record.

When we focus on the annual rankings, the differences between the temperature analyses can be confusing. For example, GISS previously ranked 2005 as the warmest, while the Met Office listed 1998. The discrepancy helped fuel a misconception that findings from the research groups varied sharply or contained large degrees of uncertainty. It also fueled a misconception that global warming had stopped in 1998.

“The official records vary slightly because of subtle differences in the way we analyze the data,” said Reto Ruedy, one of Hansen’s colleagues at GISS. “But they also agree extraordinarily well.”

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