File:Five-year average global temperature anomalies from 1880 to 2010.ogv

Groups of 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 data collected by temperature monitoring stations spread around the world and make an announcement about whether the previous year was a comparatively warm or cool year. This analysis concerns only temperature anomalies, not absolute temperature. Temperature anomalies are computed relative to the base period 1951—1980. The reason to work with anomalies, rather than absolute temperature is that absolute temperature varies markedly in short distances, while monthly or annual temperature anomalies are representative of a much larger region. Indeed, we have shown (Hansen and Lebedeff, 1987) that temperature anomalies are strongly correlated out to distances of the order of 1000 km. For more information about this dataset, see http://data.giss.nasa.gov/gistemp

NASA's announcement this year — that 2010 ties 2005 as the warmest year in the 131-year instrumental record — made headlines. But, how much does the ranking of a single year matter?

Not all that much, emphasizes James Hansen, the director of NASA's Goddard Institute for Space Studies (GISS) in New York City. In the GISS analysis, for example, 2010 differed from 2005 by less than 0.01 °C (0.018 °F), a difference so small that the temperatures of these two years are indistinguishable, given the uncertainty of the calculation.

Meanwhile, the third warmest year — 2009 — is so close to 1998, 2002, 2003, 2006, and 2007, with the maximum difference between the years being a mere 0.03 °C, that all six years are virtually tied.