The map shows temperature changes for the last decade--January 2000 to December 2009--relative to the 1951-1980 mean. Warmer areas are in red, cooler areas in blue. The largest temperature increases occurred in the Arctic and a portion of Antarctica. Image: NASA
NASA has just announced that the period from January 2000 through December 2009 has been the hottest decade since record-keeping began in 1880. Furthermore, 2009 was the second-hottest year on record for the planet as a whole and in the southern hemisphere, the hottest. Last year was only a fraction of a degree cooler than the hottest year, 2005.Focus on Long-Term Trend, Not Just Yearly Records
But don't fixate on yearly data--which year is hottest, or second hottest--says Goddard Institute for Space Studies director Dr. James Hansen. Doing so "usually misses the point." In short, it's the long-term trend not the year-to-year variability that we should be focusing on.
Hansen points out that while there's always interest in that record, "There's substantial year-to-year variability of global temperature caused by the tropical El Niño-La Niña cycle. But when we average temperature over five or ten years to minimize that variability, we find that global warming is continuing unabated."
The NASA data shows that surface temperatures over the past 30 years have increase about 0.2°C each decade.
Since records began being kept, average temperatures have increased 0.8°C, or just under halfway to the 2°C critical threshold beyond which sea level rise, ocean acidification, polar and glacial ice melt and a number of factors begin creating a climate far different than the one which humanity has known since the dawn of civilization.
Natural Variability Doesn't Account for Observed Temperature Increase
In it's press release announcement, NASA points out that while there are other factors than greenhouse gases contributing to the amount of warming observed--changes in the sun's irradiance, oscillations of sea surface temperatures in the tropics, changes in aerosol levels in the atmosphere--these factors are not sufficient to account for the temperature increases observed since 1880.
Backing that up, NASA says that 1) sea surface temperature fluctuations (El Niño-La Niña) can cause global temperature deviation of about 0.2°C; 2) solar maximums and minimums produce variations of only 0.1°C, warmer or cooler; 3) aerosols from natural sources such as volcanic eruptions (Mount Pinatubo for example) have caused average cooling of 0.3°C, but recent eruptions have had not had significant effect.
As far as manmade aerosols in the atmosphere, such as from burning of fossil fuels, GISS estimates that these offset about half of the warming produced by man-made greenhouse gases, but that better measurements are needed to determine this.
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