How Hot Was 2009? Was It the 2nd Warmest Year, or the 5th Warmest Year?

NASA's temperature estimates for Arctic, shown here in December 2009, are one of the factors that made the agency produce different global temperature estimates for 2009 than other science agencies.

Photo: National Snow and Ice Data Center

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After Climate Gate, when some top climate scientists were found to be behaving badly via email, and Glacier Gate, when the Nobel Prize-winning U.N. climate panel was found to have included a bogus "fact" about the threat to Himalayan glaciers in its landmark climate report, climate skeptics have seeded some new doubts about the veracity of global warming science. Investor's Business Daily, no stranger to climate science skepticism, had this charming headline on the subject: "Climate Flimflam Flaming Out." Of course, even a cursory look at the facts reveals these two sensational bits of news to be insignificant, when compared to the multiple lines of carefully analyzed data that shows the world warming because of the accumulation of greenhouse gases.

But it might add to the fire that the two U.S. agencies that do the most research into global warming – NASA and NOAA (the National Oceanographic and Atmospheric Administration) – both put out authoritative press releases on the same day last week, each stating just how hot 2009 was in comparison to the historical record, each coming to a different conclusion ... and neither explaining the discrepancy.

NASA's conclusion: "2009 was tied for the second warmest year in the modern record."

NOAA's conclusion: "For 2009, global temperatures tied with 2006 as the fifth-warmest on record."

So which is it, the second-warmest or the fifth-warmest?

We asked both NOAA and NASA to explain.

First up, Reto Ruedy, of NASA's Goddard Institute for Space Studies in New York City. Ruedy makes clear, first, that all this data comes from real observations in the real world: "These global mean temperature data are estimates based on temperature reports from many weather stations and other sources." But not all weather stations are equally distributed across the globe, and not all weather stations have complete data or historical records, and not all weather stations report data that is free of errors. "Different techniques are applied to deal with these problems," Ruedy said, explaining how the same or similar data can yield different estimates. For instance, NASA estimates temperature for a larger swath of the Arctic based on surrounding weather stations than NOAA does; because the Arctic was particularly warm in 2009, NASA's global temperature estimate is warmer, too. The graph below demonstrates just how closely NOAA and NASA data jives over the longterm, and it puts the latest discrepancy into perspective.

Next up, Derek "Deke" Arndt, chief of the Climate Monitoring Branch at NOAA's National Climatic Data Center in Asheville, N.C. Since his explanation is so colloquial, here it is in full:

"There are methodological differences between the major global datasets (NOAA, Hadley, and NASA). (Hadley is the U.K.'s Hadley Center for Climate Change, which produces a third estimate for global temperature, which in this case agreed with NOAA's estimate that 2009 is the fifth-warmest year on record.) These are related to the details of, for example: how individual "grid cells" are filled; how missing data is handled, the exact quality assurance routines used to accept/reject data, how the polar regions are handled, if and how much satellite data is used (and where), and so on.

"Also, the concept of using ranks may add to the dissonance. Just like when watching a race, if a cluster of runners crosses the finish line together, small differences can make a big difference between, say, 2nd place and 5th place. That's what's happening here. Our 3rd through 6th place years were all quite close.

"Finally, as you likely know: In the context of step-back-and-take-stock-of-the-big-picture ... the important thing is that three different datasets are saying essentially the same thing, even if some of the details differ slightly. In essence, the three datasets are singing the same song, in a (slightly) different key.

"This phenomenon - the relative independence of the methods producing essentially the same big-picture results - is an indicator that the findings are fairly robust. While the concept of three different estimates of global-averaged temperature understandably confuses many people, it is quite healthy to examine planetary-scale temperatures using slightly different metrics."

So there you have it. Two (or three) different estimates for global temperature that are actually nearly the same. The main message is that multiple scientists, independently assessing sets of data are coming to nearly the same conclusions: The world keeps heating up as we pump more greenhouse gases into the atmosphere.

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