Northern-hemisphere temperatures over the past decade are likely to have been warmer than at any time in the past 1,300 years -- perhaps over the past 1,700 years if tree rings have anything useful to say about it. Either time span embraces the warmest years of the so-called Medieval warm period.
That's the latest from a team led by Penn State University's Michael Mann, who heads the university's Earth System Science Center. And the graph illustrating the take-home message? It still looks a lot like the much-battered, but still rink-ready stick of 1998. Today the handle reaches further back and it's a bit more gnarly. But the blade at the business end tells the same story.
The temperature data, which actually span the globe, come from weather records dating back to 1850. To reach further back, the team relied on natural stand-ins, or proxies, that include ice cores, ocean and lake sediments, mineralized "rings" from slices of stalactites or stalagmites in caves, coral growth rings, and tree rings. Data from the southern hemisphere are too sparse to reach deep into the past with much certainty, either for the southern hemisphere or globally. The team points out that the past decade's warming has been unusual for both, at least over the past 1,500 years. But because of the paucity of data south of the equator, they can't rule out the possibility that for brief periods in the past, recent warming might have been topped. Overall, the researchers use the word "likely" in describing their conclusion regarding northern-hemisphere temperatures in the same way the UN-sponsored Intergovernmental Panel on Climate Change does: There's a 66 to 90 percent chance we're right. The work appears in this week's edition of the Proceedings of the National Academy of Sciences.