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.
In gathering and analyzing the data, the team says it incorporated recommendations for improving such temperature reconstructions from the National Research Council. The NRC offered up the recommendations in a report in 2006. That the NRC got involved at all highlights the challenge scientists can face when their work leaps from the pages of scientific journals few mere mortals read to become an icon for a politically controversial issue -- global warming. Congress requested the report after an acrimonious hearing in 2005 over the the hockey stick and the way Mann and his team derived it. Ironically, a number of other reconstructions since 1998 have reinforced his basic conclusion, although subsequent studies and their alternate ways of looking at the data led to graphs with different wiggles and jiggles along the way.
For all the thunder and fury the original hockey stick generated (and I trust Mann and his team have their hard hats at the ready again), it's not the hinge-pin argument for global warming. For that, researchers say, one turns to physics and the radiative properties of carbon dioxide, which Swedish chemist Svante Arrhenius painstakingly calculated -- longhand -- just before the turn of the 19th century. He estimated that if CO2 concentrations in the atmosphere double, global average temperatures would rise by 5 to 6 degrees Celsius. The latest estimates suggest a 2- to 4.5-degree average increase globally.
So why bother with contentious reconstructions? Gavin Schmidt, a scientist with the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York notes several reasons, not the least of which is to serve as a reality check on climate models.
Note: Eoin O'Carroll is on vacation. He will return Sept. 2.