Over at Greenwire, Paul Voosen has reported a very nice article on the debates over why average global temperatures have remained essentially flat for nearly a decade now. Here's a tempting excerpt:
Why, despite steadily accumulating greenhouse gases, did the rise of the planet's temperature stall for the past decade?
"If you look at the last decade of global temperature, it's not increasing," Barnes said. "There's a lot of scatter to it. But the [climate] models go up. And that has to be explained. Why didn't we warm up?"
The question itself, while simple sounding, is loaded. By any measure, the decade from 2000 to 2010 was the warmest in modern history. However, 1998 remains the single warmest year on record, though by some accounts last year tied its heat. Temperatures following 1998 stayed relatively flat for 10 years, with the heat in 2008 about equaling temperatures at the decade's start. The warming, as scientists say, went on "hiatus."
The hiatus was not unexpected. Variability in the climate can suppress rising temperatures temporarily, though before this decade scientists were uncertain how long such pauses could last. In any case, one decade is not long enough to say anything about human effects on climate; as one forthcoming paper lays out, 17 years is required.
For some scientists, chalking the hiatus up to the planet's natural variability was enough. Temperatures would soon rise again, driven up inexorably by the ever-thickening blanket thrown on the atmosphere by greenhouse gases. People would forget about it.
But for others, this simple answer was a failure. If scientists were going to attribute the stall to natural variability, they faced a burden to explain, in a precise way, how this variation worked. Without evidence, their statements were no better than the unsubstantiated theories circulated by climate skeptics on the Internet.
"It has always bothered me," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research. "Natural variability is not a cause. One has to say what aspect of natural variability."
Over at her blog, Georgia Institute of Technology Judith Curry offers some tart observations about what the hiatus means for "consensus" over man-made global warming.
To see the scientific process in action, the whole excellent Greenwire article is well worth reading.