Isolating a single factor as the main cause of any phenomenon is usually a tricky disposition for scientists. Particularly in climate science, where there still resides much uncertainty over the relative contributions of individual weather characteristics to the climate patterns we've witnessed over the last few years, attempting to draw a direct line between a single event and a specific factor is almost always inadvisable. Yet that is exactly what four NOAA scientists have done in a paper in which they claim that greenhouse gases are behind over half of last year's record-breaking heat wave in the U.S.
While NOAA had originally announced in a press release that El Niño and increasing GHG levels were both to blame for 2006's unprecedented warmth, it hadn't specifically designated one as having a greater effect over the other. A group of scientists from NOAA's Earth System Research Laboratory in Boulder, Colorado, decided to find out which, if any, had a larger impact. After scrutinizing the effect of 10 El Niño warmings of the tropical Pacific on U.S. temperatures, they found that these events had actually had a slight cooling effect on northern states. In separate trials using two distinct climate models to simulate the same El Niño effect, they confirmed this cooling trend. They thus concluded that "it was very unlikely that El Niño either caused or materially contributed to the record 2006 warmth." To assess the contribution of greenhouse gases to the warming, they analyzed simulations from 18 models that had GHG levels rising from the late 19th century to the present time.
They determined that the simulated greenhouse warming, averaged over the models, spanned the entire contiguous U.S., much as did last year's unprecedented warmth. Greenhouse gases thus accounted for "more than half of the observed warmth," they concluded.
Though greenhouse gas levels are expected to continue reaching new highs, the NOAA scientists don't expect to see back-to-back record warmths, placing the odds of such a phenomenon at 16%. Yet given the fact that spring 2006 was the fifth warmest spring on record and that GHG emissions will continue their dramatic rise in subsequent years, we're more likely than not to see additional record warmths in the near future.
Via ::Science: Record U.S. Warmth of 2006 Was Part Natural, Part Greenhouse (magazine, subscription required)