Scientists studied whether China’s increased coal production had released enough sun-reflecting sulfur particles to counteract the warming effect from steadily rising CO2 emissions.
Emitting excess greenhouse gases is only one of the ways in which human industry, agriculture, and transport are affecting the atmosphere. As this study
indicates, human activity releases sulphate aerosols — tiny particles — but deciphering their precise impact is not straightforward. Other activities such as forest clearance also have an impact on local and global climate. Scientists are trying to measure the various forces that are changing the climate and to forecast what may happen in years to come, but it's complex. Still, just understanding that there are several impacts to take into account gives people insight into why temperatures aren't rising uniformly and constantly, even though greenhouse gas emissions continue to increase.11:48:33 AM
I'll be watching:
Understanding the aerosol contribution to global temperature change, and integrating it into climate models, is likely to be a major advance in the next Intergovernmental Panel on Climate Change report, due out in 2013.
The main author, Robert Kaufmann, gave an unusual insight into why he'd started this research: at a talk he'd given, someone had quoted Fox News as saying temperatures hadn't risen for a decade.
Most of the coverage I've seen of this study seems to just accept the result as though it's definitive. It's hardly that; it's a reasonable shot at explaining the slowdown in warming in the 2000s, but it's not the final explanation by any means. Other explanations — such as a theory that excess heat was being stored temporarily in the deep ocean — have been proposed, and scientists have pointed out that while coal use is indeed growing in China, it’s decreasing in other parts of the world. Hiram Levy, a climate modeler at NOAA’s Geophysical Fluid Dynamics Laboratory near Princeton, N.J., estimates an overall increase of 10 percent in sulfate emissions over the last decade, which wouldn’t be enough to explain the slowdown.
Anthropogenic warming has never been predicted to be steady. Understanding episodes of faster and slower warming has always been a goal of climate scientists.
Human vs. Nature:
Another paper, published in Science this week, comes to a different, but not necessarily contradictory, conclusion about what caused the plateau. While the PNAS paper posits that the culprit is human-made aerosols in the lower atmosphere, the Science paper says it could be human-made or natural aerosols (such as sea salt, dust, and volcanic ash) — or both — higher in the stratosphere.