New research suggests that sulphur emissions from China’s coal-fuelled power stations have masked global warming in recent years. From The Guardian:
The last decade was the hottest on record and the 10 warmest years have all occurred since 1998. But within that period, global surface temperatures did not show a rising trend, leading some to question whether climate change had stopped. The new study shows that while greenhouse gas emissions continued to rise, their warming effect on the climate was offset by the cooling produced by the rise in sulphur pollution. This combined with the sun entering a less intense part of its 11-year cycle and the peaking of the El Niño climate warming phenomenon ….
The cooling effect of sulphur pollution on climate has long been recognised by scientists studying volcanic eruptions, which have, for example, caused failed crops and famines in the past. Sulphur dioxide forms droplets of sulphuric acid in the stratosphere, which increases the reflection of the Sun’s heat back to space, cooling the Earth’s surface ….
[The authors] emphasised the rapid increase in coal burning in Asia, and in China in particular, noting that Chinese coal consumption doubled between 2002 and 2007: the previous doubling had taken 22 years.
Michael E Mann, at Pennsylvania State University and not part of the research team, said the study was “a very solid, careful statistical analysis” which reinforces research showing “there is a clear impact of human activity on ongoing warming of our climate”. It demonstrated, Mann said, that “the claim that ‘global warming has stopped’ is simply false.”
At OnEarth, however, Mike Lemonick presents some dissenting views:
It all sounds logical, and, says Hiram Levy, a climate modeler at NOAA’s Geophysical Fluid Dynamics Laboratory near Princeton, NJ, “the idea is physically sound.” But he’s not convinced that this is what’s really happening. Coal use is indeed growing in China, but it’s decreasing in other parts of the world. Globally, he estimates an overall increase of 10 percent in sulfate emissions over the last decade, which wouldn’t be enough to explain the slowdown. “At the same time,” he says, “there’s been a 10 percent increase in black carbon [emissions]” — soot, essentially, which tends to absorb sunlight and warm the air ….
Even worse from, [Kevin] Trenberth’s perspective, is the fact that the new paper cites the period from 1998 to 2008 as the span over which temperatures were relatively flat. But as climate scientists have explained ad nauseam, 1998 was an unusually warm year, thanks to a strong El Niño event in the Pacific. Choosing 1998 as a starting point (as many climate skeptics do) inevitably makes any temperature increase that follows look artificially small.
Trenberth, Levy and Kaufmann all agree, however — as do virtually all climate scientists — that slowdowns in global warming, some lasting as long as a decade or more, are not just expected, but inevitable as manmade greenhouse-gas emissions heat the planet. They further agree that these can come from natural effects as well as artificial ones like sulfur emissions from coal-fired power plants.
The deliberate injection of sulphur compounds into the upper atmosphere has been mooted as a possible means of counteracting temperature increases. Such schemes were controversially featured in the 2009 book Superfreakonomics: see an argument against the proposals from MIT’s Technology Review, a defence by the book’s authors, and a related Q&A from Wired magazine on geoengineering, “A Bad Idea Whose Time Has Come”.