Has “China Sky” Helped Slow Global Warming?

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When I lived in Hong Kong, I used to travel across the border into the People’s Republic, mostly for stories—like this one about the sex toy king of China—or for simple travel. (Particularly memorable were the all-night raves on the Great Wall of China which, sadly, have since been banned.) When I’d flip through my photos after I returned home, something always stood out: China sky. In both cities and the countryside, from the north to the south, the color of the sky even on supposedly clear days was a chalky white, the sun a sickly yellow disc barely visible. China sky was the unfortunate side effect of the country’s rapid economic growth, the smog partially a result of the new coal plant a week that was being built to power that transition. All that coal—in 2009 China consumed more than 3.5 billion tons of it, by far the most in the world—adds to climate change through the carbon released into the atmosphere, but it also leads to truly devastating health consequences from air pollution.

As it turns out, however, China sky may actually have another, surprising impact on global warming. For a while now scientists have been somewhat perplexed that the rise in the Earth’s temperatures paused for a time during the 2000s. It’s not that the Earth cooled—the last decade was the hottest on record—but global surface temperatures stopped showing a continuing rising trend even as carbon emissions grew year by year. Something had to be acting to offset the warming that should otherwise have been caused by increasing carbon concentrations in the atmosphere.

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According to a study published this week in the Proceedings of the National Academy of Sciences, we can blame—or thank—China and its coal industry. The authors of the study—led by Robert Kaufmann of the Center for Energy and Environmental Studies at Boston University—noted that during the time period there was an 11-year decline in solar input, as well as a cyclical shift from an El Nino to a La Nina climate pattern, which is associated with cooling. But the larger effect might have come from the rapid growth in Chinese coal combustion, which doubled between 2003 and 2007—, leading to an increase in sulfur emissions and that white China sky.

Sulfate particles can have a cooling effect on global temperatures because they can reflect sunlight back into space—something seen most recently in 1991, when the volcano Mt. Pinatubo erupted in the Philippines, spewing up to 30 million tons of sulfur dioxide high into the atmosphere. That led global temperatures to fall about 0.5 C in 1992 and 1993, before the sulfur eventually fell from the atmosphere. The sudden spike in sulfur from Chinese coal combustion over the past decade could have had a similar cooling effect that would have offset at least some of the expected warming from rising greenhouse gas emissions. It wouldn’t even be the first time that had happened—there was a similar slowdown in warming during the 30 years following World War II as the global economy boomed on the back of fossil fuels, only to see warming pick up as pollution controls kicked in and companies installed scrubbers in coal-fired power plants.

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The PNAS paper hasn’t convinced everyone, as my colleague Michael Lemonick writes in Climate Central:

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.

Other scientists question whether the oceans—which absorb far more heat than the atmosphere—might be playing a more important role in the slower warming. But it’s important to remember that, whatever the cause, this pause in increased global warming could be brief indeed. 2010 is already tied with 2005 as the hottest year on record, and global carbon emissions last year were the highest on record. Meanwhile China—more because of worries about health and pollution—is working to install scrubbers on its coal-fired power plants, which will remove sulfur and other conventional pollutants, but leave the carbon. So warming is likely to pick up again, as Professor Joanna Haigh of Imperial College London told the Guardian:

The researchers are making the important point that the warming due to the CO2 released by Chinese industrialisation has been partially masked by cooling due to reflection of solar radiation by sulphur emissions. On longer timescales, with cleaner emissions, the warming effect will be more marked.

Ironically, one of the most popular possible methods of geoengineering—directly cooling the climate to compensate for the greenhouse effect—would involve shooting sulfur particles into the stratosphere, to create an artificial, global version of China sky. At least now we know it will probably work—even if the results, judging from the People’s Republic, aren’t pretty.

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