Chinese coal consumption doubled between 2003 and 2007, emitting large amounts of sulfur compounds that reflect some sunlight back into space, counteracting the warming effect from rising amounts of greenhouse gases. Credit: Frederic Brown/AFP/Getty Images.
Some popular myths about climate change are like zombies. You can stab them, shoot them, beat them about the head with a baseball bat, and they just get up and keep going. One such undead notion is the idea that if humans are the cause of global warming, temperatures should be going steadily upward. Any significant slowdown in the temperature increase, the argument goes, and the whole theory collapses.
That’s nonsense, according to most climate scientists. The climate system has all sorts of moving parts — ocean currents, wind patterns, changes in vegetation and ice cover and dust particles in the air. Heat moves around in complicated ways. Global warming theory predicts an overall temperature rise over decades, but slowdowns, and even brief periods of cooling are not only possible; they’re inevitable. (So, for that matter, are periods of faster-than-average warming).
The decade that just ended is a case in point. While it was the warmest ten years on record, temperatures didn’t rise as fast during the 2000’s as they did for the previous 30 years or so. And while they aren’t at all surprised by this, scientists are still trying to figure out why it happened. “The fact is that we can't account for the lack of warming at the moment,” wrote Kevin Trenberth of the National Center for Atmospheric Research in one of the “Climategate” emails, “and it is a travesty that we can't.” (This quote was widely taken out of context to suggest — falsely — that he was expressing doubt in the underlying theory that human emissions of greenhouse gases, such as carbon dioxide (CO2), are helping to warm the planet.)
Trenberth himself has suggested that the slowdown in the 2000’s happened because excess heat was being stored temporarily in the deep ocean. But a new paper published in Proceedings of the National Academy of Sciences offers a different explanation. The cause, argues lead author Robert K. Kaufmann, of Boston University’s Center for Energy and Environmental Studies, is another type of pollution — specifically, sulfur emissions from Chinese power plants. “Chinese coal consumption,” he points out, “has doubled in just four years, and they were using plenty to begin with.”
Graph of surface temperature departures from average since 1880, from four different data sources. Note the overall increase in temperatures, despite the shorter periods with level temperatures or slight cooling. Credit: Climate Central and NASA.
If unchecked by pollution controls, the sulfur creates atmospheric particles that reflect some sunlight back into space, counteracting the warming effect from rising amounts of CO2. “The short story,” says Kaufmann, “is that all of a sudden the human effects on climate cancel out during this period.”
It wouldn’t be the first time this has happened. During the middle of the 20th century, the planet went into a slight cooling phase that lasted for about 30 years. The cause: a rise in sulfate particles as the world’s economy boomed after World War II. Anti-pollution measures that went into effect starting in the 1970’s cleaned up the air — and warming started to take off again. Now, Kaufmann points out, China is beginning to deal with its own polluted air by installing scrubbers on the smokestacks of coal-fired power plants, in part because sulfur emissions are contributing to acid rain.
Without sulfate particles to counter the effects of CO2, says Kaufmann, “there could be a big increase in warming — and we’ve already seen that happening in 2009 and 2010. The hiatus is over.”
Nobody's saying the Chinese should stop cleaning up their smokestacks, of course. Even if reflected sunlight is an unintended benefit of pollution, the harm from acid rain and other noxious substances is enormous. And while some proponents of “geoengineering” have suggested lofting sulfur compounds into the atmosphere on purpose to block the sun, the unintended consequences of such a step could be disastrous.
But not everyone accepts Kaufmann's analysis in the first place.
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.
“If climate change were driven only by radiative forcing,” says Levy, referring to changes in how much energy arrives on Earth from the Sun, and how much escapes back to space, “I would expect all of those [temperatures] to be leveling off.” But changes over a period as short as a decade, he says, “can also involve ocean stuff.”
Kevin Trenberth is equally unimpressed. “Ninety percent of the energy imbalance ends up in the ocean,” he wrote in an email, “and so it is the SSTs [sea-surface temperatures] and upper ocean heat content that matter for surface temperatures. The model they have does not appear to consider any of this and is not physically correct.”
Even worse from, 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 fact that they disagree about what went on in the 2000’s doesn’t take away even a little from this underlying agreement, nor does it detract from the consensus view that manmade emissions of greenhouse gases are causing global temperatures to increase over the longer-term.