Has Sulfate Pollution from Asia Masked a Decade of Warming?

Warming overwhelms the cooling effect of sulfates by about 2045 even if China and India continue to grow rapidly and delay pollution controls. Radiative forcing is a measure of influence that a climate factor has in altering the balance of Earth’s incoming and outgoing energy. Positive forcing tends to warm the surface, whereas negative forcing tends to cool it. A more detailed definition of radiative forcing is available here.

Science News, the Washington Post, and Climate Central have all written about a new study, published this week by the Proceedings of the National Academy of Sciences, that suggests a decade-long lull in global warming, which has caused some commentators to question the scientific underpinnings of climate change, stems from large increases in sulfur dioxide emissions in Asia. Between 2003 and 2007, global sulfur emissions have gone up by 26 percent. In the same period, Chinese sulfur dioxide emissions have doubled.

While burning coal is best known for emitting carbondioxide, a greenhouse gas, the sulfur dioxide the same process generates leads to the formation ofreflective sulfate particles that have the opposite effect on the climate. Releasing sulfates might seem, then, like a reasonable way to counteract global warming, but there’s a catch. Sulfates also cause acid rain and health problems. The World Health Organization estimates that air pollution, including sulfates, causes as many as 2 million premature deaths each year.

The combination of the contradictory coal burning impacts leaves policy makers in a bind: clean up the sulfates and accelerate the pace of global warming or allow sulfates to build up and people will die directly of air pollution. Reducing sulfate is relatively cheap and the health benefits don’t take long to realize, so most industrialized countries end up adopting pollution controls that reduce sulfate emissions. The United States, as well as industrialized European countries and Japan, cut sulfate emissions significantly in the 1970s and 1980s, and there’s little reason to believe that China will follow a different path.

In fact, the Chinese government is already in the midst of an effort to reduce sulfate pollution. A team of researchers, including NASA Goddard’s Mian Chin, used satellite imagery and other data about emissions to estimate sulfate emission trends in China in a 2010 paper published in Atmospheric Chemistry and Physics They found that sulfur dioxide emissions increased dramatically between 2000 and 2005, particularly in Northern China. But they also found that sulfur dioxide emissions in China, which I wrote about in an earlier post, began to decline in 2006 after the government began installing large numbers of flue-gas desulfurization (FGD) devices in coal power plants.

Since 2006, flue-gas desulfurization (FGD) devices in coal power plants have caused sulfur dioxide emissions from power plants in China to begin declining.

What does it all mean for the climate? In 2010, Drew Shindell and Greg Faluvegi of NASA’s Goddard Institute for Space Studies simulated a number of emission scenarios for China and India to find out. They looked, for example, at how the climate would respond if the Chinese and Indian economies continue to expand rapidly or only grow at a moderate pace. Likewise, they modeled what would happen if China and India instituted sulfate pollution controls immediately or waited a number of decades before doing so.

In their paper, Shindell and Faluvegi present their results, shown in the line graph at the beginning of this post, as a suite of projections. The strength of warming predicted depends on whether the economies continue to grow quickly and whether sulfate pollution slows, but there is one common – and concerning – similarity between all of the projections: regardless of how fast China or India grow or put off sulfate pollution controls, it’s not enough to mask warming from carbon dioxide in the long term, particularly in the mid-latitudes of the Northern Hemisphere where the climate impacts of sulfates from Asia are the most noticeable.

Here’s how the GISS authors explained the situation:

We find that while the near-term effect of air quality pollutants is to mask warming by CO₂, leading to a net overall near-term cooling effect, this does not imply that warming will not eventually take place. Worldwide application of pollution control technology in use in Western developed countries and Japan along with continued CO₂ emissions would lead to strong positive forcing in the long term irrespective of whether the pollution controls are applied immediately or several decades from now. Continued emissions at current (year 2000) pollutant and CO₂ levels may have little near-term effect on climate, but the climate ‘debt’ from CO₂ forcing will continue to mount. Once pollution controls are put into place as society demands cleaner air it will rapidly come due, leading to a “double warming” effect as simultaneous reductions in sulfate and increases in CO₂ combine to accelerate global warming. The only way to avoid this would be not to impose pollution controls and to perpetually increase sulfur-dioxide emissions, which would lead to a staggering cost in human health and is clearly unsustainable.