This is a guest post by Jamie L. Vernon, Ph.D., a research scientist and policy wonk, who encourages the scientific community to get engaged in the policy-making process

While many of us were howling about global warming over the last decade, Earth’s surface temperature actually failed to significantly increase. Yes, I said it. Global surface temperature showed little warming between 1998 and 2008. But, don’t go and broadcast the demise of the global warming movement quite yet. The reasons for the cooling trend are not encouraging. In fact, they are quite threatening. And, if environmentalists have their way (and I think they should), global warming will reemerge and may do so at an alarming rate.

A team of researchers led by Harvard professor James Stock have determined that gases resulting from human activities in conjunction with natural variables can explain the “1999-2008 hiatus in warming.” Using published statistical models, they were able to demonstrate that a rapid increase in coal consumption in Asia likely generates sufficient sulfur emissions to reduce global surface temperatures. They write,

We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations.

In other words, despite the influence of other natural variables, sulfur dioxide is the major driver of recent temperature fluctuations. Sulfur dioxide is a natural by-product of burning coal. Accumulation of sulfur dioxide aerosols in the atmosphere reflects the sun’s rays leading to a cooling effect on global surface temperatures. Because emissions from human activities greatly exceed natural production, increased dependence upon coal-based energy production can lead to sulfur dioxide-driven cooling effects that counteract the warming caused by increasing carbon dioxide.

The authors cite China’s growing dependence on coal as an energy source to explain the increase in sulfur emissions. From 2003 to 2007, Chinese coal consumption more than doubled. Prior to that, it took 22 years for China to double its coal usage. Whereas global coal consumption increased by 27% from 1980 to 2002, the recent Chinese growth rate which occurs over a 4 year period (5 times the previous rate) represents 77% of the 26% rise in global coal consumption.

So why not rely on sulfur dioxide as a geoengineering tactic for regulating global warming?

The sulfur dioxide produced by these coal-fired power plants is a pollutant that contributes to the production of acid rain. Forests, crops, buildings, aquatic life and human health are all negatively impacted by acid rain. In 1963, motivated by the environmental movement, the U.S. Congress passed the Clean Air Act (CAA), which established standards for regulating pollutants such as sulfur dioxide. However, it took until 1990 for Congress to strengthen CAA enough to force the coal industry to significantly cut or trap sulfur emissions. This legislation successfully decreased sulfur dioxide emissions by 40% from 1990 levels. As a result, the reduction of sulfur dioxide emissions dramatically reduced the cooling effects associated with these gases. Thus, removing sulfur dioxide to protect crops, forests, wildlife and human health resulted in the warming trend observed between 1990 and 2002.

Given the negative effects of sulfur dioxide on the environment and human health, we should expect Chinese environmentalists to act to reduce these pollutants. Indeed, China has already made some moves in this direction. Subsequent temperature increases will very likely be more dramatic than those observed during the 1990’s, because the global community has done little to reduce the warming effects that will occur due to continual accumulation of carbon dioxide in the atmosphere.

For those who would like to use this as evidence against global warming, I invite you to closely examine the following graph from the paper described herein. You’ll notice that the window of time during which this temperature stabilization was observed occurs at the high end of a 100 year warming trend. The temporary cooling does not suggest that warming observed since 1910 has been reversed or that we shouldn’t expect additional warming in the future.

Follow Jamie Vernon on Twitter or read occasional posts at his personal blog, “American SciCo.”