The fact that the average global temperature hadn’t budged in 10 years had long puzzled scientists as well, including Robert Kaufmann, professor in the Department of Earth & Environment. When an audience member raised the issue at a public lecture he gave on global climate change to senior citizens in New Jersey, Kaufmann resolved to find out why. Suspicious that something was counterbalancing the warming effects of GHGs, he pored over U.S. Department of Energy data on global coal consumption, and soon found what appeared to be a smoking gun: a 26 percent rise in global coal consumption between 1998 and 2008.

Taking a closer look at the data, Kaufmann was astounded to discover that during that same period, China had significantly increased its use of coal-burning power plants, doubling its coal consumption in only four years (2003–2007). The plants filled the sky with sulfur dioxide particles, reflecting solar energy away from Earth and thereby cooling the ­atmosphere.

“There was a tremendous increase in sulfur emissions and a slight waning in solar activity, and those things offset to some degree the increased warming due to greenhouse gases,” says Kaufmann, disparaging climate skeptics’ exclusive focus on GHGs. “If you’re just focusing on greenhouse gases, all you see is those things going up and up and up, but you don’t see the sun weakening or the cooling effect of sulfur aerosols. When you put all those factors together, a good climate model will show a hiatus in warming.”

Kaufmann’s next step was to show that by taking all relevant factors into account, the theory of anthropogenic, or human-impacted, climate change would account for the pause in warming between 1998 and 2008. Toward that end, he sought to demonstrate that a statistical model of global climate estimated with observations before 1998 would replicate the global temperatures that meteorologists had recorded between 1999 and 2008.

At a climate science meeting at the Massachusetts Institute of Technology, where an MIT physicist described a new spectral technique to analyze the impact of human activity on global temperature, he ran into a very well-respected econometrician from Harvard University, James Stock. Stock had to leave early, but on his way out, he turned to Kaufmann and said, “Contact me. We can do a lot better than this.” The two scientists subsequently met and Kaufmann became confident he could work with Stock to develop a statistical model of the global climate that they could apply to the problem at hand.

Working with Stock, a researcher from the University of Turku in Finland, and then-BU PhD student Michael L. Mann, Kaufmann took the model and ran it for the designated time period using data they assembled on both anthropogenic factors, including GHGs and sulfur emissions, and natural influences such as incoming solar radiation and El Niño and La Niña, cyclical warming and cooling patterns across the Pacific Ocean.

“We found that the observed temperature was well within the confidence range of the forecast generated by our model,” says Kaufmann, who eventually obtained the same results with three other statistical models. “Our model, based upon statistical relations before 1998 and using the factors that anthropogenic climate change argues are important, could account for the lack of warming during that decade.”

Kaufmann and colleagues published their results in Proceedings of the National Academy of Sciences in July 2011. Despite this scientific evidence that climate change is indeed influenced by human activities, conservative pundit Rush Limbaugh trashed the study. Hate mail to Kaufmann followed. Undaunted, the climate scientist is now collaborating with another econometrician to model global climate change over the past 400,000 years.

“A lot of people have tried to raise doubts about whether humans are responsible for climate change,” says Kaufmann, “but we found that there’s no scientific basis to doubt that humans are responsible for the rise in global temperature over the past 150 years and the need to take action to address it.”

Looking ahead, Kaufmann reports some good and bad news. The positive is that China now incorporates scrubbers in its coal-burning power plants to reduce sulfur emissions, raising expectations that the atmosphere will become cleaner. However, as GHG concentrations continue to rise, this “cleaner coal” may usher in a period of rapid global warming.