China's soaring coal consumption in the last decade held back global warming as sulfur emissions served as a coolant, according to a study that takes head-on a key argument of climate skeptics.

While 2005 and 2010 are tied as the hottest years on record, skeptics have charged that an absence of a steady rise from 1998 to 2008 disproves the view that people are heating up the planet through greenhouse gas emissions.

Robert Kaufmann, a professor at Boston University, said he was motivated to conduct the study after a skeptic confronted him at a public forum, telling him he had seen on Fox News that temperatures had not risen over the decade.

"Nothing that I had read that other people have done gave me a quick answer to explain that seeming contradiction, because I knew that carbon dioxide concentrations have risen," Kaufmann told AFP.

The US-Finnish study, published in the Proceedings of the National Academy of Sciences, named a culprit -- coal.

The burning of coal jumped in the past decade, particularly in China, whose economy has grown at breakneck pace. Coal emits sulfur, which stops the Sun's rays from reaching the Earth.

Kaufmann said that there was a precedent -- greenhouse gas emissions also soared in the post-World War II economic boom in Western countries and Japan.

"What happened was at the same time, sulfur emissions increased very rapidly, thereby canceling much of the greenhouse gas effect," Kaufmann said.

Global temperatures rose after the early 1970s when major developed nations started to take action to curb sulfur emissions, the study said. Global coal consumption again rose by 26 percent between 2003 and 2007, with China accounting for more than three quarters of the increase.

China remains the world's largest greenhouse gas emitter and its output keeps rising. But it has also started to take action to address rampant pollution, including by installing scrubbers on its coal plants.

"So we already see temperatures starting to increase again. It rose in 2009, it rose in 2010 and that may be one reason for that increase," Kaufmann said of the Chinese moves.

Both the US and Chinese governments want a a future for coal, a major industry. But while sulfur serves as a temporary coolant, it also contributes to major problems, such as acid rain and human respiratory problems.

Turning to sulfur to curb global warming is like saying, "We'll pick our poison," Kaufmann said.

"You could certainly make that argument, but I don't think many people would view that as a very satisfactory solution, especially if it meant living in a very polluted atmosphere like in China," he said.

The study's co-authors included Michael Mann, a prominent member of the UN scientific panel whose landmark 2007 report warned that climate change was unequivocal and mostly caused by humans.

The study also found additional factors that limited warming in the period, including a natural dip in solar activity and the effects of the El Nino and La Nina ocean patterns.

Joe Romm, a fellow at the left-leaning Center for American Progress, criticized the study for speaking of a hiatus in surface temperatures. He pointed to record temperatures in 2005 and 2010 and a rise in ocean heat.

"There has been no hiatus in global warming," Romm wrote on his blog, saying that the years 1998 and 2008 were "the favorite cherry-picked endpoints of the deniers" due to outside factors such as El Nino and La Nina.

Climate change skepticism has been on the rise in the United States. Leading lawmakers in the Republican Party, which triumphed in last year's congressional elections, argue that the science is unproven and that action would be too costly to the economy.

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