Trend in surface temperature departures from average since 1880, from four different data sources. Note the pause in warming from the 1940s through the 70s, and again from about 1998 to 2008. (Climate Central)

A new study published this week offering a fresh take on what may have driven a temporary, 10-year slowdown in global warming reinforces the scientific hypothesis that human activities are contributing to long-term global warming.

Researchers have published several studies seeking to explain a relative plateau in global warming that took place between 1998 and 2008, compared to the longer-term period since the beginning of the industrial revolution during which the climate has warmed significantly. Some studies have focused on “internal climate variability” that is poorly understood and poorly anticipated, others on changes in water vapor in the upper levels of the atmosphere.

But the new study, by American and Finnish researchers, says the answer lies mainly in the quickening pace of coal consumption in Asia, particularly in China, and the air pollution that is causing.

Reflecting China’s rapid economic growth, Chinese coal consumption more than doubled in just four years between 2003 and 2007, causing a 26 percent jump in global coal consumption during this period, the study states. This added coal use, much of it in the absence of stringent air pollution controls, caused more sulfur emissions to go into the atmosphere, the study states. Unlike greenhouse gases such as carbon dioxide, which are also emitted by coal-fired power plants, sulfate aerosols act as a cooling influence on the climate, by reflecting incoming solar radiation back toward space.

The study finds that the surge in these emissions, plus a weakening phase of the 11-year solar cycle, and an assist from La Nina conditions in the Pacific Ocean, helped to temporarily reduce the warming effects of greenhouse gases.

“What’s going on is, human activities do two things: They cool the planet and they warm the planet. People normally just focus on the warming effect of CO2 (carbon dioxide), but during the Chinese economic expansion there was a huge increase in sulfur emissions,” study lead author Robert K. Kaufmann of Boston University, told the Associated Press.

A map showing the average amount of sulfate aerosols worldwide between 2000 and 2006. Note that the highest levels are across eastern China. (NASA)

Natural processes quickly rid the atmosphere of sulfur emissions, whereas carbon dioxide remains for centuries or longer. Therefore, as sulfur emissions drop again, global temperatures should rise at a faster pace. Last year, for example, tied for the warmest year on record, according to the National Oceanic and Atmospheric Administration and NASA.

Here’s one of the key excerpts from the study:

The finding that the recent hiatus in warming is driven largely by natural factors does not contradict the hypothesis: “most of the observed increase in global average temperature since the mid 20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations (14).” As indicated in Fig. 1, anthropogenic activities that warm and cool the planet largely cancel after 1998, which allows natural variables to play a more significant role. The 1998-2008 hiatus is not the first period in the instrumental temperature record when the effects of anthropogenic changes in greenhouse gases and sulfur emissions on radiative forcing largely cancel. In-sample simulations indicate that temperature does not rise between the 1940’s and 1970’s because the cooling effects of sulfur emissions rise slightly faster than the warming effect of greenhouse gases.

Predictably, climate skeptics are hailing the study as a partial vindication. Finally, they’re cheering, someone is acknowledging that there wasn’t much warming between 1998 and 2008 (the study doesn’t state there wasn’t warming at all during this period, but instead finds there was “little” warming).

David Whitehouse, a researcher with the Global Warming Policy Foundation, a British group of climate skeptics, wrote: “It is good news that the authors recognise that there has been no global temperature increase since 1998. Even after the standstill appears time and again in peer-reviewed scientific studies, many commentators still deny its reality.”

Skeptic blogger Anthony Watts dismissed the study by saying, “My take on it from the paper – We don’t know what’s going on, but we aren’t going to admit that.”

However, Watts, Whitehouse and others ignore the main finding of the study – that the small amount of warming that occurred between 1998 and 2008 is completely consistent with the hypothesis that human activities are helping to warm the climate over the longer-term. And with climate change, it’s the longer-term that matters.