Ocean Currents and Sulfur Haze Deliver Global Warming Hiatus: But Increasing Temperatures Remain in Long-Term Forecast

Article excerpt

Climate scientists say it's a puzzle but no surprise that global temperatures haven't risen as fast since 2000 as in the few decades before that.

Several new studies may solve at least part of that puzzle. The warming slowdown boils down, in part, to several factors: Coal-burning power plants, particularly in Asia, have spewed more light-reflecting particles into the atmosphere and cooled the planet. So too have volcanic eruptions. Meanwhile, natural climate fluctuations have combined with one another to squirrel away heat deep in the oceans.

Together, these observations bolster scientists' understanding of how different physical processes affect Earth's temperature.

The bottom line for climate remains the same: The buildup of greenhouse gases is causing more of the sun's energy to be trapped in the atmosphere instead of radiated back into space. Global temperatures have been rising as a result.

Yet the 2000s, even though they were the warmest decade on record, did not warm as fast as the three decades before. "After around 2001 the temperatures seem to flatten out a bit compared to the '80s and '90s," says John Daniel, an atmospheric physicist with the National Oceanic and Atmospheric Administration in Boulder, Colo.

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Many researchers have wondered if tiny particles called aerosols could be partly responsible. Aerosols come from both man-made sources, like coal-burning power plants and biomass burning, and natural sources like sea spray, dust and volcanoes.

Daniel and his colleagues, led by recently retired NOAA atmospheric scientist Susan Solomon, looked at aerosol measurements taken from satellites and from the Mauna Loa observing station in Hawaii. The team found more aerosols than expected in the stratosphere, or upper atmosphere, between 2000 and 2010. Some of those extra particles probably came from power plants, but some had to have come from relatively small volcanic eruptions like those of Soufriere Hills in Montserrat and Tavurvur in Papua New Guinea. Both erupted in 2006.

The last really big eruption that spewed aerosols high into the stratosphere was Mount Pinatubo in the Philippines in 1991. Many scientists assumed that stratospheric aerosols coming from volcanoes had dropped essentially to zero once the Pinatubo particles fell out of the atmosphere several years later.

But small eruptions can have a discernible influence on climate, Solomon's team reports online in Science on July 21. Using computer models, the researchers calculated how much those extra aerosols cooled things: about 20 percent more than would be expected without them, Daniel says. …