Predicting future weather events has always been an uncertain affair, but for climate scientists today, one thing is certain: the earths atmosphere is getting hotter. Global average temperatures have risen steadily during the past century, due largely to the rising concentration of CO2 and other industrial greenhouse gas emissions. But John Daniel, climate scientist at the federal governments National Oceanic and Atmospheric Administration in Colorado, says a closer look at the data over the past decade revealed an anomaly. Since about 2001, it appears that the globally-averaged temperature has stopped going up as fast as it was going up in the decades before, Daniel said.
Meaning that the Earth is still warming, but at a slower-than-expected-pace. Daniel says neither climate scientists nor computer climate models predicted the slowdown. So Daniel, along with his U.S. and French collaborators, began to study systems that are not typically considered in atmospheric models, processes that could explain the slowing of the temperature increase. We also noticed that if you look at satellite observations, and you can also look at ground based observations from [the Hawaiian mountaintop observatory at] Mauna Loa, you see that stratospheric aerosols have been going up over this period. A lot of people in their models, after about the year 2000, neglected the impact of stratospheric aerosols, Daniel said.
Most of us are familiar with low-altitude aerosols: soot and other fine particulates from factories and vehicles that make up city smog. But Daniel says that unlike global-warming culprits such as atmospheric carbon dioxide, high-altitude aerosols actually cool the planet. The reason that these aerosols exert a cooling influence is because they reflect sunlight back to space that would have made it to the ground. Our understanding that stratospheric aerosols cool is not new. We've known that for a long time, Daniel said.
So why werent climate scientists accounting for the stratospheric aerosols in the first place? Although they are common closer to Earth, they are less abundant in the stratosphere. Terry Deshler, Professor of Atmospheric Sciences at the University of Wyoming, explains. "So a volcano such as [Mount] Pinatubo [in the Phillipines] threw a lot of aerosol particles up into the stratosphere and those probably were gone in about one year. But it also threw up a lot of sulfur gas. And all the sulfur in the stratosphere gets converted into sulfuric acid droplets. These particles are so small that gravity has a very slow [small] role so that the aerosol from the Pinatubo eruption persisted in the stratosphere for about 5 or 6 years, Deshler said.
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