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Natural bumps in the atmosphere

Temperatures at Earth’s surface have been rising for decades, but in the upper atmosphere, the temperature is slowly dropping. When it comes to explaining this climate change, scientists have generally focused on human-related ozone depletion and greenhouse gases — but a new study shows that nature should not be ignored.

Globally, the lower stratosphere, a region of the atmosphere 12 to 22 kilometers above Earth’s surface, is today generally cooler than it was in 1979, but that decline in temperature has not always been steady. Satellite and balloon-borne observations from this time period reveal a pronounced step-like pattern of general cooling interrupted by warmer pulses.

“Lots of people have been working on trying to understand the changes in the stratosphere,” says V. Ramaswamy, a climate modeler at the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Fluid Dynamics Laboratory in Princeton, N.J. “In many ways, those changes are much [greater] than what has been happening at the surface.”

Much of the existing research on stratospheric temperatures, Ramaswamy says, focuses on ozone depletion. Ozone retains heat in the atmosphere by absorbing both short-wave radiation from the sun and long-wave radiation emitted from Earth’s surface. Because ozone has been decreasing steadily for decades, it is a major player in the overall cooling trend, he says.

The decrease in ozone, however, does not explain the whole picture — particularly two sharp spikes in temperature, which correspond in time to the volcanic eruptions of El Chichon in 1982 and Mount Pinatubo in 1991, and the subsequent step-like cooling. And no existing climate models had pulled apart these natural and human-related factors to sort out what caused these features, Ramaswamy says. “It indicated to us that there was something fundamental that we were missing.”

To better understand the observed changes, Ramaswamy and his team compared satellite temperature data from 1979 to 2003 with a series of models based on multiple natural and human-induced processes. Natural processes that change stratospheric temperatures include cyclic variations in radiation from the sun and volcanic eruptions, which eject particles into the stratosphere that scatter sunlight and cool the oceans (see Geotimes, January 2006). Although the layer of volcanic particles in the stratosphere reduces sunlight reaching Earth’s surface, they also absorb the sun’s radiation, heating up the upper atmosphere while they linger. Human impacts on climate include ozone depletion, changes in forestation, and increases in carbon dioxide, methane and other greenhouse gases.

Reporting in the Feb. 21 Science, the team found that models incorporating all of these factors provided the best fit to the observed temperature data. Overall, the models suggest that human factors cooled the lower stratosphere throughout the past two decades, punctuated by inputs from natural forces, such as the two large volcanic eruptions, that modified that trend.

Various model simulations isolated the effects of each of the factors, revealing their individual impacts on temperature. The team found that natural forces caused little change to the decadal trend except from the two brief eruption-related spikes. The effect of the eruptions was transient, Ramaswamy says, because volcanic particles remain in the atmosphere for only two or three years. Changes in ozone, however, led to a gradual temperature decline.

“It’s an important result,” says Dian Seidel, a climatologist at the NOAA’s Air Resources Laboratory in Silver Spring, Md. “What they’ve done is look at an issue that hadn’t been addressed quantitatively before, but had been noticed for a long time.”

Past linear models of atmospheric temperature changes are simple and provide “a decent fit,” but the data show much more complexity, Seidel says. “We need the whole suite [of influences] to explain the variability.”

The team’s models also reflect a turnaround in ozone depletion since 1997, Ramaswamy notes. That suggests interesting scenarios for the future: If ozone recovers, global stratospheric temperatures may return to 1979 conditions within a few decades. However, prolonged cooling in the stratosphere could also prevent ozone recovery (see Geotimes online, June 2000). At colder temperatures, ice clouds form in the stratosphere at polar regions that can speed the destruction of ozone.

Carolyn Gramling
contributing writer

"Volcanoes slow sea-level rise," Geotimes, January 2006
"Clouds in cohoots with ozone thieves," Geotimes online, June 2000

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