A new report from Darryn Waugh of the Earth and Planetary Sciences Department and his colleagues suggests that future patterns of air circulation in the atmosphere may prevent ozone levels from returning to their normal levels.
Ozone, which absorbs dangerous ultraviolet radiation before it hits the Earth's surface, has been depleted in recent decades due to the release of various chemicals into the atmosphere.
Part of a much larger collaborative project with the NASA Goddard Space Flight Center in Greenbelt, Md., this ozone study was based on a computer model looking into future climate conditions.
The model was chosen for being a "middle of the road" scenario, according to Waugh, and factored in aspects such as the concentrations of carbon dioxide and greenhouse gasses, temperature and air circulation.
How accurate was this model? "We don't know," Waugh said. "If we take a different course, impacts could be different."
The model found non-uniform changes in future ozone concentrations in different regions of the stratosphere. In the tropical lower stratosphere, the main change is an increased speed of air circulation. This faster moving air makes it more difficult for the chemical reactions that produce ozone to occur, resulting in decreased ozone levels.
In contrast, increasing concentrations of carbon dioxide in the upper stratosphere (40 to 50 kilometers up) actually cool the region, opposite of what they do in the lower part of the atmosphere.
This cooling slows the rate of ozone destruction. As a result, the ozone concentrations here remain elevated for longer - the ozone builds up in the upper stratosphere.
So if the ozone destruction is increasing in the lower stratosphere but decreasing in the upper stratosphere, shouldn't the effects cancel out? Not exactly.
The global warming effects Waugh and colleagues observed in the model are regionally different. In the tropics, the increased circulation of the lower stratosphere wins out, and we see more ozone destruction.
In the Southern mid-latitudes, the increased destruction of the lower layer is cancelled out by the decreased destruction in the upper layer. In the Northern mid-latitudes, the cooling effect in the upper stratosphere dominates, and more ozone is conserved.
"As it turns out, regional differences are not based on what greenhouse gasses are produced," Waugh said. The differences really arise from changes in the rate of air circulation, which affect the rate of ozone destruction reactions in the atmosphere.
Ozone is produced in the stratosphere by chemical reactions involving UV radiation and other compounds like greenhouse gasses.
The reactions depend on the temperature and always on the rate at which the air is flowing (circulation) - if the air is moving more quickly, the chemicals have less time in contact to react with one another.
What do the regional differences in global warming tell us about "going green?" The solution is the what we have known for years - reduce our emissions globally.
Where the pollutants originated is not a factor in the stratosphere ozone reactions; they all get mixed together rather thoroughly at those heights.
It is clear from Waugh's study that there is more to the story of global warming than simply our greenhouse gas emissions. Climate change, especially variations in air circulation and temperature due to the buildup of carbon dioxide, will play a large role in dictating the concentration of ozone in Earth's atmosphere.
The study should help scientists to determine more precisely the causes and sources of regional global warming, helping us paint a more accurate picture of our atmospheric impact.
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