Global warming and ozone depletion are usually thought of as separate problems. But atmospheric scientists have discovered that they may be intertwined. A study to be published later this month in the Journal of Geophysical Research-Atmospheres finds that climate change may substantially undo international efforts to restore the ozone layer by limiting emissions of ozone-eating gases called chlorofluorocarbons (CFCs).
The ozone layer lies in Earth's stratosphere, 10 to 50 kilometers above sea level. In the high atmosphere, ozone plays a crucial role in shielding the surface from harmful levels of ultraviolet light. This protective layer began a precipitous decline in the 1980s due to emissions of several gases, including CFCs, which were widely used in refrigerators and spray products. The ozone layer was expected to fully recover by about 2040, thanks to the Montreal Protocol of 1987, which phased out the use of CFCs.
Global warming may hinder that recovery, says Drew Shindell of NASA's Goddard Institute for Space Sciences in New York City. Using a computer model of the atmosphere, Shindell's team simulated what will happen if greenhouse gases, water vapor, and excess heat find their way into the upper atmosphere via such processes as convection from large tropical storms. Methane--a common greenhouse gas--is one troublemaker. In the harsh sunlight of the upper atmosphere, methane can react with other gases to form water vapor, which then breaks down into other chemicals that destroy ozone. Moreover, the warming makes the atmosphere damper (providing still more water vapor) and may cause the stratosphere to heat up, speeding the chemical reactions that destroy ozone. After incorporating all this into the model, Shindell predicts that by 2030, the stratosphere will have recovered only about one-fourth of its lost ozone.
Some details of the new model may prove to be controversial, particularly the degree to which water vapor can waft directly into the upper atmosphere. But John Wallace, an atmospheric scientist at the University of Washington, Seattle, says that Shindell's team has made a good case that changes in the lower atmosphere may have important effects on stratospheric ozone. Already, about half the water vapor in the stratosphere comes from the breakdown of methane, Wallace says, and methane concentrations in the atmosphere as a whole are continuing to increase.