Global warming could open new holes in Earth's ozone layer at latitudes that until now have seemed immune to the ozone destruction that recurs over Antarctica and the Arctic, a new study warns.
The underappreciated keys to this conundrum: water vapor and temperatures in the lower stratosphere, where the ozone layer appears. Both, the researchers say, reach summertime values over the continental United States known to encourage ozone-destroying chemicals that are already aloft to attack ozone.
The team makes no attempt to project when significant erosion might be expected to occur. And researchers have yet to make the measurements that would confirm that the reactions the study describes are occurring. Rather, it points to conditions that are appearing and are known to stimulate stratospheric ozone destruction.
The results suggest a clear link between climate change and the amount of harmful ultraviolet radiation reaching Earth's surface, says James Anderson, a Harvard atmospheric scientist and the lead author of the study, which appears in Friday's issue of the journal Science.
When people talk about global warming, "most people would have their backyard warmer than it is now," he says. But global warming "is not about that," he adds. "It's about these feedbacks that couple the system into irreversible paths forward. That's the primary concern."
At ground level, ozone is a pollutant – a key element of smog. In the stratosphere, however, it shields organisms on Earth from destructively excessive levels of ultraviolet light from the sun.
The study by Dr. Anderson's team is based on well-established atmospheric chemistry and observations over the continental US of an underappreciated character in the ozone story: summer thunderstorms.
In essence, the team found that thunderstorms and their powerful, convective updrafts drive unexpectedly large concentrations of water vapor high into the stratosphere. The high concentrations of water vapor alter conditions in ways that encourage ozone destruction when the man-made chemicals associated with ozone depletion are present.
If the frequency and intensity of mid-latitude storms increase with time, as some global-warming models suggest, Anderson and his colleagues say they are concerned that ozone destruction at mid-latitudes could become one of those irreversible feedbacks.
"This is a really important find to nail down," says Mary Barth, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo., noting that it begs for field measurements to see how valid the warning is and how widely any effect might reach.
Dr. Barth, who was not part of Anderson's team, and colleagues have just completed a field project gathering data on thunderstorms and their impact on atmospheric chemistry, including ozone reactions.
In fact, Anderson's team initially was working on another climate problem: the formation of cirrus clouds and their effect on climate. Clouds represent one of the big areas of uncertainty in climate models. And cirrus clouds can have a warming effect on climate by allowing sunlight through but trapping heat rising up from underneath. The tops of thunderheads are one source of cirrus clouds, which assume their featherlike appearance as high-altitude winds blow across the tops of the thunderheads, giving them their anvil shape.
Researchers have known that the most powerful thunderstorms, whose strong updrafts can punch through the boundary between the troposphere (where weather happens) and the stratosphere, can deposit large amounts of water vapor in the stratosphere. But Anderson's team was using high-altitude aircraft to measure water vapor rising from the tops of a broader range of thunderstorms.
"We weren't hunting for big game here," he says. The aircraft were taking measurements from garden-variety summer thunderstorms over the Midwest and along the East Coast.
"To our huge surprise, we discovered that those convective storms were not simply distributing water in the upper troposphere," he says. Rather, they were lofting water vapor three to four miles into the stratosphere. Plumes often reached altitudes exceeding 60,000 feet.
Anderson had been deeply involved in studies of ozone depletion over Antarctica and the Arctic. What researchers learned about Arctic ozone depletion in particular triggered alarm bells. There, ozone depletion was all about the interplay between water vapor, temperature, the ozone-destroying compounds, and tiny particles known as sulfate aerosols – which act as tiny platforms on which chemical reactions can occur.
As water-vapor concentrations rose, so did the temperature at which the ozone-eating reactions could begin to take place, suggesting that the temperatures and water vapor were the key controls.
Under commonly observed conditions of water vapor and aerosol concentrations, the temperature at which ozone-depleting reactions would take place is about minus 119 degrees F., the researchers explain. Double the amount of water vapor, and the temperature at which reactions start rises by about 10 degrees F., bringing it within the range of temperatures commonly found in the stratosphere during mid-latitude summers.
Anderson's team studied data from the continental US, but it notes that similar conditions may appear over other parts of the globe.
The study points to "the potential for a pretty significant effect on stratospheric ozone at latitudes where we normally wouldn't think that would happen," said Mario Molina, an atmospheric scientist at the University of California at San Diego who shared the Nobel Prize in Chemistry in 1995 with the late Frank Rowland. The two worked out the chemistry behind stratospheric ozone depletion and chlorofluorocarbons, once widely used as refrigerants and aerosol-spray propellants.
"Many ecological systems are quite sensitive to ultraviolet radiation, and they have not evolved the repair mechanisms for more severe ozone depletion," Dr. Molina said in a prepared statement.
The 1987 Montreal Protocol and subsequent agreements have played important roles in halting production of key ozone-depleting chemicals, Anderson notes. But these compounds remain in the stratosphere for decades. Moreover, the sulfate aerosols there occur naturally: They are not affected by controls on sulfur emissions at ground level.
Without reductions in greenhouse-gas emissions – especially carbon dioxide, which itself remains in the atmosphere for centuries – ozone holes once limited to the poles could become regular summertime features over some of the most populated places on the planet, the team suggests.
With the Montreal Protocol, many people said in effect, problem solved – let's move on, Anderson says, acknowledging that even he had dealt with ozone destruction and global warming as separate issues.
People "completely forgot about the remarkable dependence of the destruction of ozone on water and temperature," he says.