Extra water vapor up high coul trigger destructive chemical reactions
Climate change may spur the destruction of ozone in unexpected parts of the globe.
In a warming world, many scientists believe, severe weather will become more common. That could be a problem in part because powerful rainstorms have the potential to erode ozone above the United States, researchers report online July 27 in Science.
“For 30 years, we’ve studied the problems of ozone loss and climate change separately,” says team leader James Anderson, a Harvard atmospheric scientist. “Now it’s pretty clear that climate change appears to be linked directly to the loss of ozone.” High-altitude ozone acts as a protective shield, blocking ultraviolet rays that can cause skin cancer.
Anderson and his colleagues stumbled on the unexpected connection while studying strong summer storms fueled by rising heat. During missions from 2001 to 2007, NASA planes flying close to the edge of space spotted water spewed high into the sky by convective storms over the U.S. The goal was to gather useful measurements for figuring out how high-altitude clouds form and trap heat.
But the data also revealed a possible threat to ozone molecules floating 15 to 20 kilometers up. Large storms often left behind extra water vapor in this part of the stratosphere, the remnants of melted ice crystals propelled upward.
That vapor could set the stage for a chemical chain reaction, Anderson says. Aerosol particles swollen with water can dissolve airborne hydrochloric acid. The acid can then react and form other chlorine compounds, including pairs of chlorine atoms. Sunlight cleaves those molecules, spitting out unstable chlorine atoms that break down ozone.
“It’s the same chemistry as that going on above the Arctic and the Antarctic,” says Anderson. Chlorine from human-made chemicals has chewed seasonal holes in ozone above Antarctica and caused thinning above the Arctic. These ozone-destroying chemicals are being phased out under the 1987 Montreal Protocol.
The team’s calculations suggest that the havoc wreaked by water vapor could continue for days after a storm, as humidity levels slowly fall. As much as 25 to 35 percent of ozone, over a horizontal distance of 100 kilometers, could be annihilated in a week.
“I was surprised that so much ozone could be removed in such a short time,” says Dale Hurst, an atmospheric chemist at the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences.
For now, the danger exists only on paper. Actual measurements tracking chlorine compounds in the stratosphere would help to confirm whether the damage is taking place and, if so, how widespread the problem may be. And while many climate simulations do call for more strong storms as temperatures continue to rise, the future is still somewhat fuzzy. A warmer atmosphere would hold more moisture but would also weaken the wind shear that whips up extreme weather.“It’s a bold idea that raises more questions than it answers,” says Andrew Dessler, a climate scientist at Texas A&M University in College Station. “But this is what great scientists do; they come with ideas that spur people to start working on new things.”
J. Anderson et al. UV dosage levels in summer: increased risk of ozone loss from convectively injected water vapor. Science. doi:10.1126/science.1222978.
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