It was Anderson’s most important contribution to science to date. And it was the final piece of the puzzle needed to move public policy, culminating in the 1987 Montreal Protocol, now signed by 197 countries that agreed to phase out CFCs.
In 2005, the United Nations lauded Anderson for “his elegant measurements and brilliant analysis of ClO radical concentrations over Antarctica,” demonstrating how CFCs are “responsible for the observed massive springtime ozone depletion.”
The rest of the world may have thought the ozone problem had been solved, but Anderson wasn’t so sure. He persisted in his high-altitude research forays. ER-2 flights from Bangor, Maine, in 1992, found “extremely high ClO over the United States,” he recalls. In 2000, flights from Sweden showed that “the arctic was beginning to emulate” the “massive ozone loss” over Antarctica, as he put it. (The Sweden mission was slightly delayed when a Russian general, who was scheduled to fly in a DC-8 chase plane with Anderson as the spy plane flew over Russia, vanished briefly. Anderson thought he had been going to the head, but the general was taking forever. It turned out he was conferring by phone with officials in the Kremlin, telling them one last time that the U-2 they’d soon notice in Russia’s skies was doing science, not espionage, and to please not shoot it down.)
Those discoveries should have served as a wake-up call that, for all the good the Montreal Protocol did, ozone loss was not a thing of the past. “But NASA [which had funded much of Anderson’s work] said we’re declaring victory against ozone loss and going after climate change by studying clouds,” he says. Among the many unknowns about how climate will change in a world warmed by a blanket of greenhouse gases—mostly carbon dioxide from burning fossil fuels—is whether clouds will slow or accelerate global warming.
Anderson decided to tackle one piece of that puzzle: the formation of cirrus clouds. Clouds, of course, are made of water vapor. On summertime flights to measure water vapor starting in 2001, Anderson’s team kept getting “deadly boring” results, the same 4.5 to 5 parts per million of water in the stratosphere. In 2005 and 2007, however, flights over Florida and then Oklahoma found “to our shock and surprise,” Anderson says, that thunderstorms were injecting water molecules as high as 12 miles into the stratosphere, reaching the ozone layer. It wasn’t a rare event, either: About half the flights found the high-altitude water. As Anderson and his colleagues wrote with the usual academic understatement in Science last summer, “What proved surprising is the remarkable altitude to which large concentrations of water vapor are observed to penetrate.”
“I went to NASA and said we have a big problem here,” says Anderson. Go away, the agency told him; we’ve moved on, now that the world had solved the ozone problem by phasing out CFC production.
Anderson persisted (again) and began writing more and more insistent letters up the NASA chain of command. He finally got a sympathetic hearing from Ken Jucks, manager for the agency’s Upper Atmosphere Research Program. Together, they wrested enough financial support for Anderson to keep his team together and analyze what the raw data from the flights were trying to tell him.
What happens is that the strong thunderstorms—those about 30 miles across—create powerful updrafts, essentially gaseous elevators that carry warm, humid air miles into the atmosphere. Usually, the gaseous elevator stops short of the stratosphere. But if a storm is strong enough, the updraft can blast through the boundary between the lower atmosphere and the stratosphere, reaching the latter and spreading 60 miles or more in all directions and remaining there for days. The concentration of water in the stratosphere more than triples.
The more water, the more ozone loss, through a sequence that begins with the fact that as the air rises, it cools. (To test this, put your hand against the window of an airplane the next time you fly.) The water vapor condenses out as liquid water, much as the steam from your shower turns liquid when it hits a cold bathroom mirror. Condensation releases heat. That raises the temperature of the surrounding air, which contains CFCs left over from the days before they were banned.
The heat alters CFC molecules in such a way as to make them more reactive; specifically, sunlight breaks apart the chlorine molecules in CFCs, producing ClO, the same free radical whose detection by Anderson’s team provided the final proof that CFCs destroy ozone over Antarctica. That free radical, Anderson’s latest work showed, is also—thanks to powerful thunderstorms—chomping its way through the ozone layer over the U.S.