No. But you don’t expect it to happen very quickly because the lifetimes of these CFCs are so long. But at least it's not getting any worse. It will take at least a decade to get better in any kind of obvious way, and that's not exactly Good Morning America type of news. It also doesn't mean the problem would have automatically fixed itself. It simply means the public attention span has been exhausted. And maybe it should be. There are still press statements that come out every year reporting on the size of the ozone hole. Last year was the third-largest ozone hole on record, depending on how you measure it. But that's what we expect. It's going to be in this quasi-stable state—slowly, slowly getting better for a long time to come.

It's interesting to draw a contrast between that history and global warming. For one thing, with global warming the effects so far are smaller relative to the variability. They are now just coming out of the noise level in the last decade or so. And we are just starting to have the tools to measure all the kinds of things we need to measure to fully understand what's going on. The ozone hole, in comparison, was focused and readily accessible to measurement. With global warming, you have to talk about clouds on a global basis; tropospheric water vapor on a global basis—a lot of things that are not easy to measure and have to be done globally, not locally. You just can't do that quickly because it is a global problem.

You said earlier that you didn't take ozone depletion that seriously at first because the effect was so small and the uncertainties so great. Isn't that still the case with global warming and the question of whether it's naturally occurring or anthropogenic?

That's a little bit different. I think you could have made some of the same arguments in the mid-1980s about global warming, but when you look at the decade of the 1990s in particular, the warming was quite remarkable. Global warming, at this time, has turned the corner on detectability. Not as dramatically as the ozone hole did. It wasn't like, all of a sudden—boom—it's huge. But over the last two decades, we have seen an incredible change in temperature, to the point where personally I am becoming convinced that there is a real signal there in the global mean. Another thing to bear in mind in the case of global warming is that you actually have a longer record to compare against. Even without dealing with ice cores and tree rings, you can easily go back to the 1800s. If you include ice cores and tree rings, you can make reasonable estimates back 1,000 years. So we know more about the long-term variability in temperature. And temperature is just much easier to quantify and measure than is ozone. And fortunately, nature has recorded the temperature information for us, and for a lot longer. All those factors have to be in mind when you look at the global warming issue. When you put it all together, the case for a detectable climate signal due to, or likely due to, human activities over the past decades is pretty good. It wouldn't have been good before that.

You left ozone behind in the early 1990s. What have you been concentrating on since then?

What I did for the next few years was to show, in a manner somewhat similar to the polar stratospheric clouds, that other kinds of surfaces could also enhance ozone depletion at lower latitudes. We were able to show, for instance that when El Chichon went off in 1981 and then Pinatubo in the 1990s, both had significant effects on the ups and downs of ozone depletion. Pinatubo, in particular, had a measurable effect in the northern mid-latitude depletion via chlorine chemistry on the volcanic particle surfaces in the stratosphere. That was a pretty major finding and really helped to explain why, at that time of history, the ozone in our latitudes looked the way it did.

I have also done some work on the issue of gases other than carbon dioxide that could contribute to global warming. This has been fascinating from the chemistry point of view. Among other things, one question my colleagues and I have probed is the role of perfluorinated chemicals like CF₄, SF₆, and others. There’s not a lot of that stuff in the atmosphere today, so I’m not saying they are significant contributors to today’s global warming. But we’ve shown that these molecules live for literally thousands of years—they may as well be immortal—and they are potent absorbers of infrared light, hence greenhouse gases. A molecule that can outlast the pyramids of Egypt might be one to think about venting to the atmosphere especially carefully. In fact, maybe one of the most interesting overlaps between ozone depletion and climate change is that the ozone-depletion issue shows the need for science to help understand not just impacts but also time scales in environmental problems. It’s all part of using science to help inform society about the questions that have to be asked—not just what is or may be happening now, but if something does happen tomorrow, how long will we have to live with it? Particularly if that something tomorrow might possibly be worse than what we are predicting. In the case of the Antarctic ozone hole, the chlorofluorocarbon-induced depletion was far greater than we had predicted, because we didn’t know about surface chemistry. But we knew all along that if ozone depletion did occur, it would last 50 to 100 years, because that’s how long the CFCs last in the atmosphere. As we think about the possibility of future climate change, I think it’s important to think about chemistry, and about the lifetimes of the things that could contribute to the climate. Carbon dioxide lives about 150 years in the atmosphere, so that fact should be part of a science-based risk assessment. Climate change is a complex problem, and this kind of scientific information is one piece of the problem.

In the past five years, I've taken a change of direction doing work more related to understanding how various molecules in the lower atmosphere actually absorb incoming sunlight. It's relevant to the climate problem, but it really addresses the fundamentals of how the system works. So I'm moving into an area that is related to the climate problem, but a little more esoteric. I'm coming back to something like my early work on auroras and things like that, helping to lay the basis for understanding how radiation propagates—how sunlight really gets in through the atmosphere. It isn't front-page New York Times kind of work. That's fine with me. I love doing it. Although I wasn't looking for headlines when we did the ozone work, either. It just kind of fell my way.