Fingers in Dikes
This month marks the 30th anniversary of the Montreal protocol that started to phase out chemicals destroying the stratospheric ozone layer. This is widely and justifiably hailed as a major international success, as concentrations of these chemicals in the atmosphere have declined and ozone levels have stopped decreasing. A side benefit is that the CFCs and related compounds that have been discontinued are strong greenhouse gases, so Montreal is in a sense the first treaty to combat climate change. Of course, what everybody would like to see is a rebound in the ozone layer.
That such a rebound isn’t obvious is the subject of a review in Nature from about a week ago. While measuring the concentration of the destructive chemicals is straightforward as is the total amount of ozone in the atmosphere, what is less clear is what else is going on and how that is affecting the changes we’re seeing. Recovery of the ozone layer is expected to be slow in any case, and natural variability both spatially and temporally complicates any signal, especially in whole atmosphere ozone measurements. There are indications that stratospheric ozone is starting to increase, but the signal is still noisy. The review’s authors explore possibilities using constraints from observations and different modeling approaches to try to untangle things. While this shows a clear and strong success from Montreal’s restrictions, other elements are creeping into the equation. Basically, the changing climate is also impacting ozone, with its role gradually increasing relative to that of the destructive chlorine compounds. Warming of the troposphere and increased carbon dioxide and methane emissions leads to cooling of the stratosphere, which is good for ozone. But increased N2O emissions work the other way. And then the changes in the strength of atmospheric circulation mean that the recovery won’t be uniform–in fact, the authors suggest that the tropics could see a decrease in overall ozone even as global ozone levels rise, which would increase UV levels over a significant part of the inhabited globe.
What all this means is that something as heavily studied as the ozone layer is a product of complex interactions between relatively unfamiliar chemistry, appearances and disappearances of chemical species both from human and natural events, and the evolution of the atmospheric circulation. Montreal wasn’t the solution; it was a finger in the dike holding back a flood of consequences from using the atmosphere as a dumping ground. Dramatic ozone loss from CFCs was unanticipated because the peculiar chemistry in stratospheric clouds was unexpected. Fortunately that oversight was caught. What we need to recognize is that this part of earth’s atmosphere needs to be monitored closely as a changing level of greenhouse gases could introduce yet another unexpected surprise. Unfortunately, such ongoing research seems to be a target of the current U.S. administration. We might not be done plugging holes in this dike…it might be good to know where to next stick our finger in.