Top: The ozone layer in 2008. Bottom: The ozone layer in 2008 had the Montreal Protocol not been signed. Credit: Chris Rowan
By signing the Montreal Protocol two decades ago, political leaders from all over the world sanctioned the first significant international effort to combat human-caused environmental stress. Since then under the treaty’s auspices, the production and use of chlorofluorocarbons (CFCs) and other substances that were depleting the ozone layer — which shields the Earth’s surface from high-energy ultraviolet radiation — have been gradually phased out. Consequently, the number of CFCs in the atmosphere has stabilized or even started to decrease.
Even so, right now this success seems rather abstract: The wound that we have inflicted on the ozone layer — the hole that opens above Antarctica at the beginning of the southern hemisphere spring — still persists. CFCs can survive in the upper atmosphere for decades, so we may well be into the second half of the 21st Century before the injury fully heals.
But how much worse would that wound have been if the Montreal Protocol had never been signed?
Using an advanced model of the Earth’s atmosphere, atmospheric physicist Paul Newman of NASA’s Goddard Center and colleagues have been playing a game of climatic “what if” by charting the evolution of ozone in the upper atmosphere in two different scenarios. The first scenario describes the world that is and will be, with emissions of CFCs declining following the implementation of the Montreal Protocol. The second shows the world that might have been: A parallel universe in which the warning about the threat to the ozone layer — first sounded by Nobel Laureates Sherwood Rowland and Mario Molina in 1974 — was ignored or negotiations failed, and CFC emissions had continued to rise up to and beyond the present day.
Comparing the results of these two models leads to the cheering conclusion that our world is already reaping the benefits of the Montreal Protocol. The “world that is” model shows that whilst the ozone hole may not be noticeably shrinking, the fact that the amounts of CFCs accumulating in the upper atmosphere are leveling off, the hole that formed above the Antarctic in September 2008 was no larger than any of the others observed in the last decade.
In contrast the authors’ “world avoided” model reveals that more CFCs in the atmosphere would have created a much deeper ozone hole. (See image) Although our ozone layer hasn’t fully healed, cutting back on CFC emissions has clearly limited the extent of the damage.
What’s more, when the authors modeled decades to come, they found that the divergence between our world and the world that might have been only increases with time. Our predicted future is a tale of slow recovery. The concentration of CFCs will eventually begin to fall, and as it does, the ozone layer will finally begin to regenerate. By 2060 this healing is almost complete.
Meanwhile as CFCs continue to accumulate in the atmosphere of the world we avoided, its story takes a much grimmer turn. By 2040 there is not only an ozone hole above the Antarctic for the entire year, but severe ozone depletion at much lower, more populous latitudes. In fact the average concentration of ozone is everywhere lower than the value that used to define the present ozone hole. By 2060 the ozone layer is effectively extinct and without it, the Earth’s surface is a much less friendly place: You would only have to be outside for about five minutes to get sunburned and the incidence of skin cancer would increase substantially.
So, we really have saved the ozone layer, getting our environmental act together just about in time to dodge a rather carcinogenic bullet. In the run up to the climate change conference in Copenhagen in December, it’s a timely reminder that international agreements to curb our wilder environmental excesses can be successful.
The challenges of regulating emissions of greenhouse gases, however, are far greater. Which raises the question: If we run a similar modeling exercise for carbon dioxide 20 years from now, will we be celebrating again the world we avoided — or looking enviously at the world we wish we had? — Chris Rowan is a geologist and Marie Curie Fellow at the University of Edinburgh. He blogs at Highly Allochthonous on ScienceBlogs.
Originally published April 22, 2009