How Much Ozone Depletion Has Occurred over the Past Two Decades?

Ask a Scientist - July 2011

K. Roumph, of Fremont, NE, asks: “How much ozone depletion has occurred over the past two decades, and to what degree are more harmful UV rays now reaching the Earth?” She is answered by Peter Frumhoff, Ph.D., director of science and policy at UCS.

Let me first address your concern: Earth’s ozone layer, high in the atmosphere, which had thinned significantly by the 1980s and early 1990s, has actually stabilized over the past two decades. Its recovery is uneven and complicated by global warming. But overall, a scientific and political success story averted a much more serious threat from ultraviolet radiation, or UV rays.

Back in the 1980s, scientists discovered that the ozone layer was thinning, with particularly dramatic loss—known as the infamous “ozone hole”—over the Antarctic during that region’s springtime in September and October. Scientists rightly recognized that this phenomenon resulted from people’s increasing use of a category of chemical refrigerants and aerosol propellants called chlorofluorohydrocarbons, or CFCs. These chemicals were causing a reaction in the stratosphere that was depleting ozone. Using the best available science for guidance, the world’s nations mobilized in 1989 to negotiate an international treaty known as the Montreal Protocol that phased out ozone-depleting chemicals worldwide. The evidence is now in: the treaty has been a striking success in avoiding more severe damage to the ozone layer and putting it on a path toward recovery.

Despite that heartening story, however, there is an awful lot of confusion about ozone, global warming, and human health. So, let me try to draw some clear distinctions.

First of all, the ozone hole is not the mechanism of global warming. Ultraviolet radiation represents less than one percent of the energy from the sun—not enough to be the cause of the excess warming scientists are now documenting on Earth. Rather, human activities are the main cause of global warming because we are collectively overloading the atmosphere with carbon when we burn gasoline to run our cars or burn coal, gas, and oil to generate electricity. These carbon emissions spread around the planet like a blanket, trapping solar heat that would otherwise be radiated out into space.

Finally, it is important to understand that there is an entirely distinct type of problem caused when ozone is created near Earth’s surface. This so-called ground-level ozone is a human health irritant and component of smog. Found in the lower atmosphere, or troposphere, ground-level ozone also has nothing to do with the “ozone hole.” It does, however, have a closer connection to global warming because warmer temperatures increase ground-level ozone.  That’s why we most often hear warnings of “bad air days” due to ozone pollution during the summer. A recent report by the Union of Concerned Scientists estimates that, within the next decade alone, higher ground-level ozone concentrations caused by global warming could lead to an average of 1.4 to 2.8 million more occurrences of acute respiratory symptoms like asthma, result in thousands more hospitalizations of seniors and infants, and cost Americans $2.7 to $5.4 billion in additional health costs.

Unfortunately, unlike its high-flying counterpart, ground-level ozone is cause for mounting concern and yet another reason why it is so vital that we reduce the emissions causing global warming as quickly as we can.

Dr. Peter C. Frumhoff is the director of science and policy at the Union of Concerned Scientists (UCS), and chief scientist of the UCS Climate Campaign. A global change ecologist, he has published and lectured widely on topics including climate change impacts, climate science and policy, tropical forest conservation and management, and biological diversity. He is a lead author of the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) and the 2000 IPCC Special Report on Land Use, Land-use Change and Forestry. He holds a Ph.D. in Ecology and an M.A. in Zoology from the University of California, Davis and a B.A. in Psychology from the University of California, San Diego.

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