Title: "What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated?"

Speaker's website: http://acdb-ext.gsfc.nasa.gov/People/Newman/
Host: Daniela Domeisen
Faculty host: Alan Plumb

Abstract: Imagine a world where a 5-minute July walk would give you a sunburn, or where an extended December stroll on the National Mall would also give you a sunburn. Such a world could have happened if human-produced chlorofluorocarbons (CFCs) had never been regulated. Stratospheric ozone screens ultraviolet (UV) light that causes sunburn. In 1974, Mario Molina and Sherwood Rowland hypothesized that human-produced CFCs could break down in the stratosphere and release chlorine to attack the ozone layer. Considerable work followed their publication - strengthening their hypothesis. The nations of the world acted by negotiating The Montreal Protocol on Substances that Deplete the Ozone Layer in 1987. This agreement controlled the production and consumption of ozone depleting substances such as CFCs. All of the nations of the world have now signed the Montreal Protocol. CFCs are now declining in our atmosphere, and stratospheric ozone levels are showing some signs of healing.
In this presentation two simulations will be shown: our expected future under the Montreal Protocol, and a future where ozone depletion substances (ODSs) were never regulated. In this “world avoided” simulation, ODS levels increase by 3% per year, and by 2065 the chlorine levels were approximately 40 times greater than natural levels. Two-thirds of the ozone layer has disappeared by 2065. In the “world avoided”, large ozone depletions in the polar region become year-round rather than just seasonal as is currently observed in the Antarctic ozone hole. Very large temperature decreases are observed in response to circulation changes and decreased shortwave radiation absorption by ozone. Ozone levels in the tropical lower stratosphere remain constant until about 2053 and then collapse to near zero by 2058 as a result of heterogeneous chemical processes (as currently observed in the Antarctic ozone hole). The tropical cooling that triggers the ozone collapse is caused by an increase of the tropical lower stratospheric upwelling. In response to ozone changes, ultraviolet (UV) radiation increases, tripling the erythemal (sunburn) radiation in the northern summer mid-latitudes by 2065. We will also show some of the significant changes in tropospheric circulation and chemistry and discuss the limitations of this model study.
Without the Montreal Protocol, substantial ozone depletion would have led to extreme UV levels. For a modest ODS growth of 3% per year from 1974-2065, UVI would have exceeded 25 over much of the Earth.