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 I will show two simulations: 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. The reduced ozone screens less UV, leading to a tripling of the erythemal (sunburn) radiation in the northern summer mid-latitudes by 2065. I will contrast this “world avoided” with our “expected future.”