By 1970 it was well established that ultraviolet light from the sun contributes to skin cancer. Fortunately, much of the sun's most damaging ultraviolet radiation is screened out by a thin, diffuse layer of ozone—a toxic form of oxygen—in the stratosphere, 10 to 25 miles above the earth's surface.
During the 1970s, however, public policy makers worldwide were alerted to the fragility of the ozone layer through the pioneering research and advocacy of two Nobel Prize-winning scientists, Mario Molina and F. Sherwood Rowland. In the absence of pollutants, stratospheric ozone concentrations should remain stable over time, with natural production and destruction of the gas in rough equilibrium. Molina and Rowland showed how manufactured chlorofluorocarbons (CFCs)—highly volatile chemicals, millions of tons of which had been used each year in products such as aerosol sprays and refrigerants—chemically attack and deplete the ozone layer, diminishing its effectiveness as a shield against ultraviolet radiation. Studying two freon gases—types of CFCs—they observed that, when released into the lower atmosphere (troposphere), these gases slowly diffuse upward into the stratosphere. There, subjected to massive ultraviolet radiation, they break down into their constituent elements, including chlorine. The resulting increase in the concentration of chlorine in the stratosphere is devastating to the ozone layer. Chlorine and ozone chemically react in a way that both destroys the ozone and regenerates the chlorine atoms. As a result of this chemical reaction, each chlorine atom could destroy as many as 100,000 ozone molecules before becoming inactive.
In 1974 the two scientists estimated that the atmosphere contained the accumulation of five years of global CFC production. This meant that, given the rate of diffusion and breakdown of CFCs in the atmosphere, the depletion of the ozone layer would continue for years, if not decades, even if the production and use of CFCs were to cease immediately. Recognizing this as a pressing environmental threat, Molina and Rowland became public advocates for a prompt and proportionate public policy response. As a result, Molina was invited to testify before the U.S. Congress and was later appointed to the U.S. National Science Foundation Committee on Fluorocarbon Technology Assessment.
Predictably, the work of Molina and Rowland and their advocacy of dramatic policy changes were subjected to attacks by critics, especially scientists with ties to the CFC industry. However, over time their views were corroborated, especially by the discovery of a hole in the ozone layer over Antarctica, and this led to the development of an international agreement (the Montreal Protocol of 1987) to ban the production of ozone-depleting gases. In North America, CFCs were banned in the late 1970s, leading to a transformation in packaging for consumer spray products and the development of more environmentally friendly refrigerant chemicals.
Which one of the following, if true, would most strengthen the conclusions of Molina and Rowland concerning the long-term effects of CFCs in the stratosphere?
The hole in the ozone layer over Antarctica continued to grow for years after CFC emissions had almost ceased.
This strengthens by showing that what Molina and Rowland predicted would occur actually occurred.
Other manufactured chemicals have been found to diffuse upward into the stratosphere when released into the troposphere.
Molina and Rowland’s conclusions concerning the long-term effects of CFCs didn’t relate to other chemicals besides those in CFCs, so (B) has no clear impact on their conclusions.
Ozone has been shown to react more violently with chlorine than with many other chemicals.
What matters is whether after CFC use is ceased, the ozone layer continues to be depleted. This is the long-term effect predicted by Molina and Rowland. The violent nature of ozone’s reaction to chlorine doesn’t help establish the long-term effect predicted by Molina and Rowland.
Many scientists who in the 1970s were highly critical of the research methods of Molina and Rowland have come to accept the soundness of their methods.
What other scientists have come to accept doesn’t help establish that the predictions of Molina and Rowland concerning the long-term effects of CFCs are correct.
Current CFC levels in the troposphere suggest that not all nations currently abide by the Montreal Protocol.
Whether other nations abide by the Montreal Protocol doesn’t help establish that the predictions of Molina and Rowland concerning the long-term effects of CFCs are correct. What matters is whether, even after stopping CFC use, the ozone layer will continue to be depleted for years.
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