ability and the associated policy implications for the United States and other nations. Additionally, the NAS/NRC have simultaneously considered climate change and variability within the broader context of global change. The CGCR, author of this report, and CGCR's predecessor, the Board on Global Change, have been charged with providing continuing guidance to national and international global change efforts. In 1995, CGCR undertook an initial assessment of the scientific programs of the USGCRP, reviewed the specific role of NASA's Mission to Planet Earth/Earth Observing System, and issued a report with recommendations (the “La Jolla” report)19 and a follow-up report on the government response.20 The present study significantly expands that effort.

Scientific Roots of Stratospheric Ozone Research

A related history of research concerns another pressing environmental issue—depletion of the stratospheric ozone layer that shields us from damaging ultraviolet radiation. In the early 1970s, proposals to build a fleet of supersonic transports raised questions about possible damage to the ozone layer from engine emissions in the stratosphere. A major U.S. research and assessment program was launched, and the NRC was commissioned to conduct a series of studies.21 But soon Rowland and Molinaa made the startling discovery that chlorofluorocarbons (CFCs), not airplanes, were the frightening threat to our ozone shield. Eventually, an international assessment was conducted under the auspices of the World Meteorological Organization and other international bodies. 22

The discovery by Rowland and Molina reminds us that studies and reports often do not adequately address the complexities of the real world. Indeed, they can even significantly miss the mark. Studies of ozone depletion had focused on slow incremental changes and had sought incremental improvements through corresponding models and parametric analyses. Meanwhile, observations extending back to the 1950s had been tracking the amount of ozone over the Antarctic each year through its seasonal cycle. In the late 1970s an anomalous deficit was observed in the total amount of ozone over the southern hemisphere in late winter observations. Then in 1985 the British Antarctic Survey reported dramatic—and rapidly worsening—ozone losses in springtime ozone concentrations over Halley Bay.

Theories about the cause of this unprecedented and unexpected loss blossomed. Explanations ranged from the hypothesis of the simple redistribution of stratospheric ozone by atmospheric motion to proposed chemical reactions initi-

a

The Swedish Academy of Sciences awarded the 1995 Nobel Prize in Chemistry to F. Sherwood Rowland, Mario Molina, and Paul Crutzen for their work in atmospheric chemistry. Rowland and Molina published an article in Nature in 1974 that showed that CFC releases into the atmosphere cause stratospheric ozone depletion. Paul Crutzen had previously shown the importance of nitrogen oxide catalytic chain reactions in controlling the amounts of stratospheric ozone.



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