A BRIEF HISTORY

The earth's stratosphere contains a thin, but crucial, layer of ozone that filters out many damaging forms of solar radiation and makes life as we know it possible on the earth's surface. Beginning in the 1970s, scientists became concerned that certain human-produced chemicals, known as chlorofluorocarbons or CFCs, could diminish the stratospheric ozone layer.

In the 1980s, atmospheric concentrations of CFCs continued to increase, and decreases in stratospheric ozone began to be detected. Stratospheric ozone concentrations over Antarctica plummeted at a remarkable rate during the Antarctic springtime--a phenomenon now referred to as the Antarctic Ozone Hole. Meanwhile, atmospheric scientists continued working to refine their models of ozone depletion, which had not predicted losses nearly as large as those observed over Antarctica.

As a result of a cross-cutting scientific effort, including theory, modeling, and observations, a sound scientific basis for the protection of stratospheric ozone was established. In response to the scientific findings, an international agreement was reached to halt production of the most destructive ozone-depleting chemicals.

The decisive response of the world community to the stratospheric ozone threat was a tribute to a combined international scientific and policymaking effort. Under the scientific leadership of Jerry Mahlman, GFDL played an important role in that process. Mahlman's group developed a three-dimensional atmospheric model that was tailored to study the interaction of chemistry, dynamics, and radiation in the stratosphere. Their extensive calculations were necessary for evaluating the simpler models used in the policy assessment studies, as well as for understanding the climatic impact of the Antarctic ozone hole.

Percentage reduction of total ozone as a function of latitude and time in a 4 1/2 year "ozone hole" experiment using the GFDL SKYHI three-dimensional atmospheric model with highly simplified chemistry. Note the northward progression from year to year of the area of reduced ozone, showing how ozone losses in the Antarctic ozone hole may affect ozone across even wider regions of the globe. [Source: Jerry Mahlman, et al., Journal of Atmospheric Sciences, 15 February 1994.]

REMAINING QUESTIONS

Many questions are still unanswered about the future of stratospheric ozone. Will an ozone hole like the one over Antarctica develop over the Northern Hemisphere in the coming years? How will greenhouse-gas induced climate change interact with stratospheric ozone chemistry? GFDL scientists will continue to attack these complicated and important questions.

ATMOSPHERIC CHEMISTRY MODELING

As part of GFDL's major research effort in atmospheric chemistry modeling, development is underway on comprehensive models that include chemical calculations for both the troposphere and stratosphere. The goal is a completely interactive simulation of the dynamical, radiative, and chemical processes in the atmosphere. Such a model will be essential in future studies of, for example, tropospheric trace constituents such as nitrogen oxides, ozone, and sulfate aerosols. Nitrogen oxides are believed to control the production and destruction of tropospheric ozone, which, in turn, both controls the chemical reactivity of the lower atmosphere and is a significant greenhouse gas. Tropospheric sulfate aerosols, on the other hand, are believed to significantly affect the earth's radiation budget by scattering solar radiation.

Near-surface ozone, in contrast to stratospheric ozone, is a harmful pollutant that can damage both plant and animal life. GFDL model calculations indicate that near-surface summertime ozone levels have increased substantially since pre-industrial times (a vs. b) and will continue to increase in the coming decades (c). These increases are a result of fossil fuel combustion and fertilizer use. The color scale indicates the approximate degree of hazard to crop yields from increased surface ozone: yellow indicates ozone values on the threshold of possible crop damage; orange indicates levels that will begin to damage crops; red indicates extremely high levels that are a clear hazard to crop yields. Ozone estimates shown (in units of parts per billion by volume) are based on the GFDL Global Atmospheric Chemical/Transport Model and assume a constant per capita level of pollution emissions. [Source: adapted from Hiram Levy II, et al., in Preparing for Global Change: A Midwestern Perspective, SPB Academic Publishing, Amsterdam, 1995.]