Understanding the Direct and Ozone-Mediated Impacts of (CO2) and Ozone Depleting Substances (ODS) on the Climate System

The ozone hole and greenhouse gas-induced global warming are distinct environmental problems. The ozone hole matters because reductions in stratospheric ozone lead to increases in harmful ultraviolet radiation reaching the surface. The loss of ozone in the hole is caused by a small set of ozone depleting substances (ODS, primarily chlorofluorocarbons) used in a specific set of applications such as spray cans and cooling equipment. Global warming, on the other hand, is caused by the heat-trapping effect of greenhouse gases emitted throughout the world economy, principally the carbon dioxide (CO2) emitted in the burning of fossil fuels.

Despite the clear distinction between global warming and the ozone hole there are important physical pathways that link the two. One is that most ODS are also greenhouse gases. ODS concentrations are minuscule compared to CO2 but they are thousands of times more effective as absorbers of infrared radiation, thus their heat-trapping effect is substantial. Another connection is that CO2 increases can change the amount and distribution of ozone in the stratosphere. Although CO2 warms the troposphere it cools the stratosphere, a difference caused by their opposite temperature profiles (cold aloft, warm below in the troposphere and vice versa in the stratosphere). Cooler stratospheric temperatures can increase ozone concentrations by reducing the reaction rates for ozone-depleting chemistry. But a CO2 increase can also cause a speed-up of the stratospheric overturning circulation, which in turn decreases the lifetime of stratospheric ozone.

This work explores the climatic effects of CO2 and ODS, recognizing that they affect climate both directly and through their impact on stratospheric ozone. In addition to global temperature the study considers regional influences on sea surface temperature, sea level pressure, jet stream wind speed, and other variables. Preliminary work suggests, for example, that the greenhouse warming due to ODS is greatest in the subtropics of the North and South Pacific, presumably as a consequence of the uneven geographical distribution of ODS. A further consideration is that ODS can affect climate either directly, as a powerful greenhouse gas, or indirectly through the changes in climate induced by ozone loss. For example, previous work by the PI and others shows that the ozone hole has affected the winds, temperature, and precipitation of the Southern Hemisphere. The work also examines changes in stratospheric ozone due to CO2 increases. The primary research tool of the project is the Whole Atmosphere Community Climate Model developed at the National Center for Atmospheric Research.

The work has broader impacts due to the societal impacts of ozone depletion and global warming. Efforts to address these impacts will benefit from a better physical understanding of the processes involved. The work also fosters international scientific collaboration given the participation of researchers at the Swiss Federal Institute of Technology in Zurich. The project also provides support and training to a graduate student and a postdoctoral research associate, thereby promoting the future workforce in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.