I have been a Research Professor at the Johns Hopkins University Department of Earth and Planetary Sciences since mid-2010. Before that I was a research scientist at the NASA Goddard Space Flight Center for more than 30 years. I worked on all aspects of the stratospheric ozone issue. I developed some of the early models for the impact of chlorine chemistry on the ozone layer. I have worked with satellite measurements of ozone to describe the structure and variability of the Antarctic ozone hole and to derive global trends in stratospheric ozone. Recently, I have been involved in the development of comprehensive chemistry-climate models that describe the response of atmospheric chemistry and dynamics to perturbations of the Earth system.
My interest in the open Earth-system modeling project is the role of the atmosphere in Earth history. As atmospheric oxygen evolved, an ozone layer began to form, shielding the surface from the ravages of solar ultraviolet radiation. The knowledge that we have gained from three decades of intense research on the ozone layer can now be applied to the ancient atmosphere. I am exploring the development of the ozone layer and its dependence on oxygen, carbon dioxide, methane, nitrous oxide and other atmospheric components using a modern two-dimensional model of atmospheric composition and dynamics.