Steven's scientific expertise is in the domain of atmospheric composition, including the representation of transport processes in models and data assimilation systems. He is author or co-author of more than 150 peer-reviewed publications. His early work focused on the stratosphere, including the ozone layer. Over time his interests have moved closer to the ground to include air pollution and the carbon cycle. He was the founding co-lead of the GEOS Chemistry-Climate Model (CCM) project and spearheaded early work on representing the carbon cycle in GEOS systems. More recently, he has overseen the introduction of the GEOS-CF (Composition Forecasting) system into the suite of GEOS products.