My research interests are in the dynamics of climate change. A general theme of much of my research is to use physical modeling and mathematical methods to address societally relevant problems. I use a variety of models ranging from idealized mathematical representations of climate phenomena that can be solved with pencil and paper to comprehensive climate models that are run on massive computer clusters, also drawing on observations. A few specific themes are described below.
Arctic sea ice is one of the components of the climate system that has changed most during recent decades, and it is projected to continue to change rapidly during the coming century. A goal of my current research is to develop physical theories to explain how the global sea ice cover interacts with other components of the climate system and how it responds to climate changes. Such theories provide a framework for interpreting the observed sea ice retreat and constraining the widely differing future sea ice projections from current state-of-the-art climate models. Related areas of my research include investigating the possibility of bifurcation thresholds during sea ice retreat ("tipping points"), as well as the impact of cloud simulation errors on sea ice in climate models.
Greenland ice core temperature reconstructions reveal a 1500-year abruptly punctuated cold period which interrupted the transition from last ice age to modern conditions. This period, known as the Younger Dryas, is one of the most dramatic incidents of past abrupt climate change reconstructed from paleoclimate proxy records. I have worked on understanding the mechanism that caused this cold period, and I recently proposed a theory in which the receding glacial ice sheets cause a change in the atmospheric hydrological cycle, which leads to reduced ocean heat transport, expanded sea ice cover, and widespread cooling. My other paleoclimate research includes work on abrupt climate change events during the course of the last ice age, the Late Ordovician glaciation 450 million years ago and accompanying mass extinction, the global glaciation hypothesized to have occurred 650 million years ago ("Snowball Earth"), and the response of tropical atmospheric circulation to past changes in solar radiation.
My research also involves other aspects of atmosphere-ocean dynamics. For example, I have worked on El Niño, which is the largest source of interannual variability in the global climate system. Rapidly varying weather has been frequently suggested to drive El Niño variability. I have investigated the extent to which this weather is modulated by El Niño itself, thereby giving rise to a positive feedback which can amplify El Niño events.
Although my research has not directly involved field work, I have occasionally had the opportunity to take part in research in the field (some photos).