This image shows global ocean surface temperatures as measured by the NASA MODIS satellite (image from science.hq.nasa.gov/oceans/physical/SST.html). What did this picture look like 5 million years ago when the Northern Hemisphere was essentially ice-free? My work focuses on reconstructing how ocean surface temperatures have evolved from a climate state with unipolar ice which existed approximately 5 million years ago to the present climate state with bipolar ice.

My research is driven by an interested in understanding how and why Earth’s climate has changed through time. I study the evolution of climate over the past 5 million years, an interval spanning the last major climate transition in Earth’s history, which was marked by the rapid expansion of ice across landmasses in the Northern Hemisphere, the so called “intensification of Northern Hemisphere Glaciation.” My interest in this transition is inspired, in part, because the warm climate state that preceded it, a period called the Pliocene, is one of the best analogs for future climate conditions. The present concentration of carbon dioxide in Earth’s atmosphere (~380 parts per million) - recently reached as a consequence of anthropogenic activities - was last reached during the Pliocene. Additionally, global average temperatures during the Pliocene were 3ºC warmer than present, consistent with the best estimates for how much warmer Earth’s surface temperature will be in the year 2100.

My work offers a new perspective on the climate of the Pliocene and the transition to the glaciated conditions of the subsequent Pleistocene interval by using a relatively new methodology to characterize climatic variations at the sea surface. To conduct my work, I use alkenones, organic compound produced by a few species of surface ocean dwelling algae, which are preserved in ocean sediments after the algae die. By extracting these organic compounds from ocean sediments and quantifying their abundance, I am able to estimate past variations in ocean surface temperature and qualitatively characterize past changes in ocean productivity. Using this technique, I develop continuous, high-resolution records that document the evolution of ocean surface conditions over the past 5 million years. My research strives to address a number of other unanswered questions about the Earth’s climate system. Among them: How does the Earth’s climate system transition between ‘cold’ (bi-polar ice) and ‘warm’ (uni-polar or no polar ice) climate states? What mechanisms and feedbacks are involved in these transitions? Does the system respond differently to changes in solar forcing depending on the mean climate state? How does the Earth system “work” during a ‘warm’ climate state?