I was initially trained as a marine geologist. My subsequent work over many years has explored several different aspects of the field of paleoclimate. My earliest research was on orbital-scale changes in North Atlantic sediments to reconstruct past sea-surface temperatures and to quantify the deposition of ice-rafted debris. I also studied the way that vertical mixing by sea-floor organisms smoothes deep-sea climatic records.
Later, my interests turned to the cause of long-term cooling over the last 50 million years. This research led to a new hypothesis that uplift of the Tibetan Plateau has been a major driver of that cooling. Soon after, a key link suggested by my student Maureen Raymo on chemical weathering became the central part of that hypothesis. That research also demonstrated that Tibetan uplift created much of the seasonally alternating monsoon climate (warm wet summers. cold dry winters) that dominates eastern Asia today. These ideas have now entered into the ‘mainstream’ understanding of past climate changes.
Since entering ’semi-retirement’ in 2001, my research has concentrated on the climatic role farmers played during the last several thousand years by clearing land, raising livestock, and irrigating rice paddies, all of which put increasing amounts of greenhouse gases in the atmosphere. This led to the ‘early anthropogenic hypothesis’ that early agriculture caused the observed (and anomalous) reversals in the natural declines of atmospheric CO2 (carbon dioxide) near 7000 years ago and CH4 (methane) near 5000 years ago. This hypothesis provoked a major debate, with some 50 papers published for or against it in the last decade, but with a favorable shift in the last few years based on emerging archeological evidence. My research on this issue has been NSF-funded for 7 years (and the next two), despite its controversial nature.
2010 Lyell Medal, Geological Society of London
2012 Distinguished Career Award, American Quaternary Association
2013 Royal Scottish Geographical Society Coppock Research Medal