My research is interdisciplinary, arbitrarily divided here into several overlapping areas...
Specifically, I have worked on:
The nature of the Great Oxidation The transition from a reducing to oxidising atmosphere around 2.4 billion years ago, delayed some hundreds of millions of years from the origin of oxygen producing photosynthesis. I demonstrated that there are two distinct stable steady states for atmospheric oxygen in the presence of oxygenic photosynthesis: a low oxygen state with less than 1 ppm oxygen and a high oxygen state with at least 0.1% oxygen. The transition between these is non-linear and geologically rapid, occuring with the formation of the ozone layer.
The global nitrogen budget and changing atmospheric inventory We performed the first comprehensive reassessment of the global nitrogen budget for several decades. There are substantial reservoirs of nitrogen in the continental crust and in the mantle. Mantle nitrogen is not primordial, but subducted. The major mechanism for putting nitrogen in rocks is substitution of NH4+ for K+, indicating that the nitrogen in the solid Earth is geological in origin. The existence and nature of these reservoirs implies that atmospheric nitrogen has varied over time and may well have been two to three times the present level during the Archean.
The Faint Young Sun Paradox The Earth received around 20% less energy from the Sun in the Archean than today, yet geological evidence is for a temperate climate. The mechanism of this warming, through a stronger greenhouse effect and/or a lower albedo, remains subject of intense debate. I have contributed to this debate in two ways. First, by showing that a higher nitrogen inventory was likely on early Earth, and that this would have caused net warming through pressure broadening of the adsorption lines of greenhouse gases. Second, I conducted a comprehensive evaluation of the possible role that clouds could have within solutions to the Faint Young Sun paradox. By fully exploring phase space, I was able to put constraints on which proposed solutions are, and are not, feasible.
Presently, I have a major NASA funded project to study the runaway greenhouse from a planetary atmospheres perspective. The runaway greenhouse is the case with an optically thick steam atmosphere, where outgoing thermal emission becomes decoupled from surface temperature. This is only escaped when the planet gets hot enough (~1400K) to emit in the visible and near-infrared, where water vapour is not a strong adsorber.