The runaway greenhouse If a terrestrial planet becomes sufficiently warm, its atmosphere will become water vapour rich and a hard limit on the amount of thermal radiation which can be emitted will emerge. If more solar radiation is absorbed than the maximum thermal emission, the planet will heat uncontrollably, the so-called runaway greenhouse. Along the way, the whole ocean will evaporate and all life will become extinct. This is the history of Venus and the future of Earth. I have showed that the runaway greenhouse may be possible with amount of energy that Earth receives from the sun today, and that either decreasing the amount of cloud reflection or strongly increasing greenhouse gas concentrations could trigger it.
Specifically, I have worked on:
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.
Climatic effects of clouds Clouds have huge leverage on planetary climate, but are fickle beasts. Introducing condensate into the atmosphere has a strong and non-linear effect on the radiation field and thus climate. They both absorb thermal radiation, contributing a greenhouse effect and a warming, and reflect sunlight, reducing absorbed energy and thus cooling. Whilst a clear-sky atmosphere can be treated as a coupled thermodynamics-radiative transfer problem, clouds should require that dynamics are included to find where condensation occurs. This makes simple climate models less applicable, but non-simple climate models contribute a host of difficulties. I've used explored the parameter space for clouds for Early Earth climate (see above), and contributed that treating clouds on brown dwarfs as patchy (as Earth clouds are) gives a better fit to observations.
Radiative transfer codes I've run a handful of different codes. What developers of these know, but many users seem not to, is that any "off the shelf" RT code will have been designed for specific conditions. Take the code outside its design range, and performance will deteriorate. Best to talk to the developer, or test the code yourself against something you know will work.
Physical Oceanography I was on the shipboard party of the RRS James Clark Ross for cruise JR97 to the Weddell Sea and Fimbul Ice Shelf doing physical oceanography. Physical Oceanography was a major focus as an undergraduate.
Meteorology The other half of my undergraduate degree, now coming back to be part of my work more. Victoria has one of the densest urban meteorology networks available through the UVic Victoria Schools Weather Network - so there is lots of interesting potential associated with this.