Orbital Modulation of Eocene Carbon Cycle and Climate (OMECCC)

How sensitive is our climate system to the emission of greenhouse gases such as carbon dioxide and methane? Might we reach a 'tipping point', when natural processes start to release more and more greenhouse gases, greatly amplifying the warming that humans are already causing? Because of the complexity of the Earth's climate system, and sometimes simply because of our lack of imagination about all the different ways in which the Earth can respond to being poked, a comprehensive answer to these questions is extremely difficult to achieve, even with our best climate models. It would be a great help to us in testing and improving our computer models and predictions of future climate change if we could identify and understand events recorded in the geological past involving a massive release of greenhouse gases to the atmosphere. One of the most promising events we have discovered so-far is called the Palaeocene-Eocene Thermal Maximum (or 'PETM'). It occurred about 55 million years ago, some 10 million years after the last dinosaur walked the face of the Earth, and is associated with a sudden and substantial warming of both the Earth's land surface and all the oceans. It stands out clearly in the geological record by extinctions amongst organisms dwelling on the deep ocean floor, by red, clay-rich sediments unusually poor in the shells of microscopic plankton who lived in the surface far above, and by sudden changes in the ratios between different elements and isotopes imprinted into these shells (from which geochemists can reconstruct the temperature of the ancient ocean and all sorts of useful things). The correspondence between indicators of a sudden disruption of the global carbon cycle and increasing surface temperatures is the hallmark of greenhouse warming, and this is what has gotten scientists so excited. But look closely along the meters and meters of cylinders of mud drilled from the bottom of the ocean and you might see something more subtle going on -- minute fluctuations in color. And if you had an expensive mass spectrometer, you could also find tiny fluctuations in carbon isotopes as well. It seems that the global carbon cycle periodically 'wobbled'. Why? And what is the relationship with the PETM -- could this massive event have been set off by a small carbon cycle wobble? Understanding the reasons for these wobbles has a special importance as they seem to occur at the same frequency as some of the wobbles in the Earth's orbital, and paleoceanographers (a fancy term for scientists who study million year old stinking mud) use the relatively regular changes recorded in the sediments as a means of measuring the passage of time as the sediments build up. In this project we will test ideas for what caused the ancient wobbles in the global carbon cycle. As we do not have spare copies of our planet on which to experiment and test ideas, nor a time machine to go back and observed the cause, our research tool is a computer representation of the Earth system. This model accounts for ocean circulation and greenhouse warming, as well as the cycling of carbon and nutrients within the ocean and exchanges with the underlying deep-sea sediments. We will use this model to predict what the geological record would look like if any of the various hypotheses proposed for the observations were correct. The one that fits the closest we will assume is also closest to the truth (although we could never know what happened absolutely for sure). The result of our work will be an improved understanding of how and why the global carbon cycle fluctuated in response to the Earth's orbit in the ancient past, increase our confidence in using the observed wobbles to tell time, and hopefully may find clues to the trigger of the PETM event.