TRAcing the fate of Glacial-Interglacial Carbon ('TRAGIC')

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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 better understand events recorded in the geological past involving changes in atmospheric greenhouse gases and climate. Our focus here is prompted by painstaking measurements that have been made for more than 30 years of the amount of carbon dioxide (CO2) contained in minute bubbles trapped in ancient ice. These measurements reveal that the concentration of CO2 in the atmosphere has undergone large fluctuations over the course of the past million years, falling in approximate step with the growth of massive ice sheets and cold glacial periods and rising as conditions get warmer. As CO2 is a greenhouse gas, lower concentrations clearly help explain why climate was much colder during glacial intervals than today. So what then drives CO2 lower during glacial times (and up during inter-glacial periods such as today)? Amazingly, although scientists have been searching for the answer for over 30 years, still no-one knows for sure. Many different hypotheses have been forwarded; some involving changes in ocean circulation, others the supply of nutrients (such as iron) to life at the ocean surface. Some suggested changes will turn out to be unimportant, others may be key parts of the puzzle. How can we choose the correct answer from such a variety of possibilities? We believe that new information generated by a PhD student at the University of Bristol holds the key. He has measured the composition of the shells of minute organisms living at the bottom of the ocean. Various elements and compounds are incorporated from seawater into the shells as they form, and the exact amount of the element boron and its isotopic composition depends on ocean acidity. Hence, it is possible to reconstruct how the acidity of the deep ocean has changed since the last glacial. This is important because much of the 'missing' atmospheric carbon during glacial times may have been stored in dissolved form in the deep ocean, and since adding CO2 to water makes it more acidic, it is possible to reconstruct how much carbon there really was down there. In this project we will test ideas for what caused the glacial-interglacial changes in atmospheric CO2. 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 make direct measurements of carbon storage in the deep ocean, 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 glacial-interglacial cycles, increasing our confidence in being able to predict how (fossil fuel) carbon may be exchanged between different reservoirs in the future, and how changes in climate and ocean circulation may modulate this.