Timing, Causes and Consequences of the Decline in Pliocene pCO2

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Human activity has led to an increase in atmospheric carbon dioxide levels from 275 to 285 parts per million (ppm) in preindustrial times to >380 ppm today. This greenhouse gas contributes more to the human-induced warming of the planet than any other gas. Therefore, understanding the relationship between carbon dioxide and climate is a major research focus of Earth Scientists predicting future climate change. Although General Circulation Models are the primary tool by which we anticipate future change, they remain imperfect tools that require validations. Therefore, the study of ancient climate is now an integral part in informing policy makers on climate change issues. If climate models successfully reproduce large scale climate changes that occurred in the past, this will give us more confidence in their prediction for the future. The most informative analogues will be in the recent geological past where geographical configurations, ocean currents and ecosystems are similar to today. The Mid-Pliocene (about 3.3 to 3.0 Ma) is the most recent time in Earth's history when mean global temperatures were substantially warmer and sea levels much higher than they are today. Immediately following this period of comparative warmth, the Earth cooled dramatically and ice sheets grew: the modern ice sheet on Greenland is testament to our current 'icehouse' climate. A reduction in the concentration of CO2 is widely believed to be the cause of this critical climate transition, yet this understanding depends on knowing exactly what atmospheric CO2 concentrations were in the Pliocene, how they changed and how those changes impacted the climate. However, this is currently based on a few extremely low resolution and limited studies. Therefore, we propose to determine Pliocene CO2 concentrations with particular emphasis on determining the timing of its decrease with respect to the glaciation of the northern hemisphere. This will involve revised estimates of CO2 concentrations and globally widespread estimates of sea surface temperature, environmental parameters that cannot be directly determined for the past. We will study the fossil remains of sea-dwelling microscopic organisms, the foraminifers and coccolithophorids. These organisms are very abundant in the mud on the floor of the oceans, providing an invaluable archive of past ocean climate data, and by looking at the chemical composition of their shells or the organic compounds they biosynthesise we can determine how warm or how acidic the ocean was. And from such parameters, we can also deduce how much CO2 was in their environment. We will use several complementary approaches to determine CO2 concentrations, including the boron isotopic composition of foraminifera and the carbon isotopic composition of organic compounds. Methodological advances have only recently made such a multi disciplinary approach possible. Armed with this improved understanding of the evolution of CO2 in the atmosphere over the last ~3 million years, we will determine what natural processes resulted in changing its concentration. We will initially compare the timings of CO2 change to tectonic events such as the closure of the Indonesian Seaway and changes in global ice volume to examine a) what might have triggered the decrease in carbon dioxide, and b) if that truly was the main driver of global cooling and the expansion of the great northern hemisphere ice sheets observed at that time. Finally, we will test those ideas using climate models. First, we will use a coupled ocean-atmosphere model to test whether the magnitude and timing of carbon dioxide change is consistent with associated cooling. Second, we will couple that model to a model of the carbon cycle to test if positive feedbacks during global cooling played an important role in carbon dioxide removal from the atmosphere.