Climate Variability in a Warm Past: the Mid-Pliocene as an Analogue for the Future?

The Earth is warming, this is due to human actions, it has life-threatening consequences, and there are things we can do to stop it. The climate of the future depends on future emissions, but also on the dynamics of the Earth. The internal behaviour of the climate causes climate variations such as El Niño. Simulations of the future do not always agree on how these kinds of climate variations change, and we do not have measurements of the future to compare with model simulations. Fortunately, we can go back to the distant past. In the past, the Earth also saw periods with high CO2 concentrations and increased temperatures, even though reasons were different. In addition, we have measurements and reconstructions of these that we can compare with our model simulations. One of the periods that are comparable to the future in terms of CO2 concentrations and warming is the Pliocene, a period about three million years ago, after the dinosaurs and before the ice ages. In this thesis we investigate climate variations in model simulations of the Pliocene, and the role that elevated CO2 had in the climate of that time. We find that El Niño, but also other climate variations, were considerably weaker than today, partly due to shifts in tropical circulation patterns. These changes are partly related to elevated CO2, but also to other conditions in the Pliocene such as smaller ice sheets and closed ocean gateways. Ultimately, we better understand what drove changes in climate variations in the past, but we also see that direct comparisons with the future should be carefully constructed.