PaleoGradPhan: Paleoclimate meridional and zonal Gradients in the Phanerozoic

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The temperature of the Earth today is not uniform everywhere, but, in general, is warmest near the Equator and coldest at the poles (the "meridional temperature gradient"), and varies from west to east, for example with warmer waters in the tropical west Pacific than in the tropical east Pacific (the "zonal temperature gradient"). These gradients are a fundamental property of the climate system - for example being controlled by, and controlling, atmospheric and ocean circulation, water vapour, clouds, and sea ice. The temperature gradients are also crucial for understanding the role of feedbacks (such as the interactions between ice and climate), as well as being important for determining the distribution of many land-surface properties, environments, and ecologies.The primary reasons for these modern temperature gradients are relatively well understood; however, as we go back millions of years into Earth's history, knowledge and understanding of temperature gradients decreases rapidly. Knowledge of past gradients comes primarily from the geological record - estimates of temperature from indirect sources such as fossils. Understanding of the physical mechanisms that controlled past gradients comes primarily from climate models, which can be configured to run for intervals in the past and whose results can be interrogated to determine the reasons for past changes in gradients. However, estimates of past gradients are limited to a few "snapshots" of time that have been studied in detail, and results from many models have been inconsistent with the geological record for those time periods. Because temperature gradients are such a fundamental property of the climate system, without a robust knowledge and understanding of past gradients, we cannot begin to claim to understand the climate history of our planet - which is a first-order blue-skies question.Several recent developments mean that we are now in a position to make substantial progress on this "grand challenge" question. Firstly, there has been a recent push from the geological community to collate, quality-control, and make available an extensive database of past temperatures over the last 500 million years. This database provides an ideal source for reconstructing temperature gradients over time. Secondly, recently we have been developing a version of the UK Met Office climate model that can produce temperature gradients in agreement with the geological record at selected snapshots in time which have been studied, and which can run fast enough on a supercomputer to allow us to carry out the required number of simulations. As such, with these new tools at our disposal, we are ideally placed to study this question.In particular, in this proposed project we will analyse the new database to reconstruct temperature gradients through the Phanerozoic, carry out new climate model simulations through the same time period, and evaluate the temperature gradients from the model with the temperature data. In addition, for selected time periods that show substantial changes in gradients, we will collate published data to evaluate some of the key mechanisms in the model that lead to the modelled temperature gradients, in particular ocean circulation, the hydrological cycle, and vegetation. We will also interrogate the model to ascertain the reasons for the modelled changes in gradients, including a focus on the role of changes in ocean circulation. Finally, we will integrate our model simulations and the geological data using a statistical framework that we have previously developed to explore global average temperature changes, and communicate our findings to other scientists and to the general public.Overall, by the end of the project we will have made a step-change in our knowledge and understanding of past temperature gradients over the last half a billion years, and therefore be closer to our ultimate goal of fully understanding the climate history of our planet.