Palaeo Constraints on the 1.5C World: What does the Pliocene tell us about the long-terms effects of atmospheric CO2 at ~400 ppmv

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"HE IMPORTANCE OF THE 1.5C (WARMER) WORLDBy the end of the 21st century global temperatures will have increased, the oceans will be more acidic and the hydrological cycle will be invigorated (IPCC, 2013). Understanding the geographical patterns, magnitudes and societal impacts associated with these changes is a huge challenge and responsibility. Success in this endeavour is crucial in order to develop robust policy-driven mitigation and adaptation strategies. Prior to the 2015 Paris Agreement the policy position, based on the best available scientific evidence of environmental and societal impacts, was that CO2 emission and stabilisation scenarios should not permit a global annual mean temperature increase of more than 2C by the end of this century. This target was reaffirmed as an outcome of the Paris Agreement. However, due to more recent studies that demonstrated the likely impacts associated with a 2C warmer world, a tougher policy stance was adopted, whereby the global annual mean surface temperature increase should be brought below 1.5C of the pre-industrial era's baseline by 2100 AD. This scientifically informed decision has rapidly developed into a nexus for research across all disciplines that share the common aim of understanding climate change and its societal impacts (Mitchell et al., 2016). THE IMPORTANCE OF EARTH HISTORY AND THE PLIOCENE EPOCHOne of the greatest challenges in understanding what society must do in terms of emissions to limit global temperature change to a specific value (i.e. 1.5C) is resolving the long term sensitivity of the climate system to a given change in CO2 forcing. Warm intervals in the geological past provide science with a unique natural laboratory in which to investigate long-term environmental change, and climate models have been used to simulate past climate, greatly enhancing our understanding of atmospheric, oceanic and ice sheet behaviour (Haywood et al., 2016a; Haywood et al. 2019). The most recent interval in the past known to have had a comparable atmospheric carbon dioxide (CO2) level to today (~410 ppmv) was the Pliocene epoch (~3 million years ago). It was an interval known to be warmer than the pre-industrial era, and shares a number of parallels to model predictions of climate for the end of this century (Haywood et al., 2016a; Burke et al., 2018; Tierney et al. (2019).In the context of an international climate modelling effort (see international partners section below), this project will use a new suite of climate model simulations, and perform new experiments as needed, to investigate the nature of the critical components of the Earth system during the Pliocene to greatly enhance our knowledge and understanding of the long term consequences of atmospheric CO2 being at or above 400ppmv.OBJECTIVESThe student will be given significant academic freedom to follow their interests and explore different components of the long-term climate and Earth system response to CO2 forcing. Potential avenues for investigation include:- Analysis of the behaviour and variability of the jet stream and Arctic climate change- Behaviour of the hydrological cycle and variability in the global monsoon - The nature of, and changes to, temperature gradients and patterns of large-scale atmospheric and oceanic circulation- Modes of climate variability, such as the El Niño Southern Oscillation (ENSO)- The analysis of seasonality, climate and weather extremes- Terrestrial environmental/vegetation responses in the tropics and high latitudes