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

到21世纪末，全球气温将会上升，海洋的酸性将会增强，水文循环将会活跃（IPCC, 2013）。了解与这些变化相关的地理格局、规模和社会影响是一项巨大的挑战和责任。这一努力的成功对于制定强有力的政策驱动的缓解和适应战略至关重要。在2015年《巴黎协定》之前，根据现有的关于环境和社会影响的最佳科学证据，政策立场是，到本世纪末，二氧化碳排放和稳定情景不应允许全球年平均气温上升超过2摄氏度。这一目标作为《巴黎协定》的成果得到了重申。然而，由于最近的研究表明，全球变暖2摄氏度可能带来的影响，因此采取了更强硬的政策立场，即到公元2100年，全球年平均地表温度增幅应控制在工业化前时代基线的1.5摄氏度以下。这一科学决策已迅速发展成为所有学科研究的纽带，这些学科的共同目标是了解气候变化及其社会影响（Mitchell etal ., 2016）。地球历史和上新世时代的重要性要了解社会在排放方面必须做些什么才能将全球温度变化限制在一个特定的值（即1.5摄氏度），最大的挑战之一是解决气候系统对二氧化碳强迫的特定变化的长期敏感性。过去地质时期的温暖间隔为科学研究长期环境变化提供了独特的自然实验室，气候模型已被用于模拟过去的气候，极大地增强了我们对大气、海洋和冰盖行为的理解（Haywood et al., 2016a; Haywood et al. 2019）。在过去已知的最近的一个与今天相当的大气二氧化碳（CO2）水平（~410 ppmv）是上新世（~ 300万年前）。这是一个已知比工业化前时代更温暖的间隔，与本世纪末的气候模型预测有许多相似之处（Haywood等人，2016a； Burke等人，2018；Tierney等人（2019））。在国际气候模拟工作的背景下（见下文国际合作伙伴部分），该项目将使用一套新的气候模式模拟，并根据需要进行新的实验，研究上新世地球系统关键组成部分的性质，以大大提高我们对大气二氧化碳达到或高于400ppmv的长期后果的认识和理解。学生将有很大的学术自由，根据自己的兴趣探索长期气候和地球系统对二氧化碳强迫响应的不同组成部分。潜在的调查途径包括：-分析急流和北极气候变化的行为和变率-全球季风的水文循环和变率的行为-大尺度大气和海洋环流的温度梯度和模式的性质及其变化-气候变率模式，如厄尔Niño南方涛动(ENSO)-季节性分析；气候和极端天气-热带和高纬度地区的陆地环境/植被反应

"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