Terrestrial methane cycling during Paleogene greenhouse climates

从前工业时代到今天，人类活动导致了二氧化碳分压和甲烷水平的增加。虽然前者的增加主要是由于化石燃料的燃烧，但甲烷浓度的增加更为复杂，不仅反映了直接的人类活动，还反映了气候系统中与温度和水文引起的甲烷排放变化相关的反馈机制。为了解开这些复杂的关系，科学家们越来越多地对古代气候系统进行研究。同样，古气候研究的主要挑战之一是了解甲烷生物地球化学在控制无冰、高二氧化碳分压温室世界（如古近纪早期（约50Ma））气候中的作用。缺乏甲烷浓度的代用物是有问题的，因为湿地的甲烷排放受降水和温度的控制，因此它们可能对气候起到重要的正反馈或负反馈作用。事实上，对过去甲烷水平（pCH4）的唯一估计来自我们的气候-生物地球化学模拟，其中gcm驱动了谢菲尔德动态植被模型，甲烷通量由此得出。这些结果表明，古近纪的pCH4可能几乎是现代工业化前水平的6倍，而这样的数值会产生一种辐射强迫效应，几乎相当于二氧化碳分压的两倍，这种影响在二氧化碳水平已经比今天高得多的时间间隔内可能会特别剧烈。因此，提高对古近系pCH4的认识对于理解生物地球化学过程如何在变暖的地球上运作以及理解地球历史上这一重要时期的气候至关重要。我们建议使用改进的土壤生物地球化学算法来改进、扩展和质疑这些模型结果，进行模型敏感性实验，并将我们的结果与古湿地甲烷循环的替代记录进行比较。前者将提供更好的、以过程为导向的对生物源微量气体排放的理解，特别是CH4、NOx和N2O的排放。灵敏度实验将集中在改变二氧化碳分压水平和操纵决定云形成的大气参数；总之，这些实验将限制我们对痕量温室气体估计的不确定性。为了对这些模型进行定性测试，我们将量化脂质生物标志物并确定它们的碳同位素组成，以估计过去产甲烷和产甲烷种群的规模，并将它们与现代泥炭和全新世泥炭进行比较。我们项目的最后一个组成部分将是确定这些升高的甲烷（和其他微量气体）浓度如何对全球变暖起到积极的反馈作用。综合起来，我们的工作将验证一个假设，即始新世温室期间二氧化碳分压、大陆温度和降水的升高导致湿地温室气体排放和大气浓度的增加，并对气候产生重要的反馈，这是迄今为止大多数模型研究所缺失的。这项工作对我们了解温室气候至关重要，但这种综合方法在世界上任何其他地方都没有进行过；在这里，它由有机地球化学、气候、植被和大气模型、古植物学和煤岩石学方面的国际专家领导。这将代表着我们对古代生物地球化学循环及其对未来全球变暖的潜在反应的理解向前迈出了一大步。

Human activity has led to an increase in pCO2 and methane levels from pre-industrial times to today. While the former increase is primarily due to fossil fuel burning, the increase in methane concentrations is more complex, reflecting not only direct human activity but also feedback mechanisms in the climate system related to temperature and hydrology-induced changes in methane emissions. To unravel these complex relationships, scientists are increasingly interrogating ancient climate systems. Similarly, one of the major challenges in palaeoclimate research is understanding the role of methane biogeochemistry in governing the climate of ice-free, high-pCO2 greenhouse worlds, such as during the early Paleogene (around 50Ma). The lack of proxies for methane concentrations is problematic, as methane emissions from wetlands are governed by precipitation and temperature, such that they could act as important positive or negative feedbacks on climate. In fact, the only estimates for past methane levels (pCH4) arise from our climate-biogeochemistry simulations wherein GCMs have driven the Sheffield dynamic vegetation model, from which methane fluxes have been derived. These suggest that Paleogene pCH4 could have been almost 6x modern pre-industrial levels, and such values would have had a radiative forcing effect nearly equivalent to a doubling of pCO2, an impact that could have been particularly dramatic during time intervals when CO2 levels were already much higher than today's. Thus, an improved understanding of Paleogene pCH4 is crucial to understanding both how biogeochemical processes operate on a warmer Earth and understanding the climate of this important interval in Earth history.We propose to improve, expand and interrogate those model results using improved soil biogeochemistry algorithms, conducting model sensitivity experiments and comparing our results to proxy records for methane cycling in ancient wetlands. The former will provide a better, process-orientated understanding of biogenic trace gas emissions, particularly the emissions of CH4, NOx and N2O. The sensitivity experiments will focus on varying pCO2 levels and manipulation of atmospheric parameters that dictate cloud formation; together, these experiments will constrain the uncertainty in our trace greenhouse gas estimates. To qualitatively test these models, we will quantify lipid biomarkers and determine their carbon isotopic compositions to estimate the size of past methanogenic and methanotrophic populations, and compare them to modern mires and Holocene peat. The final component of our project will be the determination of how these elevated methane (and other trace gas) concentrations served as a positive feedback on global warming.In combination our work will test the hypothesis that elevated pCO2, continental temperatures and precipitation during the Eocene greenhouse caused increased wetland GHG emissions and atmospheric concentrations with a significant feedback on climate, missing from most modelling studies to date. This work is crucial to our understanding of greenhouse climates but such an integrated approach is not being conducted anywhere else in the world; here, it is being led by international experts in organic geochemistry, climate, vegetation and atmospheric modelling, and palaeobotany and coal petrology. It will represent a major step forward in our understanding of ancient biogeochemical cycles as well as their potential response to future global warming.