Terrestrial Methane Cycling During Paleogene Greenhouse Climates

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.

Our proposal is of wide and direct relevance to the Earth Sciences academic community and associated stakeholders. It explicitly addresses two strategic themes of the new NERC strategy for 2007-2012: Earth System Science and Biodiversity. In particular, our research meets three of the six key outcomes identified in the overarching 'Earth system science' theme, as NERC seeks to provide "knowledge of the impact of potential mitigation and adaptation strategies to forthcoming climate change", "understand tipping points and rates of change", and aims to provide "better assessment of the risk of changes in the Earth system to inform policy decisions". The results of this proposal help the UK meet the strategy challenges to "provide forewarning of abrupt changes in the Earth System", to "improve knowledge of the interaction between the evolution of life and the Earth", and to "understand the forces and feedbacks that drive the Earth System". As such, prompt and high profile delivery of our results to our scientific peers is a key component of our Impact strategy, and we have requested funds for conference attendance by the PI, CoIs and PDRAs. We have a very strong track record in conference attendance (including invited keynote presentations) and rapid, high profile publication of our work; that good practice will be continued as part of the proposed work.

Of course, our conclusions will also be of wide interest to the public, policy makers, climate-orientated NGOs and other NERC stakeholders; we, as a society, will benefit from this research because it furthers our understanding of climate change and informs our predictions for future change. As such, we have an ambitious impact programme built around two main (non-academic) themes:

1. Public engagement. The University of Bristol is a national leader in public engagement: Bristol, along with the University of the West of England, is the coordinator of the national engagement beacons; we had the first Professor of Public Engagement (K Sykes); and our two Centres of Excellence in Chemistry and Anatomy are world leading in terms of the quantity and quality of outreach activity. The PI and our Co-Is have been particularly active in this capacity, including serving on the organising committee for the Bristol Festival of Nature, hosting BBC series, giving >100 talks to fellow scientists and the wider public and writing books marketed to the general public (The Emerald Planet: How Plants Changed Earth's History by DJB). Funds to support our Impact Plan will allow us to exploit the best practices developed elsewhere, allowing us to: embed a methane component in the Bristol ChemLabS lecture series (>100 lectures to ca 30,000 people per year); develop a website, complete with interactive tools; and have a prominent display at the Bristol Festival of Nature hosted by ourselves but also Sixth Form students who will have done summer internships in our labs.

2. Stakeholder engagement: We have hosted MPs in the past, and Pancost and Singarayer have numerous connections with the Bristol City Council via our involvement in the University of Bristol centenary fund and the Bristol Festival of Nature. Also via the Festival of Nature, we have excellent connections with the Environment Agency and charitable trusts devoted to wetland awareness and conservation. We will use these connections to host events where our research is explained to and discussed with a range of stakeholders. It is our intention to make our political leaders more aware of the sensitivity of wetlands to climate change - and therefore, the need to better understand these environments. Of course, this will also be an ideal mechanism to update policy makers with the most up-to-date research on this particular 'tipping point' in the Earth System, a particularly important goal of the NERC.