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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
Brief summary: 
When did humans become the dominant influence on the methane budget?
Importance of the area of research concerned: 
Methane has a global warming potential ~28 times higher than that of carbon dioxide over a 100 yr timescale. Methane levels in the atmosphere are currently increasing more rapidly than expected, with grim implications for the goals of the UN Paris Agreement. Incomplete understanding of the global methane budget undermines our ability to predict future trends. The precise impact humans on the methane budget is unknown with recent work arguing that methane emissions from fossil fuel burning by humans are substantially higher than accepted estimates [1], balanced by a lower contribution from natural geological sources. However, other research argues for the importance of natural geological sources at the expense of emissions from wetlands [2] or biomass burning. By extending the atmospheric record back in time using the archives of ancient air trapped in ice core bubbles we can examine the pre-industrial atmosphere without significant fossil fuel burning. This offers the opportunity to resolve the methane budget for the pre-industrial and present-day atmospheres in a consistent way and ask the question: when did humans become the dominant influence on the methane budget?
Project summary : 
This project uses a holistic approach to better constrain the global methane budget by including constraints from ice core gas measurements as well as recent atmospheric observations. Previous attempts to balance the present-day methane budget using box modelling have concentrated on only the last few decades rather than using the additional information ice cores provide about pre-industrial methane emissions. Furthermore, many used out-dated d13C-CH4 source signatures with large uncertainties [2], which limited the ability to distinguish between emissions from different sources such as wetlands and biomass burning. The evolution of atmospheric methane and its isotopic signature will be simulated using box-modelling to obtain realistic scenarios for source-sink changes over the last 1000 years. Methane budget predictions will then be tested using a sophisticated chemistry-climate model.
What will the student do?: 
The student will begin by harmonising all the data relevant to the atmospheric methane budget over the last 1000 years from ice cores and observational networks. These data will be the target for modelling efforts and include not just methane and d13C-CH4 but also d14C-CH4, dD-CH4, carbon monoxide ethane. Carbon monoxide and ethane are both co-emitted by certain methane sources, while carbon monoxide also influences the strength of the main methane sink. Using dD-CH4 and 14C-CH4 in addition to d13C-CH4 improves delineation between source contributions. The student will use a newly-developed box model capable of simulating all these variables and will implement a range of source/sink optimizations. The optimum scenarios will be tested using the Unified Model-UK Chemistry Aerosol (UM-UKCA) model that includes realistic fluxes and isotopic signatures of methane sources [3]. There will also be the option to run a version of UM-UKCA with full atmospheric chemistry to examine sink changes. Model simulations will be analysed against the atmospheric data collected to reconstruct the evolution of the global methane budget from the pre-industrial to present-day.
References - references should provide further reading about the project: 
[1] Hmiel, B., Petrenko, V.V., Dyonisius, M.N., Buizert, C., Smith, A.M., Place, P.F., Harth, C., Beaudette, R., Hua, Q., Yang, B., Vimont, I., Michel, S.E., Severinghaus, J.P., Etheridge, D., Bromley, T., Schmitt, J., Faïn, X., Weiss, R.F., Dlugokencky, E., 2020. Preindustrial 14 CH4 indicates greater anthropogenic fossil CH4 emissions. Nature 578, 409–412.
[2] Schwietzke, S., Sherwood, O.A., Bruhwiler, L.M.P., Miller, J.B., Etiope, G., Dlugokencky, E.J., Michel, S.E., Arling, V.A., Vaughn, B.H., White, J.W.C., Tans, P.P., 2016. Upward revision of global fossil fuel methane emissions based on isotope database. Nature 538, 88–91.
[3] Heimann, I., Griffiths, P.T., Warwick, N.J., Abraham, N.L., Archibald, A.T., Pyle, J.A., Methane Emissions in a Chemistry-Climate Model: feedbacks and climate response. J. Adv. Model. Earth Syst. In Press.
You can find out about applying for this project on the Department of Earth Sciences page.
Dr Rachael Rhodes
Department of Earth Sciences Graduate Administrator
Dr Anna Jones