This project was funded by the Natural Environment Council (NERC) with the grant reference - NE/L010992/1 - led by Dr Anita Ganesan (University of Bristol).

Methane is a powerful Kyoto Protocol regulated greenhouse gas and has the second largest radiative forcing after carbon dioxide. Globally, a large fraction of methane emissions are naturally occurring from sources such as wetlands and termites. In South Asia, natural wetlands are a smaller but still significant source of methane compared to the larger anthropogenic sources such as rice paddies, ruminants, fossil fuel emissions and biomass burning. Global average emissions of methane are generally well constrained but in order to specifically quantify emissions from South Asia, measurements taken in close proximity of South Asian sources are required. To fulfill this objective, a measurement site in Darjeeling, India was established in 2011 to measure methane on-site and collects over 70 measurements per day. This project quantified methane emissions from each source sector (fossil fuel, wetlands, cattle, etc) by using surface, aircraft and satellite observations of methane concentration over South Asia.

Satellites currently measure methane concentrations around the globe and are powerful tools because of their global coverage, while surface sites only measure at one location. The main limitation with satellites is that they can be prone to errors in areas where clouds and dust can mask the methane signal. One of the avenues of research that was done in this project was to combine all of the surface, aircraft and satellite measurements together and use the combined data to provide information on the quality of the satellite observations over South Asia. This helped to better understand how these satellite observations can be used over this region in the future.

One of the main sources of atmospheric methane in India comes from rice paddies, also known as anthropogenic wetlands. The process that governs these emissions is similar to that occurring in natural wetlands. When water covers the surface of the soil, oxygen is deprived to lower layers of the soil. In these anoxic zones, bacteria metabolize organic matter in the soil through a pathway that leads to methane production. However, the human influence on water management, fertilizer use and other agricultural practices makes rice paddy methane production a very different problem from a naturally driven wetland process. For example, humans artificially alter the water table level through the use of irrigation. Farmers also add additional nutrients into the soil, providing more organic matter and nitrogen for bacteria to utilize than would be found in a natural wetland. Furthermore, rice paddies are often created in areas that wouldn't otherwise contain wetland and thus are artificially introduced.

One of the main objectives of this research was to improve our understanding of the processes that drive methane emissions from wetland sources in India (from both natural wetlands and rice paddies). A wetland model was used and modified to include agricultural practices, to simulate emissions from the ground as well as a model of atmospheric transport to link the processes producing methane in the soil to the measurements that we make in the atmosphere. These atmospheric observations were used to improve the processes in the wetland model. Using this tool, the project produced the most accurate and up-to-date methane emissions estimates, from all sources, from the Indian subcontinent. Finally, projected emissions of methane from rice agriculture in India to the year 2100 were created using projected rice-growth and future climate scenarios such as those used in the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5).

The overall objective of this project was to improve understanding of the sources and processes that govern methane emissions from South Asia. The work accurately quantified South Asian methane fluxes from all source sectors and elucidate the drivers of South Asian wetland variability using the complete set of surface, aircraft and satellite observations from South Asia.

The main objectives of this study were to:
1. Quantify methane emissions from all source sectors in South Asia
2. Present a robust validation of the GOSAT satellite product over South Asia using surface and aircraft measurements
3. Elucidate the drivers of wetland/rice paddy variability in South Asia using atmospheric observations and a dynamic vegetation model modified to include agricultural practices
4. Predict future South Asian wetland/rice emissions and the climate feedbacks and response to this forcing
5. Continue an active measurement campaign at a newly developed in situ station in India and deploy new measurement systems (halocarbons, isotopologues)
6. Apply the novel model framework developed for South Asia to estimate East Asian methane emissions using AGAGE observations from China, Korea and Japan.
7. Incorporate nitrous oxide observations into the model framework to improve understanding of the nitrogen cycle