South Asian methane emissions, inferred from surface, aircraft and satellite observations

甲烷是京都议定书规定的一种强大的温室气体，具有仅次于二氧化碳的第二大辐射强迫。在全球范围内，很大一部分甲烷排放来自湿地和白蚁等自然来源。在南亚，与稻田、反刍动物、化石燃料排放和生物质燃烧等较大的人为来源相比，天然湿地是一个较小但仍然重要的甲烷来源。甲烷的全球平均排放量通常受到很好的限制，但为了具体量化南亚的排放量，需要在靠近南亚来源的地方进行测量。为了实现这一目标，2011年在印度大吉岭建立了一个测量站点，用于现场测量甲烷，每天收集70多个测量数据。在这个项目中，将利用地面、飞机和卫星对南亚地区甲烷浓度的观测，对每个来源部门（化石燃料、湿地、牛等）的甲烷排放量进行量化。目前，卫星可以测量全球的甲烷浓度，由于其覆盖全球，卫星是强大的工具，而地面站点只能测量一个地点。卫星的主要限制是，在云层和灰尘掩盖甲烷信号的地区，卫星容易出错。在这项建议中将进行的研究途径之一是将所有地面、飞机和卫星测量结合起来，并利用合并后的数据提供关于南亚卫星观测质量的信息。这将有助于更好地了解未来如何在该地区使用这些卫星观测结果。印度大气中甲烷的主要来源之一来自稻田，也被称为人为湿地。控制这些排放的过程与发生在自然湿地的过程类似。当水覆盖土壤表面时，下层土壤中的氧气就被剥夺了。在这些“缺氧”区，细菌通过一条导致甲烷产生的途径代谢土壤中的有机物。然而，人类对水管理、肥料使用和其他农业实践的影响使得稻田甲烷产生与自然驱动的湿地过程非常不同。例如，人类通过灌溉来人为地改变地下水位。农民还向土壤中添加了额外的营养物质，为细菌提供了比自然湿地更多的有机物质和氮。此外，稻田通常是在原本没有湿地的地区种植的，因此是人工引入的。这项研究的主要目标之一是提高我们对驱动印度湿地源（天然湿地和稻田）甲烷排放过程的理解。我将采用一个“湿地”模型，将农业实践纳入其中，模拟地面的排放，并采用一个大气运输模型，将土壤中产生甲烷的过程与我们在大气中进行的测量联系起来。这些大气观测将用于改进湿地模式中的过程。使用这个工具，我将从印度次大陆的所有来源得出最准确和最新的甲烷排放估计。最后，我将根据政府间气候变化专门委员会第五次评估报告（IPCC AR5）中使用的预测水稻生长和未来气候情景，预测印度水稻农业到2100年的甲烷排放量。

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. In this project, methane emissions from each source sector (fossil fuel, wetlands, cattle, etc) will be quantified 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 because they can be prone to errors in areas where clouds and dust can mask the methane signal. One of the avenues of research that will be done in this proposal is 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 will help 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 is to improve our understanding of the processes that drive methane emissions from wetland sources in India (from both natural wetlands and rice paddies). I will employ a "wetland" model, 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 will be used to improve the processes in the wetland model. Using this tool, I will produce the most accurate and up-to-date methane emissions estimates, from all sources, from the Indian subcontinent. Finally, I will project emissions of methane from rice agriculture in India to the year 2100 using projected rice-growth and future climate scenarios such as those used in the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5).