Methane emissions from inland waters: Quantifying the largest uncertainty in the global methane budget

甲烷（CH4）是仅次于二氧化碳的第二大温室气体，相对于1850- 1901年，2010-2019年温室气体驱动的变暖占35%。甲烷排放量继续以每年18.1 ppb的速度增加。在全球范围内，水生生态系统约占甲烷源的一半，内陆水的排放即使不是最不确定的，也是全世界最不确定的甲烷源之一2。与内陆水域甲烷排放相关的不确定性来源包括，人们对不同大小和河流顺序的河流排放的经验驱动因素如何随着不同的河流流量、河流管理制度（如筑坝）、包括温度和光照可用性在内的季节变化以及土地利用类型的变化而变化的认识不足，这些变化会影响营养物浓度，进而影响生态系统代谢。由于缺乏大规模河网数据产品（包括河段尺度的流量数据），解决跨越空间和时间的甲烷排放的全系统驱动因素受到限制。最近出版的全球河段尺度的河流流量模型MERIT-Hydro4和35年的相关流量数据（GRADES）5现在允许开发生物地球化学模型，以以前不可能的分辨率跟踪全球内陆水域的级联通量和溶解成分（如甲烷）的转化，从而使局部结论和全球结论都能得出。项目目标本研究的主要目标是使用结合模型和实地的方法来量化大规模（国家、大陆和/或全球）内陆水域（包括河流、水库、湖泊、湿地和河口）甲烷排放的控制和驱动因素，并预测未来排放将如何变化。建模组件将依靠高分辨率水文和GIS数据产品作为系统的主干，并结合学生收集的经验观测现场数据、机械或动力学数据以及机器学习方法来约束甲烷排放机制。实地数据将包括在战略采样点安装在河流和水体上的浮动温室气体通量室的使用。跨学科的方法将允许学生开发一个项目，整合(1)水文建模，(2)生物地球化学建模，(3)气候建模的要素。项目成果将直接关系到IPCC和全球碳项目的利益相关者（https://www.globalcarbonproject.org/），以及旨在实现气候排放目标的地方和国家政府。

Methane (CH4) is the second most important greenhouse gas after carbon dioxide, accounting for 35% of the greenhouse gas-driven warming in 2010-2019 relative to 1850-19001. Methane emissions continue to increase annually at a rate of 18.1 ppb/yr. Globally, aquatic ecosystems account for approximately half of methane sources, with inland water emissions among the most, if not the most, uncertain worldwide methane source2. Sources of uncertainty associated with the methane emissions in inland waters include a poor understanding of how the empirical drivers of emissions change across rivers of varying sizes and stream orders in response to different river flows, river management regimes (e.g. damming), seasonal changes including temperature and light availability, and land use types, which influence nutrient concentrations and in turn, ecosystem metabolism3. Addressing the whole-system drivers of methane emissions across space and time has previous been limited by the absence of large-scale river network data products that include flow data at the reach scale. The recent publication of the global reach-scale, river flow model MERIT-Hydro4 and 35 years of associated flow data (GRADES)5 now allows for the development of biogeochemical models that track the cascading fluxes and transformations of dissolved constituents such as methane through inland waters worldwide at previously impossible resolutions, enabling local conclusions to be generated as well as global.Project goalsThe primary goal of this research is to use a combined model- and field-based approach to quantify the large-scale (national, continental, and/or global) controls and drivers of methane emissions from inland waters, including rivers, reservoirs, lakes, wetlands, and estuaries, and forecast how emissions will change in the future. The modelling component will rely on the high-resolution hydrological and GIS data products now available as the system backbone, with methane emission mechanisms constrained using a combination of empirical observation field data collected by the student, mechanistic or kinetic data, and machine learning approaches. Field data will include the use of floating greenhouse gas flux chambers installed on rivers and water bodies at strategic sampling points. An interdisciplinary approach will allow the student to develop a project that integrates elements of (1) hydrological modelling, (2) biogeochemical modelling, and (3) climate modelling. The project outcome will be directly relevant to IPCC and Global Carbon Project stakeholders (https://www.globalcarbonproject.org/), as well as local and national governments aiming to meet climate emissions goals.