Murray, Lee: University of Rochester (Project Lead)
Project Funding:
2017 - 2020
NRA: 2016 NASA: Interdisciplinary Research in Earth Science
Funded by NASA
Abstract:
Methane (CH4) is an important greenhouse gas and tropospheric ozone precursor that has more than doubled since the preindustrial era. The total source of CH4 to the atmosphere is well known due to good observational constraints on growth rate and loss processes, but the contribution of different source types remains poorly constrained. This limits our ability to predict future trends in atmospheric CH4, which are needed for robust climate forecasts.
The work proposed here aims to reevaluate the atmospheric CH4 budget within the context of the NASA GISS ModelE global chemistry-climate model (CCM), making two significant advances over previous efforts. First we will exploit the unique isotopic signatures of different sources by directly simulating a suite of CH4 isotopologues, allowing us to better refine the source contributions at regional scales, to match isotopic constrains in space and time. Second, we will incorporate new spatially resolved estimates of CH4 outgassing from the ocean and freshwater lakes, which have recently
been identified as major sources of uncertainty but are traditionally neglected in atmospheric budgets.
Methane isotopes have proven to be powerful tools for quantifying individual fluxes in box models, but have rarely been implemented within global CCMs due to the computational burden of tracking multiple isotopologue tracers. To overcome this, we will develop a new offline configuration of the ModelE CH4 cycle that is capable of conducting fast isotope simulations and sensitivity analysis. Outgassing from the ocean to atmosphere will be estimated in a new model of ocean CH4 biogeochemistry, which will simulate methanogenesis in sediments and the surface mixed layer, dissolution of methane clathrates, and oxidation of CH4 in the water column. A computationally efficient ocean circulation model will be used to facilitate data-assimilation and optimization of the CH4 cycle, using a large compilation of dissolved CH4 observations (MEMENTO database), and existing and upcoming isotopic measurements. Outgassing from freshwater lakes will be represented using a statistical parameterization, derived using observations from an ongoing survey of CH4 in the Great Lakes. The atmospheric model, oceanic model, and freshwater parameterization will developed in parallel during the first two years of this project, then combined in the third year to perform a final assessment of the atmospheric CH4 budget, with particular emphasis on placing upper and lower bounds on the oceanic and freshwater sources.
This project is ideally suited to the criteria for the Interdisciplinary Research in Earth Science (IDS) program, combining the expertise of two atmospheric chemists and two chemical oceanographers, who are further subdivided between the modeling and observational branches of their communities. Our objectives address Subelement 1 of the IDS solicitation. Understanding the Global Sources and Sinks of Methane and specifically match the stated interest in exploiting suborbital isotope measurements, and constraining oceanic and freshwater contributions to methane budget. These efforts will make extensive use of NASA remote sensing data, primarily in the use of ocean color measurements for constraining the flux of CH4 from the oceans and freshwater, and in evaluation of the global CH4 budget of a chemistry-climate model. Beyond the specific goals of this proposal, our work will also make important contributions to NASA’s core science objectives. Expanding ModelE to simulate CH4 isotopes and oceanic and freshwater sources will equip NASA with the most complete tool for diagnosis and prediction of the atmospheric methane budget to date, strengthening their contribution to international assessments of near-term climate change.
More details may be found in the following project profile(s):