Carbon-Climate Feedbacks Through The Global Mathane Cycle

Abstract:

Methane, a critical greenhouse gas, is increasing rapidly but neither the causes of this increase, nor the recent large variations in the rate are understood. The tropics are the largest natural source for methane, and a likely focus of much recent variation. We will propose two hypotheses to explain why tropical fluxes may be changing, one linked to recent trends in the carbon cycke, and the other suggesting a connection to trends in the wate cycle. We carried out an observing system experiment using model-data-fusion approach and simulated fluxes arising from both of these hyotheses using a diagnostic model, forced with satellite observations, and then translated them into concentration patterns using an attmspheric transport model. We then sampled these simulated concentrations, taking into account cloud cover and estimated the resulting sampling pattern and radiances, using a full-physics radiation model. This allowed us to simulate the performance of alternative approaches for observing XCH4 from space. We then estimated fluxes usng an inverse model to determine the required characteristics of a satellite mission able to distinguish between our two hypotheses. Geostationary observations, obtained at least 3x/day with an uncertainty of 1% are required to determine regional patterns of tropical methane flux with sufficiently high confidence to distinguish between alternative hypotheses. The key instrument characteristics are determined by the need to obtain adequate data in the presene of clouds. Reducing carbon cycle uncertainty in the tropics requires more sampling than low earth orbit missions are likely to provide.