Quantifying the Known Unknown: Including Marine Sources of Greenhouse Gases in Climate Modeling

Researchers have recently estimated that Arctic submarine permafrost currently traps 60 billion tons of methane and contains 560 billion tons of organic carbon in seafloor sediments and soil (Sayedi et al. 2020), a giant pool of carbon with potentially large feedbacks on the climate system. For comparison, humans have released a total of ~500 billion tons of carbon into the atmosphere since the Industrial Revolution. Unlike terrestrial permafrost, the submarine permafrost system has remained a known unknown because of the difficulty in acquiring samples and measurements. Consequently, this potentially large carbon stock never yet considered in global climate models or policy discussions, represents a real wildcard in our understanding of Earths climate. This presentation will detail current work towards quantifying Arctic methane gas releases from the sediments to the water column, and potentially to the atmosphere, where positive climate feedback may occur. Newly developed modeling capability at Sandia National Laboratories and the U.S. Naval Research Laboratory now gives us the ability to probabilistically map gas distribution and quantity in the seabed by using a hybrid approach of geospatial machine learning, and predictive numerical thermodynamic ensemble modeling. The novelty in this approach is its ability to produce maps of useful data in regions that are only sparsely sampled, a common challenge in the Arctic, and a major obstacle to progress in the past. By applying this model to the circum-Arctic continental shelves, and integrating the flux of free gas and dissociating gas hydrates from the sediment column under climate forcing, we aim to provide the most reliable estimate of a time-varying source term for greenhouse gas flux that can be used by global oceanographic circulation and Earth system models. The result will allow us to better understand the wildcard of the submarine permafrost carbon system, and better inform us about the severity of future national security threats that sustained climate change poses. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., for the U.S. Department of Energys National Nuclear Security Administration under contract DE-NA-0003525. SAND2021-9330 A.