Temperature sensitivity and time dependence of the global ocean clathrate reservoir

We assess the impact of anthropogenic warming on the 1-100kyr timescale stability of the methane clathrate reservoir in the oceans. Our assessment is based on model-predicted pCO₂ and temperature responses to anthropogenic CO₂ release, coupled to a mechanistic model for the steady-state inventory of clathrate. The model predicts that the steady-state inventory under present-day conditions would contain ~5000 Gton C, and that the steady-state inventory is very sensitive to temperature. Future anthropogenic warming may trigger the release of carbon from the clathrate reservoir, which oxidizes to CO₂ and contributes to further warming of the deep sea. The strength of this amplifier depends on how quickly the reservoir responds to temperature, because the anthropogenic temperature spike decays with time. The strength of the amplifier also depends on the fraction of methane that escapes from sediments during the meltdown of clathrates. Both of these parameters are still highly uncertain, but plausible values for both predict a global clathrate reservoir which would be unconditionally unstable at the present time, melting down periodically every few million years. If the timescale for meltdown is 10 kyr (conservatively consistent with the Paleocene / Eocene Thermal Maximum event 55 Mya, if that was truly a clathrate carbon release event), then a 50% release fraction is marginally stable. Decomposition of clathrates has the potential to double or triple the anthropogenic perturbation to atmospheric CO₂ 100 kyr from now. The model predicts glacial / interglacial changes in the methane clathrate reservoir of order 10² Gton C, enough to impact the deglaciation carbon and carbon isotopic budgets.