Ocean methane hydrates as a slow tipping point in the global carbon cycle

Abstract

We present a model of the global methane inventory as hydrate and bubbles below the sea floor. The model predicts the inventory of CH(4) in the ocean today to be approximately 1600-2,000 Pg of C. Most of the hydrate in the model is in the Pacific, in large part because lower oxygen levels enhance the preservation of organic carbon. Because the oxygen concentration today may be different from the long-term average, the sensitivity of the model to O(2) is a source of uncertainty in predicting hydrate inventories. Cold water column temperatures in the high latitudes lead to buildup of hydrates in the Arctic and Antarctic at shallower depths than is possible in low latitudes. A critical bubble volume fraction threshold has been proposed as a critical threshold at which gas migrates all through the sediment column. Our model lacks many factors that lead to heterogeneity in the real hydrate reservoir in the ocean, such as preferential hydrate formation in sandy sediments and subsurface gas migration, and is therefore conservative in its prediction of releasable methane, finding only 35 Pg of C released after 3 degrees C of uniform warming by using a 10% critical bubble volume. If 2.5% bubble volume is taken as critical, then 940 Pg of C might escape in response to 3 degrees C warming. This hydrate model embedded into a global climate model predicts approximately 0.4-0.5 degrees C additional warming from the hydrate response to fossil fuel CO(2) release, initially because of methane, but persisting through the 10-kyr duration of the simulations because of the CO(2) oxidation product of methane.

Fig. 1.

Maps from the bathymetric model. (A) Active (red) versus passive (purple) margin classification. (B) Column-integrated inventory of methane in hydrates. (C) Column-integrated methane in bubbles. (D) Area average bubble fraction of the pore volume. (E) Area average bubble fraction upon melting of the hydrate at the base of the stability zone. (F) Column inventory of releasable methane, defined as in text.

Fig. 2.

Depth/latitude sections of methane distribution, from the bathymetric model and from CLIMBER.

Fig. 3.

Basin inventories of methane in hydrate (A) and bubbles (B). There are 4 versions of the bathymetric model (the full-default configuration and 3 sensitivity studies: uniform T, uniform O₂, and all active margin type) and CLIMBER.

Fig. 4.

Depth distribution of methane in hydrates (A) and bubbles (B) from the bathymetric model, and from CLIMBER (C).

Fig. 5.

Results from the bathymetric model. (A) Methane inventories binned according to the organic carbon content of the surface sediment. (B) Bubble volumes averaged in bins of organic carbon content. The line labeled “detect.” indicates a 0.5% seismic delectability threshold.

Fig. 6.

Sensitivity of the total ocean inventory of methane from hydrates plus bubbles to uniform changes in ocean temperature, from the bathymetric model and from CLIMBER.

Fig. 7.

Amount of methane released by the bathymetric model as a function of a uniform change in ocean temperature (x axis), and the value of the critical bubble fraction enabling gas escape from the sediment column (symbols as indicated in the figure).

Fig. 8.

Transient response of the ocean methane hydrate reservoir within the CLIMBER model subjected to fossil fuel CO₂ forcing of 1,000 and 5,000 Pg of C. The model assumes a critical bubble fraction of 2.5%.