All of the conditions that may have led to the methane hydrate release 15,000 years ago do not exist today. Sea levels were much lower and thus, the pressure on the sediments was less. However, there is some evidence that ocean currents that impinge on ocean sediments are getting warmer, especially in the Arctic. Global warming is thus a possible triggering mechanism for massive methane hydrate release in today’s climate.
What causes release of methane hydrates is still poorly understood. Warm waters may destabilize the hydrate zone. Hydrates on the surface of the ocean floor on a ridge may then degass. The sediment may then become unstable and slide down the ridge, exposing other layers of methane hydrate, accelerating the release. As an example, the Storegga slump off the coast of Norway 8000 years ago could have released between 1 and 4 GtC as methane.
Alternatively, an undersea earthquake today, say off the Blake Ridge or the coast of Japan or California might loosen and cause some of the sediment to slide down the ridge or slump, exposing the hydrate layer to the warmer water. That in turn could cause a chain reaction of events, leading to the release of massive quantities of methane.
Another possibility is drilling and other activities related to exploration and recovery of methane hydrates as an energy resource. The hydrates tend to occur in the pores of sediment and help to bind it together. Attempting to remove the hydrates may cause the sediment to collapse and release the hydrates. So, it may not take thousands of years to warm the ocean and the sediments enough to cause massive releases, only lots of drilling rigs.
Returning to the 4 GtC release scenario, assume such a release occurs over a one-year period sometime in the next 50 years as result of slope failure. According to the Report of the Methane Hydrate Advisory Committee, “Catastrophic slope failure appears to be necessary to release a sufficiently large quantity of methane rapidly enough to be transported to the atmosphere without significant oxidation or dissolution.”
In this event, methane will enter the atmosphere as methane gas. It will have a residence time of several decades and a global warming potential of 62 times that of carbon dioxide over a 20-year period.
This would be the equivalent of 248 GtC as carbon dioxide or 31 times the annual man-made GHG emissions of today. Put another way, this would have the impact of nearly 30 years worth of GHG warming all at once. The result would almost certainly be a rapid rise in the average air temperature, perhaps as much as 3°F immediately. This might be tolerable if that’s as far as things go. But, just like 15,000 years ago, if the feedback mechanisms kick in, we can expect rapid melting of Greenland and Antarctic ice and an overall temperature increase of 30°F.