Note:This rather alarming projection of the likely course of climate change during the next century is a from a note at the end of Will Barnes' essay "What Are Our Chances: Is there an Emancipatory Resolution of the Converging Crises in Nature and Society?" A site with a variety of his writings is at this link.

If it is permitted to initiate, a runaway warming will start from a dissociation of methane hydrates, otherwise known as clathrates. Commonly thought to form through the bacterial breakdown of organic matter in low oxygen environments, clathrates are also continuously created as a result of shifting tectonic movement: An overlay of a map of methane hydrate deposits worldwide on a global map of tectonic plates shows that clathrates are densest along those continental coastal regions where plates are most active, and where areas of [either] subduction or divergence are littered with active volcanoes (e.g., the Pacific and North American plates along the western continental coast, the Aleutian Trench, the Pacific and Eurasian plates in the area of Japan and the Sea of Japan, and the divergent plate boundary in the Arctic between the North American and Eurasian plates known as the Gakkel Ridge).

Tectonic processes (and, to be sure, volcanic ones as well) emit gases, including carbon dioxide, which, in contact with hydrogen radicals in ocean waters, form methane. In this cold and with [the] pressure at these depths, they, like the methane excreted by bacteria consuming dead organic materials, are frozen in water-molecular cage-like solid structures. These structures are clathrates. Intense pressure or extreme cold, or both, are the (only) conditions under which clathrates are stable. They will become unstable as the oceans warm, particularly in shallow waters such as those in the Arctic Ocean, e.g., along the coast of northern Siberia. (A similar situation obtains with [the] partially decomposed organic matter [from] 50 million year old tropical rainforests... that is buried under the Arctic permafrost.)

Methane hydrates have been settling in the shallow sea sediment in vast quantities since the late Pliocene (i.e., for well over 2 million years) as ocean waters rapidly cooled. There have been limited releases from 16,000 to 12,000 years ago, and from roughly 130,000 to 115,000, 250,000 to 245,000, 350,000 to 340,000, etc., i.e., during the rapidly warming phases of interglacials, over at least the last 800,000 years. In these instances, the release has been a consequence of cosmological determinism or orbital forcings (i.e., the closer orbital position of the Earth to the sun). Their release has, accordingly, been “slow” enough (and limited [enough]) that the methane has been almost entirely oxidized.

Today, the situation is different, in fact unprecedented. First, if the warming is rapid enough some (not all) of the clathrates will not form the less deadly greenhouse gas CO2 (i.e., they will not be oxidized in the water). Instead these will be released directly to the atmosphere as methane. Occurring at 10,000 times the rate of an orbital forcing, “anthropogenic warming,” i.e., warming induced by the movement of capital, here and now we are witnessing a release in a contemporary (not historical, and certainly not geological) time frame for which there is no analog. Second, because these releases have been limited, the quantities of clathrates have grown absolutely over geological time and grown enormously.

The highest latitudes play a decisive role in warming, especially the Arctic.... At the moment of the last mass extinction “event” at the end of the Paleocene [and] inaugurating the Eocene (some 55 million years ago), an enormous warming was generated by volcanic eruptions of carbon dioxide. What is particularly significant and portentous is that clathrate release was not of the same order of significance to warming then as it is now. (One has to return to the end Permian extinction “event” for a geological analog to the situation we face.)

As clathrates dissociate, some more rapidly than others, two events transpire. As the clathrates in the warmest, shallower waters are released from pressure the gas trapped in them will not be oxidized but will release directly to the atmosphere and expand to 160 times its volume at the original moment of rupture. Clathrates in deeper water, that warm more slowly, will be consumed by ancient photosynthetic anaerobic bacteria (methogens) that inhabit sea water sediments far below the chemocline (that point at which oxic waters above are separated from anoxic waters below). These bacteria metabolize sulfur and methane (as we do oxygen) and produce hydrogen sulfide as a waste byproduct. The chemocline will begin to rise (it already has) closer to the surface as the oceans warm, giving these bacteria greater access to the sun, allowing them to thrive. Enough warming and ocean waters will visibly exhibit vast stretches of sulfur bacteria at the surface, as great bubbles of hydrogen erupt into the atmosphere. Carbon dioxide is poisonous, methane is far more poisonous, and hydrogen sulfide is qualitatively [even] deadlier: one molecule in a thousand of air kills human beings. A single breath can be deadly. There is as much methane within clathrates as the aggregated amounts of all the gas and oil reserves on Earth. If this is not enough to fuel a methane-based, hydrogen sulfide mass extinction, then, to boot, the atmospheric presence of hydrogen sulfide destroys ozone (H2S + O3 ? H2O + O2 + S). As it has in the geologically reconstructed past mass extinctions, lethal UV sun rays will kill off the plant and animal life that survives hydrogen sulfide poisoning...

Global warming will bring intense pressure to bear on the Earth's crust through thermal expansion of tectonic plates, leading to earthquakes that will exacerbate the clathrate situation further, by insuring destabilization of its ice crystalline-like structure in the deepest waters. A direct release of methane into the atmosphere will, in turn, vastly heat it up, further warming the oceans, releasing more methane to the atmosphere, further warming the oceans, releasing more methane now in deeper waters in a vicious cycle that would not be broken until, most significantly, Canfield oceans, the prevalence of anoxic bacteria, and a vast increase in atmospheric hydrogen sulfide content form the essential features of a new Earth. This would constitute a runaway warming with its deadliest consequence, a methane-based, hydrogen sulfide mass extinction destroying aerobic life as such.

There has been one generally recognized methane-based hydrogen sulfide extinction “event”, and perhaps at least three more in the geologically-reconstructed past. Far and away the most deadly (perhaps as much as 95% of all species life on Earth disappeared), this extinction defined the boundary between the Permian and the Triassic about 251 million years ago. Others are likely to have occurred at the end Triassic, middle Cretaceous and late Devonian. Methane release to the atmosphere is now occurring again. It is transpiring in enormous amounts in the Arctic, for example [during late summer of 2011], at levels that portend a mass extinction of aerobic life (which, of course, includes human life).