The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures (and/or falls in sea level) can trigger the sudden release of methane from methane clathrate compounds buried in seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to further temperature rise and further methane clathrate destabilization – in effect initiating a runaway process, as irreversible once started as the firing of a gun.

In its original form, the hypothesis proposed that the "clathrate gun" could cause abrupt runaway warming in a timescale less than a human lifetime.
http://en.wikipedia.org/wiki/Clathrate_gun

Paleocene–Eocene Thermal Maximum

The Paleocene/Eocene boundary, 55.8 million years ago, was marked by the most rapid and significant climatic disturbance of the Cenozoic Era. A sudden global warming event, leading to the Paleocene–Eocene Thermal Maximum (PETM, alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene Thermal Maximum",^[1] (IETM/LPTM)), is associated with changes in oceanic and atmospheric circulation, the extinction of numerous deep-sea benthic foraminifera, and a major turnover in mammalian life on land which is coincident with the emergence of many of today's major mammalian orders. The event saw global temperatures rise by around 6°C (11°F) over 20,000 years, with a corresponding rise in sea level as the whole of the oceans warmed.^[2] Atmospheric carbon dioxide (CO₂) concentrations rose, causing a shallowing of the lysocline. Regional deep water anoxia may have played a part in marine extinctions. The event is linked to a negative excursion in the δ¹³C isotope record, which occurs in two short (~1,000 year) pulses. These probably represent degassing of clathrates ("methane ice" deposits), which accentuated a pre-existing warming trend. The release of these clathrates, and ultimately the event itself, may have been triggered by a range of causes.
http://en.wikipedia.org/wiki/Paleocene-Eocene_Thermal_Maximum