By Dr. Robert Thorson

The “thing” lay there on the deck of the fishing trawler, hissing, fizzing and melting into sludge before the crew’s eyes. Frightened and amazed, they shoveled it back to the sea for fear of being asphyxiated. The place was west of Vancouver Island in Canada. The date was fall 2002. The “thing” was an unusual chunk of ice composed of water and methane, the other naturally occurring carbon gas.

Marine sediments rich in organic matter produce methane when they are decomposed by anaerobic bacteria (those that don’t require oxygen). Anaerobes take over from oxygen-aerobic ones when buried below the sea floor to a depth of several feet, especially where the sediments are slimy. Under these conditions methane gas is released. If it doesn’t leak up to the water column, it becomes trapped either in gas pockets, or as the weird form of ice found by the fishermen. The “thing” was a chunk of methane hydrate ice.

Methane ice could be good news for the energy scene, because it’s a possible alternative to petroleum and coal. But it’s bad news for the climate because the released methane gas (CH[subscript 4]) is 25 times more effective in contributing to greenhouse warming than carbon dioxide (CO[subscript 2]). Climate scientists are beginning to worry about uncontrolled releases of seabed methane because they could amplify the more well known CO[subscript 2] warming. We don’t know how much seabed methane is out there, where it is concentrated and how fast it will be released.

Ominously, methane already has been implicated in one of the most dramatic climate warmings ever to occur in the geologic record. This took place about 55 million years ago during the transition from the Paleocene to Eocene periods, long before humans evolved. Over about 100,000 years, the climate warmed abruptly before returning to normal. Air and sea temperatures spiked at least 4 degrees Celsius at mid-latitudes and more near the poles.

Massive die-offs took place as plankton species became extinct. Sea floor creatures suffocated in the warmer water, less able to hold oxygen. Tropical and fair-weather terrestrial plants migrated toward the poles. And creatures migrated over the Bering land bridge from Asia to North America, including the seed stock for three orders of mammals that remain important today: cattle and deer (even- toed ungulates), horses and rhinos (odd-toed ungulates) and the monkeys (primates). An order of extinct carnivores (hyaenodontids) also came with them.

The only plausible explanation for the Paleocene warming was a globe-sized belch of seabed methane. Given the size of the release, the source was almost certainly a chain-reaction meltdown of methane ice. Imagine the floor of the world ocean fizzing like a carbonated beverage before the heat wave sets in.

Much of the seabed methane being released today is a delayed consequence of human activities that increased the amount and organic content of river sediment deposited in estuaries, deltas and on the sea floor beyond. Rapid deforestation since the 18th century caused the erosion of soil humus, which was deposited on the sea floor, and the release of biomass nutrients that stimulated the growth of algae in rivers, lakes and the ocean.

Specific agricultural activities, for example cultivation, the use of artificial fertilizers and the disposal of animal wastes, also contributed. Finally, there are the centuries of sewage dumped offshore from enlarging cities.

The human connection may help explain why the modern hot spots for seabed methane occur generally offshore from rivers that have been strongly affected by human activities — notably the Mississippi Delta region, the Northeastern Seaboard of North America, northwestern Europe, the gulfs of Arabia and China and the mouth of the Amazon. (Kotzebue Sound in Alaska is a notable exception.)

The bottom line is that human-related sediment buried centuries ago may only now be reaching the depth where methane release begins.

Thanks in part to Al Gore, the main greenhouse gas, carbon dioxide, has finally gotten serious attention. I hope that seabed methane will also soon become be a household word, either as a potential energy source or a potential climate changer. The last thing we need now is a global belch of gas, or worse.