nature.net
May 2006
Gas Trap
By Robert Anderson
The steady blue flame glowing beneath my pancake griddle is, for the most part, burning methane. A simple molecule with four hydrogen atoms bonded to a central carbon (CH4), methane is a clear, odorless gas. Although it cooks our comfort food and is a vital source of energy, methane also has a dark side. It is a powerful greenhouse gas, trapping twenty times as much heat in the atmosphere as a similar volume of carbon dioxide.
The real worry is the methane now trapped in ice. At low temperatures (around thirty-two degrees Fahrenheit) and moderate pressures (about thirty atmospheres), methane and water form an ice called methane hydrate. Relatively unknown until the 1970s, most methane hydrate is locked up in sediments along continental shelves. A smaller amount is buried in Arctic permafrost. Together, those deposits hold twice as much carbon as all the Earth’s reserves of coal, oil, and natural gas combined. If even a small percentage of that methane hydrate melts, it could dramatically increase global warming.
At the National Oceanic and Atmospheric Administration’s Ocean Explorer Web site, Peter A. Rona, a marine geologist at Rutgers University in New Brunswick, New Jersey, conducts a virtual tour to the depths of the eastern Atlantic. There, as he shows the viewer, are vast reserves of methane hydrate. At the bottom of the Web page is a diagram comparing the simple lattice of ordinary ice with the more complex one of methane hydrate. The methane molecules in the complex ice are locked inside “cages” of frozen water.
Methane hydrate does have a good side. Entire communities of deep-sea creatures have evolved the means to thrive on its energy. Texas A & M University’s Web page Lair of the “Ice Worm” describes a colony of polychaete worms whose home is a hydrocarbon seep at the bottom of the Gulf of Mexico.
If investigators figure out how to tap methane hydrate safely, the deposits may help meet humanity’s insatiable demand for fossil fuels. Go to Mining Methane to hear Scott Dallimore, a geotechnical engineer at the Geological Survey of Canada, describe his team’s exploration of hydrate deposits in the Arctic. For a world map of known methane hydrate deposits, go to USGS’s Global Inventory of Natural Gas Hydrate Occurrence and choose the poster at the left in a format for either screen-viewing or printing.
The big fear of many climatologists is that some poorly understood part of Earth’s climate system, pushed to a tipping point by human emissions, might trigger a runaway greenhouse effect. Perhaps the most ominous clues to the threat posed by methane hydrate come from the distant past. According to a NASA news item, 55 million years ago a large release of the frozen seafloor gas heated the Earth by thirteen degrees F. In his article “Ocean Burps and Climate Change?” Gavin A. Schmidt, a NASA climatologist, explains how ratios of isotopes of carbon suggest that methane hydrate was to blame for that dramatic warming.
The most chilling scenario of all, however, comes from the end of the Permian period, when Earth underwent its largest mass extinction. Dan Dorritie, a paleontologist in California, regularly updates his online book, Killer in Our Midst, on the role that he, among others, thinks methane hydrate played in that horrendous event. Dorritie warns, it could happen again.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2006