Viernes 12 de Diciembre de 2008

Global boiling
Por Kirsten Weir

By now we all know what's in store for us if we continue on our emissions-happy path: increasingly hotter days, horrific droughts and floods, angrier storms, acidic ocean waters that will dissolve coral reefs, and a surging sea level that will swallow our coastal cities. Still, that scenario is a virtual sunny day by the pool compared to the cataclysmic climate picture being drawn by some scientists. Never mind carbon dioxide emissions. Let's talk about the vast stores of carbon hidden deep beneath our feet.

During the last year, geoscientists have held several workshops and conferences to discuss what is known -- and the great deal that isn't -- about the "deep carbon" cycle. Next week, at the annual meeting of the American Geophysical Union, scientists plan to hold a special session devoted to one potentially frightening aspect of that cycle: a strange little substance known as methane hydrate.

Methane hydrates, or clathrates, are icelike gas deposits buried under permafrost and deep below the seafloor. Some researchers fear that the hydrates are on the verge of melting en masse and belching out a cloud of methane gas that will send global temperatures skyrocketing.

The doomsday scenario goes something like this: If global temperatures keep rising, some methane hydrates will melt, sending methane gas bubbling up through the ocean and into the atmosphere. Like any good greenhouse gas, the methane will trap heat close to Earth's surface, causing temperatures to climb even higher. Hotter temperatures will melt more hydrates, and on and on. In other words, methane hydrates could trigger the mother of all feedback loops. The story, says David Archer, a geophysicist at the University of Chicago, "has a great apocalyptic side to it."

Methane is the same natural gas that we burn for fuel. Under the right combination of intense pressures and chilly temperatures, the gas becomes trapped inside icy cages of hydrogen bonds. These methane hydrates look like chunks of ice, with the nifty difference that they eagerly burst into flame when sparked. Methane hydrates are also a lot less stable than your average ice cube. If the temperature rises or pressure eases, the hydrates essentially melt to form methane gas.

Methane hydrates aren't unusual, astronomically speaking. They exist on Mars, inside comets, and on at least a couple of Saturn's frosty moons. Here on Earth, they form deep below permafrost and under seafloor sediments, where temperature and pressure conspire to keep the structures stable. It's not certain how much methane is locked up in hydrates, but some estimates put the total as high as 10,000 gigatons, says Gerald Dickens, a professor of earth sciences at Rice University. To put it in perspective, he says, "the estimates for all of the oil, gas, and coal [on Earth] is about 5,000 gigatons."

As a greenhouse gas, methane is in the big leagues, some 20 times as potent as carbon dioxide. If all the methane trapped underground were to wind up in the atmosphere, you could kiss your winter boots goodbye. "There is so much [methane hydrate] in the ocean that if you gave the planet a big shake and it came out all at once, it would be a climate disaster far worse than anything we have with carbon dioxide," Archer says.

Are we giving the planet that kind of shake? To predict the future, climate scientists begin by peering into the past. Human-induced global warming may be a new trend, but Earth has certainly experienced rapid and dramatic climate changes in its ancient history. Methane hydrates may have played a role in a period of abrupt warming 635 million years ago, according to a paper published in Nature last spring. The researchers, from UC-Riverside and Flinders University in Australia, point to high levels of methane present in the atmosphere at that time.

Around 55 million years ago, Earth again shifted abruptly from snowy to steamy. Many researchers have fingered hydrates in that warming spell, too. "Methane hydrates may not be the only explanation, but very likely played a large role," says Carolyn Ruppel, a research geophysicist with the U.S. Geological Survey, who will co-chair with Dickens the upcoming American Geophysical Union panel on hydrates.

James Kennett, a professor of earth sciences at UC-Santa Barbara, is a vocal proponent of the idea that methane hydrates have played a role in past climate changes. He also fears they are poised do so again. "The gas hydrates are inherently unstable with warming of the oceans. I can't see why [melting hydrates] would not be inevitable," he says. "The question is just how sensitive the system is."

Kennett argues that much of the geological research community has turned a blind eye to the evidence of methane hydrate's role in climate change. "It's a paradigm problem. The community is not prepared at this time to make a paradigm shift," he says. "[Climate change] is the biggest issue of our time. I think we need to look at this."

He suggests we start by taking a cold, hard look at the Arctic, where a great deal of methane hydrate exists in permafrost and under the continental shelf. Because of the extreme cold, hydrates are stable at shallower depths in the Arctic than anywhere else on Earth. Warm up the Arctic a bit, and these shallow hydrates will be the first to come apart, Kennett warns. "Is this already happening? Are we living in it now?"

Kennett has valid reasons for wondering. Inside the Arctic Circle, the ocean is reportedly bubbling like a freshly uncorked magnum of Dom Perignon. In September, scientists aboard a Russian research vessel described methane gas fizzing up from the seabed in several areas of the Arctic. Just a few days later, British scientists exploring the ocean west of the Norwegian island of Svalbard reported hundreds of these methane plumes.

It all sounds pretty ominous, but researchers aren't ready to attribute the recently observed methane bubbles in the Arctic to melting hydrates. Scientific reports of the plumes have not yet been published or peer-reviewed. Although Kennett is fearful of a methane catastrophe, he's not yet sure this is it. "I need to be convinced," he says.

He's not the only one. For one thing, says Archer, "there weren't observations before, so it's hard to say if it's a new phenomenon." Perhaps methane has been sputtering up from the Arctic for decades, with no one around to see it. What's more, many potential sources of methane exist. As bacteria break down thawing organic matter, they release the gas as a byproduct. "There's all this juicy organic carbon preserved in these areas," Archer points out. "These methane escapes could be from decomposing peat."

Ruppel, too, is a long way from ringing any alarm bells over the Arctic bubbles. "Perhaps people are jumping to conclusions before the story is really clear in the Arctic," she says. "My suspicion is that almost all of that methane has nothing to do with gas hydrates."

But let's imagine, for the sake of argument, that the Arctic gas plumes do turn out to be from methane hydrates. Does that mean it's curtains for life as we know it? Not necessarily.

"Methane beneath the permafrost is probably the most sensitive to change, but it's a small component of the total amount [of methane hydrates]," Dickens says. The vast majority is buried deep below the seafloor, he notes, and would be considerably harder to unlock. "At deep water depths, temperature would have to change 10 or 15 degrees Celsius to remove all the methane," Dickens estimates. "It would be very difficult for all of it to come out."

For that matter, adds Archer, it would be very difficult for even a portion of it to come out. "It would be arrogant to say it's impossible, but nobody has come up with a mechanism to get even 10 percent of this methane into the atmosphere," he says.

Even if methane hydrates did start melting, the gas would have to travel through hundreds of meters of mud and thousands of meters of water before it could mix with the air. "A lot of methane would dissolve in ocean waters," Ruppel says. "The ocean is very undersaturated with methane. It could accommodate a whole lot before the methane would get out into the atmosphere."

Furthermore, Dickens adds, it's not enough to show that methane can travel from the deep ocean to the atmosphere. One also has to consider the rate. "It is possible in the future that large amounts of methane can come out of these systems," he says. "Is it probable that significant amounts will come out in the next 100 years? Probably not."

Archer is also skeptical of the importance of methane hydrates in ancient global-warming events. "The evidence for these things being important for climate change in the past, I think, is kind of dodgy," he says. True, something released a lot of carbon into the atmosphere 55 million years ago. But maybe, he suggests, that something was a volcanic event that spewed methane gas, or a bunch of carbon-rich sediments that were suddenly lifted above sea level and exposed to the air. "There's no real clear smoking gun that it was methane hydrates," he says.

Ruppel says there's definitely more to learn. "I think the jury is still out on this," she says. But she doesn't see any reason for panic. The story that methane hydrates are a looming catastrophe "is a position some of us are working hard to counteract," she says.

In fact, much of the effort put into studying methane hydrates isn't focused on global warming at all, but on energy. The U.S. Department of Energy is taking a close look at mining methane hydrates for fuel, and they aren't the only ones. Countries including Japan, China and India are also exploring ways to turn hydrates into usable energy. "It is getting to the point now that methane hydrates could definitely become a viable commercial source for natural gas within the next 10 years," Ruppel says.

As a fuel, methane hydrate has some advantages. It's more accessible than conventional natural gas resources, Ruppel points out. And it is cleaner to burn and emits about half as much carbon dioxide as does coal. "Natural gas is probably the greenest of the fossil fuels," adds Archer.

But it is a fossil fuel, after all, and human-induced global warming is still a very real phenomenon. So will methane hydrates fuel our future, or destroy it? That may be the ultimate question, but not an easy one to resolve. For those willing to try, "it is a very interesting time in this field," Ruppel says. "We need more good science. I think we're moving in that direction, but it will be a few years before we have the answers."

In the meantime, panic over methane hydrates is probably premature. "There is a tendency in some quarters to latch on to a catastrophism scenario," she says. "That may sell newspapers, but it may not be the most responsible way to portray the science."

If you're the publisher of a sensationalist newspaper, take heart. There's still a good deal to fear when it comes to climate change. "I think the trajectory we're on with CO2 is very likely to lead to droughts that would be destabilizing to civilization. Another thing I worry about is sea-level rise," Archer says. "I think we have plenty to worry about with CO2. We don't need methane hydrates in order to be very reasonably frightened about the future of our climate."