Ten thousand feet below the Pacific's waves, 100 miles off the coast of Monterey, California, a large dollop of carbon dioxide spills over the edge of a beaker and lands on the ocean floor. But a camera in a vehicle nearby tracks the gas as it flows like transparent molasses and gradually rolls away into the frigid sea.
Peter Brewer, a senior scientist at the Monterey Bay Aquarium Research Institute, watches a video of this experiment over and over to see what ultimately happens to liquefied carbon dioxide at the bottom of the ocean. As it turns out, the gas dissolves slowly enough so it doesn't bother sea life but quickly enough so it doesn't form puddles that might prove ecologically disruptive.
This simple demonstration may contain the cure that will save the planet from a nasty fever. Every year, humans churn out 8 billion tons of carbon dioxide, almost all of which goes straight into the atmosphere. Most scientists agree this outpouring contributes to global warming, which could eventually lead to coastal flooding, extreme weather, and widespread crop loss. But with the Bush administration focused on increasing energy supplies from fossil fuels, and with rapid development in China and other nations, global CO2 emissions are likely to keep rising. Brewer and others like him are trying to find a way to lock away carbon dioxide where it can do no harm.
This year, the U.S. Department of Energy will spend $54 million to promote the endeavor, called carbon sequestration. Oil companies such as Chevron and BP Amoco, fearing future regulation on carbon emissions if global warming increases, are beginning to assist. And officials at the Department of Energy have set ambitious goals. In a 1999 report, agency scientists proposed that carbon sequestration could be part of a strategy to reduce overall carbon emissions by a billion tons a year by 2025 and 4 billion tons— half of the present world output— by 2050.
The sea is by far the biggest potential carbon storehouse, which is why Brewer has focused his efforts there. Researchers have proposed sending out a fleet of ships trailing 2-mile-long pipes to pump chilled, pressurized carbon dioxide into the ocean, where it would slowly dissolve and sink to the bottom. Earth's oceans hold 45 trillion tons of carbon, compared with 825 billion in the atmosphere and 2.4 trillion tied up in organic matter such as trees, soil, and your Aunt Martha. Earth's entire untapped fossil fuel reserves contain another 10 trillion tons of carbon. In theory, the oceans could absorb all of that and more.
But would the ocean ecology survive? "Really, the capacity is determined by how much adverse environmental impact you're willing to accept," says Ken Caldeira, who directs the ocean-sequestration project at Lawrence Livermore National Laboratory. Mixing carbon dioxide into the water makes it more acidic, which potentially could harm marine organisms. Brewer doubts the amount of carbon we're likely to add will cause much harm, as the pH of seawater already varies significantly by location. Besides, he says, nature eventually delivers 80 percent of atmospheric carbon into the sea anyway: "We already have an ocean CO2 disposal program— we put it in the air and the air puts it in the ocean." Still, the technological and ecological uncertainties mean ocean sequestration won't be in widespread use anytime soon.
A gallon of liquid carbon dioxide rises out of its beaker and spills onto the seafloor near Monterey, California. Some scientists envision land-based pipelines to disperse CO2 into the sea. "The setup is like how we get oil, but in reverse," says one researcher.
Photographs courtesy of Monterey Bay Aquarium Research Institute
So other researchers are pursuing a simpler approach, sticking the carbon back into the coal mines and oil fields from where it came. For years, energy companies have injected carbon dioxide into the ground to force out hard-to-reach oil or to squeeze natural gas from coal beds. Now Dakota Gasification Co. of Beulah, North Dakota, is collaborating with scientists at Lawrence Berkeley National Laboratory to determine whether the carbon dioxide will stay put. The theory, says Larry Myer, the lab's project manager for geological sequestration, is that underground formations that trapped oil and gas for millions of years should hold onto the carbon dioxide for an equally long time. Over the next two years, Dakota's CO2 injection project in Saskatchewan will show how well the theory works.
Depleted oil fields and coal seams could hold perhaps a few hundred billion tons of carbon, a fraction of the ocean's capacity. But the technology to deposit CO2 underground mostly exists right now, and several energy companies have embraced this approach because it lets them extract more oil and gas even as they bury carbon dioxide. Myer thinks fossil-fuel reservoirs contain enough room to ease the carbon dioxide buildup for a few decades— long enough for scientists to work out the kinks in ocean sequestration or to develop nonpolluting energy sources.
While Brewer and Myer work out the details of how to keep carbon dioxide locked away, Klaus Lackner of Columbia University and Scott Elliott of Los Alamos National Laboratory are tackling another part of the problem: how to capture greenhouse gas that is already in the atmosphere. They envision a giant absorbent strip, coated with any of the many chemicals that react with carbon dioxide, that could pull the gas from the air as it blows by. Or a set of towers could emit a mist of calcium hydroxide droplets, which would react with atmospheric carbon dioxide and rain down as a solid calcium carbonate. The carbonate could be processed to recycle the hydroxide and extract the CO2, which would be shipped to a sequestration site. "You could put the towers downwind of a large, dirty city and suck out the CO2," Elliott says. "But CO2 is everywhere, so the smart thing to do would be to put them near the sequestration site."
Elliott estimates a scrubber massive enough to process the entire world's carbon output would be the size of a large city, comparable in scope to other enormous engineering projects such as China's Three Gorges Dam or the interstate highway system in the United States.
All of this will take money, of course. Howard Herzog, a research engineer at the Massachusetts Institute of Technology, estimates carbon sequestration could add one to three cents per kilowatt-hour to the average bill for electricity, which already costs about 10 cents per kilowatt-hour. "That's not small, but it's not prohibitive either," he says. Especially when you compare the costs of not finding a way to get rid of excess carbon.
There really are only three other options. One is to let global warming run unchecked, which will most likely produce major costs of its own in property loss, hunger, disease, and environmental damage. A second way out is to find some other way to keep Earth cool. Manhattan Project alumnus Edward Teller, among others, has proposed blocking sunlight by launching a giant space umbrella or orbiting a fleet of tiny sun-blocking mirrors. Lawrence Livermore's Caldeira ran a computer climate model and found that blocking just 1.8 percent of sunlight would reduce global temperatures even if the amount of carbon dioxide in the atmosphere doubled. Nonetheless, he doesn't think such schemes hold much promise because of their enormous complexity and unpredictability.
That still leaves a third approach: conservation and the development of nonpolluting fuels. It's an effort Caldeira endorses regardless of the promise of carbon sequestration. "I think everybody agrees that the best thing would be if we could find a way to not produce carbon dioxide," he says.
The Monterey Bay Aquarium Research Institute has additional information and videos of deep-ocean CO2 release at www.mbari.org/ghgases.
A detailed Department of Energy report about carbon sequestration can be found at www.ornl.gov/carbon_sequestration.
