Over vast stretches of geologic time, earth has evolved ways to swap its treasure trove of carbon among various "accounts": the atmosphere, oceans, land surfaces - and even hoarded deep beneath its crust.
Yet since the dawn of the Industrial Revolution, humans have fueled their economies by drawing coal, oil, and natural gas from under the planet's mattress and burning them - boosting the rate at which carbon dioxide builds in the atmosphere and warms it.
Now, scientists and engineers are looking for ways to reverse the process and take carbon dioxide from fossil fuels out of atmospheric circulation. Ideas range from "fertilizing" vast patches of the ocean's surface so CO2-gobbling algae will grow faster to pumping the gas into underground rock formations or deep beneath the ocean.
The questions they hope to answer: Will these work? Will these remedies be cost-effective? And will they do more environmental harm than good?
"There is nothing wonderful about carbon sequestration," acknowledges Robert Socolow, a Princeton University engineering professor who specializes in environmental technology. But in battling climate change "there is no winner-take-all option. We'll need a mix of energy systems indefinitely at least through the 21st century."
This mix includes three broad paths toward reducing what a growing number of scientists see as humanity's impact on climate: using energy more efficiently; using energy sources that don't emit large quantities of carbon dioxide; and using fossil fuels, but keeping their carbon out of the atmosphere.
Of the three, sequestration represents "the new kid on the block," says Howard Herzog, a researcher at the Massachusetts Institute of Technology's Energy Laboratory and co-author, along with Dr. Socolow, of a 1999 US Department of Energy report on sequestration research.
The notion of sequestering or managing carbon from fossil fuels raises eyebrows among some environmental groups.
"The general problem we have is that this is an excuse to keep using fossil fuels," says Kert Davies, with the Greenpeace USA climate campaign.
In some cases, he acknowledges, separating carbon could be beneficial - for example, separating carbon from natural gas to leave hydrogen as a fuel, then pumping the carbon back underground as CO2. This could help provide a transition from a carbon economy to a hydrogen economy.
In other cases, he continues, carbon management "only derails us from our main course of action, shifting to a fossil-fuel-free world."
Yet to others, the argument for stripping carbon from fuels or from smokestack emissions is compelling.
David Wallace, who focuses on energy research and development at the International Energy Agency in Paris, notes that the carbon-reduction task the world faces is enormous. To "stabilize CO2 concentrations at twice their pre-industrial levels by the end of this century, developed countries will have to reduce their emissions to around half of the 1990 levels, or even lower," he concludes.
Storing carbon dioxide from power plants alone, he continues, could provide large and fast reductions on CO2 emissions and ease the transition from fossil fuels to alternative sources of energy.
Indeed, some economists note that for countries such as India and China, coal may be the most easily obtained and cheapest fuel available for decades to come. Sequestration technologies may be one way to minimize these nations' CO2 emissions.
Particularly since 1997, when a protocol for taking the first steps toward cutting CO2 emissions emerged from a United Nations conference in Kyoto, government and private funding for sequestration research has grown.
The US Department of Energy, for example, has put $15 million into sequestration research during the past two fiscal years and will spend another $18.8 million in fiscal 2001, according to DOE spokesman Robert Porter.
One DOE grant, announced on Monday, will underwrite two-thirds of the cost of research that will examine the role algae can play in turning CO2 from power plants into a mineral: calcium carbonate. Researchers at the California State University at San Marcos are spearheading the one-year, $300,000 project.
Meanwhile, in late October, Princeton announced that it will receive $20 million in combined contributions from Ford Motor Co. and BP-Amoco to study sequestration technologies. Last July, seven companies, including BP and Ford, launched an MIT-based consortium with a similar goal.
Researchers in and out of government are focusing on several possible approaches to clean up fossil fuels' act.
On the scale of an individual power plant, Shell Hydrogen, a subsidiary of Royal Dutch/Shell, envisions using the hydrogen in natural gas to generate electricity. Special fuel cells mix the gas with air, generate electricity, and give off water and CO2 as byproducts. The CO2 would be pumped back into underground rock formations. Because the fuel cells operate at high temperatures, their heat can be used to boil water to drive steam turbines for additional electricity. Shell estimates that the fuel cells required could be commercially available in about five years.
Initial estimates suggest that electricity from this type of facility would be half again as expensive as "dirty" power, but cheaper than electricity generated by wind-driven generators or solar cells.
Princeton's Socolow adds that separating the carbon and hydrogen in natural gas also could be used as a source of hydrogen for fuel cells as they become more abundant in other applications.
Potentially more controversial, however, are several large-scale approaches to getting rid of carbon after fuel is burned.
One notion involves adding iron to the ocean surface to "feed" phytoplankton, which are said to carry out nearly half of all the photosynthesis on earth. During photosynthesis, the plankton absorb carbon dioxide. Some researchers have suggested that these organisms could be used to soak up CO2. Then when they die, they would drift to the sea floor, sequestering the carbon they absorbed.
Earlier this fall, an international team of researchers led by Philip Boyd of New Zealand's University of Otago reported in the journal Nature that they had successfully "fertilized" a five-mile patch of the southern ocean some 1,200 miles southwest of Tasmania. The effort succeeded in demonstrating the first part of the proposition; the plankton accumulated 600 to 3,000 tons of carbon. The researchers noted, however, they had no evidence that any of the carbon was sent to Davie Jones's locker.
Sallie Chisholm, with MIT's civil and environmental engineering and biology departments, notes that while fertilizing is "seductive in its simplicity," the potential for unintended consequences is enormous. She adds that scientists probably wouldn't recognize any damage to marine ecosystems until it was too late to stop the changes.
An industrial-scale experiment already is under way in the North Sea, where since 1996, a Norwegian oil platform has pumped nearly 4 million tons of carbon dioxide back into a sandstone formation that lies about a mile below the sea floor. Results are being used to develop techniques that will help evaluate other potential storage sites as well as monitor a reservoir's performance over time.
And next year, a research team that includes MIT's Dr. Herzog plans to pump CO2 into the deep ocean off Hawaii. The goal is to test the feasibility of sending the greenhouse gas directly to the ocean floor, where the intense pressure either will capture it or allow it to diffuse into the surrounding water very slowly. The researchers are keenly interested not only in monitoring the fate of the plume, but also in seeing what effect the CO2 has on the water's acidity and on marine life at depth.
One of the biggest unknowns, however, remains public acceptance of sequestration approaches.
Unlike another thorny waste problem - radioactive waste from nuclear power plants - dealing with CO2 does allow for what Socolow calls the concept of "acceptable leakage."
"If some CO2 leaks as you sequester it, maybe a couple of percent of what you sequester, is that a big deal? Suppose we know the carbon dioxide will return to the atmosphere eventually, but at that time the world is past the fossil-fuel era, so instead of quadrupling CO2 concentrations in the 22nd century, they slowly peak over a much longer period of time. Are we ahead of the game?"
That, he says, is one of the questions the public will have to wrestle with as it decides which approaches to slowing climate change it will accept.
(c) Copyright 2000. The Christian Science Publishing Society