A fair number involve manipulating biological and ecological systems to become more voracious consumers of carbon dioxide. The problem is, these systems are hard to figure out, slow to respond to our influence, and likely to punish our temerity for trying.

The newest project for bioremediation of carbon was recently funded by the U.S. Department of Energy, in Washington, D.C. The project will sweep the seas for carbon-loving microbes, find the genes responsible for their carbon-munching ways, and transfer them to other microbes that can be cultured easily in bioreactors. That way, exhaust fumes from power plants and maybe car tailpipes could be funneled through the bioreactors and stripped of their carbon dioxide.

The proposal came from the Institute for Biological Energy Alternatives, in Rockville, Md., a creation ofJ. Craig Venter, famous for getting Celera Genomics to sequence the human genome faster and more cheaply than the government's project did. Venter has a stellar track record, and when he says he will search for carbon-munching genes in the DNA of every last microbe in the wide Sargasso Sea, you can bet that he will get the job done.

Likewise for his plan to design a microbe around said genes. The project hopes to get around some nasty surprises uncovered in earlier attempts at manipulating the ecosystem by building a carbon-crunching microbe from scratch—that is, making a bug having only the minimum genes for life in a smokestack, one that doesn't coincidentally emit a harmful substance or run amok in the environment.

It's an intriguing scientific project, one worth funding for the sake of the knowledge alone. But no one should put too much faith in its power to offset our love affair with the automobile, or even our propensity to consume electric power. The contribution these microbes would make, at whatever far-off point they debut, is likely to be small, say experts.

What's more, if the example of agricultural biotechnology is any indicator, the public (at least outside the United States) may not be thrilled that scientists are genetically modifying microbes for massive deployment.

Little more can be said for the other suggested approaches to biological carbon sequestration. One such plan would seed the open ocean with iron, sparking massive blooms of carbon dioxide-breathing algae.The problem is that not all the algae carry their carbon to the bottom of the ocean—some of it gets recycled into carbon dioxide. Meanwhile, the iron seeding yields other chemicals that raise the acidity of the ocean and deplete the ozone layer.

Then there is the plan to sequester carbon in the form of wood by planting vast forests. Yet, to reduce U.S. carbon emissions by 7 percent, as stipulated in the (apparently defunct) Kyoto Accord, we would have to plant "an area the size of Texas every 30 years," says William H. Schlesinger, dean of the Nicholas School of the environment and earth sciences at Duke University, in Chapel Hill, N.C.

The developed world can't wish away the problem by finding that area in other countries, because treeless tracts are typically infertile. You'd need nitrogen fertilizer, made by burning natural gas, releasing still more carbon dioxide. "I did the calculation some years ago," says Schlesinger, "and in nearly every case, you are worse off."

The one biological manipulation that makes sense involves changing agricultural practices so as to store more carbon in the soil—for instance, by reducing tillage. But even here we are talking petty gains compared to the seven billion-odd tons of carbon annually belched into the skies.

Why, then, does the Energy Department wax so enthusiastic about carbon sequestration? Schlesinger says the Bush administration is merely seeking ammunition to fire at environmental critics, who are gaining traction in public opinion. "To be totally cynical," he says, "they are funding research that doesn't have any chance because they can now say, 'Oh, gee, we're doing all this interesting research to solve the problem.' These programs are just talking points, and talk is cheap."