If the world is going to sharply reduce the amount of carbon dioxide pumped into the atmosphere by midcentury, then many businesses will have to go carbon neutral, bringing their net emissions of the greenhouse gas to zero.

But some could go even further by removing more CO2 than they produce. Instead of carbon neutral, how about carbon negative?

In academic and industrial labs worldwide, researchers are working on technologies to reach that goal. Success could create the ultimate green business - for example, one that produces fuel whose emissions are more than offset by carbon dioxide stored during production. The businesses would be even more successful if, as anticipated, the U.S. Congress put a tax on emissions, or joined Europe in starting a trading plan that made carbon credits valuable.

For some experts, it is not a question of whether businesses will go carbon negative, but when.

Carbon-negative technologies of some sort will be essential, said Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. For decades to come, the world is facing the certainty of vast emissions of carbon dioxide from factories already running, he said, and atmospheric concentrations must be stabilized. "We've got such a carbon overshoot looming in the future that this is going to have to happen," he said.

The United Nations Intergovernmental Panel on Climate Change has said that an 80 percent cut in carbon dioxide emissions is necessary to avoid the worst consequences of climate change. But capturing the gas from coal plant smokestacks or switching to fuels that produce less of it when burned goes only so far.

"The great problem is actually removing carbon dioxide from the atmosphere," said Geir Vollsaeter, an environment expert and former general manager of carbon dioxide at Shell International, a subsidiary of the oil giant Royal Dutch Shell.

While much engineering work would have to be done to make a business carbon negative, the outlines are clear.

Take the concept of building a coal plant that captures and stores carbon dioxide. Such a plant could have zero emissions, because the coal would be turned into gas and processed to produce hydrogen and carbon dioxide. The hydrogen, a pollution-free fuel, would be burned, and the CO2 pumped underground for permanent storage.

But Robert Williams, a research scientist at Princeton University, said that coal was not the only energy source that could be gasified; the same fuel could also be made with plant matter or other biomass sources.

And then, he said, "if you put any CO2 underground that is derived from biomass, that's negative CO2 emissions." That is because plants or trees - the raw material for the fuel - pull carbon dioxide from the atmosphere as they grow, and the gasification and storage takes that carbon out of circulation.

Williams said the more likely route would be to gasify a mixture of coal and biomass to keep the process carbon neutral. But the balance depends on the cost of separation and storage versus what kind of tax or other fee might be put on emissions.

The U.S. Department of Agriculture is considering carbon storage as a new crop, so to speak. "It's one of the many, many ways the agriculture community is going to be a viable player in the renewable-energy field as we go forward," said Edward Schafer, the secretary of agriculture, at a renewable-energy conference this month.

The method is called agrichar, in which some biological material - grass or trees grown specifically for the purpose; or a cornstalk or other agricultural waste - is cooked at a very high temperature in the absence of oxygen. That produces an oil that, with a little chemical work, can be used as a vehicle fuel.

But in a world focused on carbon, the important part may be the leftovers, a charcoal-like material that retains most of the carbon. It also has useful minerals, like potassium and phosphorus. Plowed back into a field, it helps the soil retain water and nutrients, which also enhances plant growth, said Robert Brown, director of the Bioeconomy Institute at Iowa State University.

Senator Ken Salazar, Democrat of Colorado, has introduced legislation that would pay for more research. If it works, he said, farmers would have a new crop. "You'd be growing allowances," he said.

If being carbon negative becomes important, then some existing industries may try to describe themselves that way. In some cases, it may even be true.

For example, Covanta Energy of Fairfield, New Jersey, operates plants that make electricity by burning municipal solid waste, which is about 80 percent paper and other organic materials. But Anthony Orlando, Covanta's president and chief executive, says he is hoping to collect carbon credits, because each ton burned, he said, would have otherwise been buried in a landfill where bacteria digest garbage to make methane, a potent greenhouse gas. The company is building plants in China that will qualify for credits, issued by the European Union, which companies in Europe can buy to offset their own carbon output, Orlando said. As for the United States, he said, "it depends on the legislation."

A Washington company, Solena Group, also has a carbon-negative plan, which emerged from the decision by regulators in Kansas last year to turn down a permit for two new coal-burning power plants because of the millions of tons of carbon dioxide they would produce. The regulators insisted that the builder of the plants, an electricity cooperative called Sunflower, had to permanently remove the carbon from circulation. Governor Kathleen Sebelius and the Kansas State Legislature are still arguing over whether the plants should be built.

Solena says it can use the carbon. The company employs a high-temperature process to break up anything organic into a flammable gas. The organic material could be algae, which have an extremely high energy value per pound. And algae eat carbon dioxide.

Solena is in discussion with Sunflower to build a 40-megawatt power plant that would run on gasified algae; the algae would be grown in thousands of clear plastic cylinders, 3 feet, or 0.98 meters, wide by 10 feet, or 3 meters, tall. The cylinders would sit in the Kansas sun and be fertilized with sodium bicarbonate, made with carbon captured from Sunflower's coal plant. For each 1.8 tons of carbon dioxide, the columns would yield a ton of algae.

A Solena subsidiary is already growing algae at a facility in Alicante, Spain.

If built, the system would make double use of the carbon from the coal and avoid digging more coal for more power. Alternatively, the gas could be turned into diesel fuel or other vehicle fuel, if prices favored that.