Scientists trying to battle the "greenhouse effect" have seriously proposed dumping hundreds of thousands of tons of iron into the ocean to create giant blooms of marine algae that could soak up much of the excess carbon dioxide believed to be responsible for global warming.

If the massive scheme is carried out, researchers say, it would be among the greatest manipulations of nature ever attempted.

The proposal to dump iron into the oceans has been given quiet endorsement by a special panel of the National Research Council, an arm of the National Academies of Sciences and Engineering, which is chartered by Congress to advise the government on scientific and technical issues.

The panel concluded "it is conceptually feasible to slow the increase in atmospheric carbon dioxide" by fertilizing the oceans with iron, the lack of which scientists believe limits the growth of marine algae in many environments.

In a report that has not been publicly released, the group recommended that, "after careful modeling and appropriate preliminary experiments," a large international "iron enrichment experiment" be implemented at a cost of between $50 million and $150 million. Researchers have suggested that the first experiment be conducted off the coast of Antarctica or Alas- ka.

"You give me half a tanker full of iron, I'll give you another ice age," said John Martin of Moss Landing Marine Laboratories in California, one of the leading proponents behind what scientists only half-jokingly refer to as the "Geritol Solution" to global warming.

Scientists are unsure, however, about the negative effects of such a massive program. Marine algae are the foundation of the ocean food web, and feed the shrimp-like krill that, in turn, are eaten by the ocean's fish, seals, penguins and whales. Disruption of the food chain in any way could have unknown ecological effects.

Yet based on new findings published by Martin and his colleagues, many ocean scientists believe populations of the tiny marine algae called phytoplankton are held in check only by the lack of iron, which the plants need to convert nutrients such as nitrogen into amino acids, the building blocks of proteins.

If iron could somehow be sprayed upon the otherwise nutrient-rich seas, perhaps in the form of powder or slow-release pellets, the marine plants should undergo tremendous growth, much like ordinary houseplants gorging themselves on plant food. The researchers do not think their experiment could run amok because the marine algae would grow only as long as other nutrients such as phosphorus and nitrogen held out.

Like all plants that carry out photosynthesis, the growing phytoplankton absorb vast amounts of carbon dioxide, which is accumulating in the atmosphere because of the burning of fossil fuels.

Carbon dioxide is the gas most responsible for predictions that Earth will warm on average by about 3 degrees Fahrenheit by the year 2020. The United States, because it occupies a large continent in higher latitudes, could warm by as much as 6 degrees Fahrenheit.

The marine plants remove carbon dioxide from the water. The water in turn interacts with the atmosphere to replenish the sea with carbon. The more the marine plants grow, the more carbon dioxide is absorbed from the atmosphere.

The idea of fertilizing the ocean to rescue the planet from overheating is attracting serious attention in the scientific community.

"I think it is a good idea," said Roger Revelle, the former director of the Scripps Institution of Oceanography and a researcher recognized as a father of global ocean-atmosphere studies. "I see no reason why it shouldn't work."

Anthony Michaels, a research scientist at the Bermuda Biological Station for Research, said, "There are lots of unknowns, but it is an enticing idea that is being actively pursued."

Michaels noted, however, that human intervention into biological affairs has had mixed results. "If you start playing God with the system, we don't understand it well enough to know what the outcome would be. The whole food web would be altered," Michaels said.

Yet other scientists said that the effects of massive iron fertilization might be benign.

"I don't think there would be any negative consequences," Revelle said. Others said the oceans' productivity would be increased by fertilization and that the result would be more marine life all along the food chain.

To counter global warming, which could alter not only temperatures but also rainfall and sea levels, many scientists and policymakers have been calling for a drastic reduction in the production of carbon dioxide.

But such a reduction would require a dramatic change in the way the industrial world gets and uses energy. For example, the use of coal, and perhaps automobiles, would have to be curtailed severely. Some economists fear the cost of drastic reductions in carbon dioxide would wreak havoc on the economy.

The scientific panel of the National Research Council, however, estimated that the cost of iron fertilization would be less than $1 billion a year.

"It is far more economically feasible than other options on the table," said Adam Heller, a professor of chemical engineering at the University of Texas in Austin who convened the meeting at the National Research Council.

Heller cautioned that the estimated cost of $1 billion is a "back of the envelope" calculation, but added that there appears to be "nothing fundamentally stupid" about the idea.

Scientists have estimated that the burning of fossil fuels and the destruction of tropical forests add about 6 billion tons of carbon to the atmosphere each year. About half that amount, or 3 billion tons, stays in the air, where the gas acts like a blanket and traps heat that would otherwise radiate into space. The other 3 billion tons of carbon is absorbed by the oceans, soil organisms and growing plants.

The proponents of iron fertilization believe that about 300,000 tons of high-grade iron, dispersed each year in slow-release floatable pellets, could eventually, at least in theory, remove an average of 2 billion tons of carbon from the atmosphere each year over a 50-year period. Martin said a single large tanker today could easily carry that much iron.

The reason the fertilization might work, the scientists say, is that marine plants need just a small amount of iron to grow. It was learned only recently that the oceans far from land are extremely iron deficient. In the Southern Ocean around Antarctica, for example, researchers believe that 95 percent of the iron in the water comes from dust blown off the continents of South America, Africa and Australia.

More work, Heller and others said, is clearly needed before anyone starts dumping iron in the ocean.

In addition to possible negative effects, it is uncertain what the ultimate fate of carbon absorbed by algae might be. If the plants simply soak up the carbon in the summer and release it when they die in the winter, there would be no net reduction of carbon dioxide from the atmosphere.

Proponents of fertilization, however, say the plants will be eaten by krill and other animals, which excrete carbon in the form of feces and which themselves die and sink to the bottom of the ocean. The dead plankton, too, will sink to the bottom, carrying their carbon to the ocean floor to accumulate as sediment, say fertilization proponents. Yet great uncertainties remain.

"I doubt you'll find anyone who thinks it's totally ridiculous," said Tony Knap, director of the Bermuda Biological Station.

"You've got to go out and find out if it works," said Revelle of Scripps. "It's definitely worth trying -- as an experiment first, of course."