SCIENTISTS worried about heat-trapping gases in the atmosphere are devising elaborate schemes for removing one of them, carbon dioxide.

The scientists are making plans to fertilize the oceans around Antarctica to increase microscopic plant life that uses carbon dioxide drawn from the air; to grow millions of square miles of seaweeds that consume carbon dioxide on manmade floats in the open ocean; and to pump billions of pounds of carbon dioxide collected from the world's power plants into the deepest oceans where, they hope, it would solidify and stay put.

Such plans, as bizarre and grandiose as they sound, are being given serious consideration by many of the nation's leading experts on global warming issues. "The urgency of the problem is such that we need to proceed with new ideas," said Dr. Francisco Ayala, chairman of a special Government workshop on how to go about putting such schemes into effect. "We are thinking of major tampering with the environment."

The workshop, held earlier this month by the National Academy of Sciences in Irvine, Calif., recommended that the National Research Council, an operating unit of the academy, undertake a two-year feasibility study of the schemes and, if the ideas seem sound, proceed to pilot projects in the open ocean. Worries About Ecological Effects

Critics of the plans say large-scale tampering with the oceans could lead to ecological disaster. They say efforts should focus on reducing emissions of carbon dioxide, primarily from the burning of carbon-based fuel like oil. New disposal strategies might further encourage the use of such fossil fuels, the critics say.

The proposed schemes are based on the notion that nature maintains a carbon balance, which can be manipulated by humans. Like a householder who tries to balance income and expenditures, nature seems to seek a balance between the amount of carbon dioxide released into the air and the amount absorbed by plants and other natural processes.

But when too much carbon dioxide is released into the air, it rises and traps heat, like the glass panes of a greenhouse.

Today's carbon budget is out of balance, said Dr. Karl Banse, a marine biologist at the University of Washington's School of Oceanography in Seattle. Each year an estimated seven billion tons of carbon is released into the atmopshere due to human activities, primarily the burning of fossil fuels and rain forests. About five billion tons is taken up by plants, sinks into the ocean or remains trapped in the atmosphere, he said. The rest is unaccounted for.

Despite not knowing where the missing carbon goes, scientists say there are two fundamental ways to balance the carbon budget. One, discussed at a recent international gathering in Geneva, is to decrease carbon emissions. A second, discussed by the workshop, is to increase the amount absorbed by natural processes. Trying to 'Prime the Pump'

The ocean water absorbs about two billion tons of inorganic carbon each year as air meets water all over the globe, Dr. Banse said. But the ocean also contains tiny plants that consume carbon dioxide falling into the upper layers of water. If one thinks of the plants as a biological pump for removing carbon, said Dr. Banse, "perhaps we can prime the pump" by increasing the amount of plant life in the oceans.

One scheme calls for making the Antarctic Ocean bloom with algae by adding iron as a fertilizer. It is based on a hypothesis developed by Dr. John Martin of the Moss Landing Marine Laboratories in Moss Landing, Calif.

Deep oceans in the southern hemisphere, remote parts of the equatorial Pacific and the northern Pacific are almost devoid of small marine life despite having rich sources of nutrients, Dr. Martin said. For some reason, algae do not grow well in these remote oceans.

Dr. Martin says the reason is insufficient iron. Like all plants, he said, algae cannot make chlorophyll without iron. The Antarctic and other remote oceans have very little iron, he said, and hence very little algae.

The reason those oceans have so little iron, Dr. Martin said, is that they are too far from land, which is the source of iron in the oceans. Tests Seem Promising

Dr. Martin supports his hypothesis with data other scientists have obtained from samples of ancient ice taken from Antarctica and Greenland. Ice laid down during glacial periods contained 50 times more iron-bearing dust and far less carbon dioxide than ice formed today, he said. This could mean the iron dust made algae thrive in remote oceans and use huge amounts of carbon dioxide, making the world colder, Dr Martin said.

To test his hypothesis, Dr. Martin recently collected bottles of water from the north Pacific far from the Alaska coast. He found very little algae in the water. But when he added iron to some of the bottles, the algae bloomed, producing ten times more chlorophyll than those in the untreated bottles.

This small experiment suggests that it might be possible to fertilize the oceans around Antarctica with a soluble form of iron to promote algae growth, Dr. Martin said. An estimated one billion tons of extra carbon might be pulled from the atmosphere to support the explosion of plant growth, he said.

The iron fertilization scheme has been criticized on several fronts. Dr. Banse argues that the bottle experiments do not prove that iron is the limiting factor for algae growth. Algae may remain low in southern oceans, he said, because light levels are low and because tiny marine animals constantly graze on all available algae. "It's like a pasture with many sheep," Dr. Banse said. "The grass is very short. But it is growing like crazy."

Others worry about potential ecological effects. Dr. Gustav Paffenhofer of the Skidaway Institute of Oceanography in Savannah, Ga. said the scheme might harm krill, a small crustacean eaten by whales and other animals. Krill eggs sink to a depth of about 1,000 yards, he said, where they hatch and slowly rise to the surface. This cycle could be imperiled under certain circumstances.

If the algae-eating animals are large, said Dr. Paffenhofer, their fecal pellets would fall to the bottom of the ocean. But if the animals were tiny, the pellets might dissolve at a depth of 1,000 yards and be eaten by bacteria. As the bacteria use oxygen in breaking down the organic matter, he said, they could deplete oxygen at that depth and krill eggs would suffocate and die. Plan for Collection

Despite such worries, a National Research Council panel suggested that scientists seriously study the idea of ocean fertilization and conduct an experiment on 60 square miles of open ocean in two years, if the underlying concepts are confirmed and if financial support can be found.

A second scheme for increasing ocean plant life is being promoted by the Electric Power Research Institute in Palo Alto, Calif. representing the nation's electric utilities. Even if most nations keep their carbon dioxide emissions steady, world population growth will result in higher net emissions, said Dwain Spencer, an institute analyst.

To offset this increase and to collect carbon dioxide for disposal, the United States might want to farm seaweeds in the open ocean, Mr. Spencer said. The plants would be grown on giant grids towed by ships on the open ocean. To pump one billion tons of carbon out of the atmosphere, he said, a farm would have to fill a million square miles of ocean. The seaweed would be harvested, anaerobically digested and turned into liquefied natural gas for fuel and carbon dioxide. The carbon dioxide would have to be disposed of somehow, he said, but at least it would be collected in one place.

It might cost from $50 to $200 to remove each ton of carbon from the atmosphere, Mr. Spencer said.

He said Japanese scientists may have found a way of disposing of the collected carbon dioxide, Mr. Spencer said. He said researchers exploring hydrothermal vents in the Pacific, recently discovered liquid carbon dioxide bubbling from the mantle. To their amazement, they found that the carbon dioxide behaves like a solid at depth.

When one molecule of liquid carbon dioxide combines with seven molecules of water under high pressure, Mr. Spencer said, the carbon dioxide instantly forms a solid. Utilities might scrub carbon dioxide from their smoke stacks, liquefy it and inject it to the bottom of the ocean where it would solidify, he said. But it is not yet known, he added, if the solids would stay at the bottom or gradually turn back into gas bubbles and make their way to the surface.

Whatever scientists propose, they will have to be careful, said Harold Mooney, a global warming expert from Stanford University. "If we talk about large-scale tinkering with the environment, we really have to have the science worked out. I'm supportive of investigating the ideas but we may not want to do them. We have just one Earth."