SOME SCIENTISTS and politicians -- including George Bush -- increasingly view plants as a sort of Noah's Ark in which humanity can ride out the storm of global warming. But a flood of recent evidence suggests that plants and soils might prove instruments of destruction, not salvation.

Consider the following:

Two increases -- one certain, the other less so -- in the rate of increase of carbon dioxide in the past 15 years, almost as if a second CO2 faucet had been turned on in 1976, then possibly a third in 1987. Probable cause: plants.

Discrepancies in atmospheric makeup which can be best explained by the existence of a "positive feedback" system involving CO2, a major greenhouse gas. That is: The hotter it gets, the hotter it will get. Probable cause: plants.

Unexplained, but large and abrupt increases in concentrations of methane -- 40 percent since the 1950s, which is lightning speed for the global environment -- perhaps from the same source as the CO2, but perhaps not. A likely cause: plants.

Recent direct evidence that much of the new CO2 entering the air emanates from the last place most people would expect it: plants.

One distressing possibility is that the record heat of the 1980s -- the warmest decade in recorded history -- helped trigger the recent rise in global CO2 by boosting the rate at which plants and microorganisms "respire," or extract energy from stored foods.

"The implication we assign to these observations," says George Woodwell of the Woods Hole Research Institute, "is that the surge {in CO2} is the result of the high temperatures of the 1980s, delayed by the time necessary to warm the soil."

The mean green hypothesis may prove incorrect. But it underscores the threat of "surprises" that almost certainly lie hidden in the bewildering complex of interactions subsumed under the simple label "global warming."

If asked to find evidence of warming, the average person might simply look at annual temperature records to see whether there has been a regular linear rise. But that could be the wrong thing to look for. In many cases, natural reactions are neither smooth nor predictable, particularly in "threshold" situations: Water, heated past a critical point, turns to steam rather than hotter water; avalanches topple all at once instead of in a series of gentle slides; lightning suddenly shatters the air -- all when a threshold is crossed. This can happen in the environment. The Antarctic ozone hole is precisely that sort of runaway reaction -- one that no scientist predicted despite tens of millions of dollars spent on research.

It is the possibility of such "surprises" -- a euphemism scientists prefer -- that cause some experts to conclude that if you don't know what's on the other end of the rope, quit jerking. How the Planet Breathes

Plants consume carbon dioxide through photosynthesis. But they and soil microorganisms such as bacteria also give off CO2 through "respiration."

In photosynthesis, a plant converts the energy of sunlight into a chemical form that is used to transform carbon dioxide into carbohydrates, usually molecules of sugar or starch. In respiration, the molecular bonds are, in effect, cracked in a complex, three-step process which releases small increments of energy. It is used to build new tissue, grow flowers or perform other useful work. Carbon dioxide and water are the waste byproducts, discarded by animals through their lungs and plants through their leaves.

In young and mature plants under normal circumstances, the amount of carbon fixed by photosynthesis is greater than or equal to the quantity released through respiration. Heat skews that system, accelerating the rates of both. As temperature climbs, the rate of photosynthesis peaks, then begins dropping. In contrast, the respiration rate -- and thus the amount of CO2 generated -- continues to rise. As a general rule, respiration doubles for every 18

F increase in temperature. In almost all cases, this process does not require light, and is thus called "dark" respiration.

"Raise the temperature and dark respiration will increase exponentially," says W. Dwight Billings, a Duke University specialist in arctic and mountain plant systems, "right up the point where everything dies." He estimates that two-thirds of the land's carbon is stored in soil where plants and microorganisms can release it as carbon dioxide or methane, another global warming gas. A significant global temperature increase would mean that "respiration rates will increase more than photosynthesis," according to Woodwell. The Mean Green Machine

There is no dispute whatsoever that CO2 concentrations in the Earth's air have been rising steadily -- up 12 percent in the past 30 years. But twice -- once in 1976, then again in 1987 -- the speed with which CO2 was building up jumped sharply, just as Woodwell's theory would suggest. It is possible that these jumps were due to El Nino cycles or major underestimates of the extent of tropical deforestation. But in either of those cases, the increases should have been temporary. Instead, they've persisted.

But is this CO2 from plants? Evidence collected by Charles D. Keeling of the Scripps Oceanographic Institute suggests that it is.

Plants prefer certain of the carbon isotopes over others just as "yuppies" prefer BMWs. Thus, in much the same way that an influx of young professionals into a high-rise condo could be measured by counting the BMWs in the garage, the amount of CO2 from green plants can be determined by counting the isotopes they like. Keeling -- who first began measuring CO2 concentrations at the Mauna Loa observatory in Hawaii in 1957 -- has done just that.

After sorting and counting the different types of carbon, Keeling found increases in the ratio between Carbon-12 (the isotope that plants and microorganisms prefer) and the more rare Carbon-13. The increase began in 1975 and has escalated to the present. By 1988, the volume of this "anomalous" carbon had risen to 20 percent of the world's total industrial emissions -- the rough pollution equivalent of two Chinas. What could produce such a massive increase? Keeling is convinced that "the oceans and land are doing something unexpected" and "ominous." We can, he says, "postulate that plants are being stimulated by carbon dioxide." Comparing rates of respiration and photosynthesis, Keeling says, is like "watching two runners, both running faster and faster, but one running faster than the other."

Dwight Billings has tracked the difference in speed between these two "runners," both in his lab and in the frozen Arctic tundra regions, which are his specialty. On one August day, the CO2 output from a given sample of Alaskan soil nearly doubled as the temperature rose from 2 Centigrade (35.6 F) to 6 C. Roughly 80 percent was generated in the top four inches of soil -- the depth most susceptible to global warming. As fog moved across the tundra, lowering temperatures, CO2 output dropped, as it did it the early morning hours. But during the mid-afternoon heat, emissions peaked.

To measure the impact that temperature might have on photosynthesis, Billings drilled cores from the tundra -- frozen dirt, lichens, moss and small plants included -- and shipped them back to Duke. He then monitored their performance at two temperatures: One set of samples was kept at 4 C (39.2 F), the average temperature in the Arctic summer; another set was kept at 8 C, the level often predicted for the Arctic if global warming occurs. The cooler cores consumed about 8 grams of carbon per cubic meter of soil. In the "greenhouse" samples, that rate dropped by 50 percent.

Billings cautioned in 1982 that warming of the tundra climate could change this plant-soil ecosystem from a sink to a source. Yet scant notice has been paid to this or the many other possible positive feedbacks that can nudge a gentle change into a runaway reaction.

Numerous other other natural systems may be susceptible to this same kind of "vicious cycle" phenomenon:

Oceans. Scientists believe -- but aren't certain -- that over half the CO2 emitted each year is ultimately dissolved in the ocean. But what if that dissolution rate changes because the sea waters become either saturated or warmer (the warmer the water, the less CO2 it can absorb) -- or if they're mistaken about where the carbon is stored? That would force more of the gas into the air, where it would in turn accelerate warming.

Also, ocean currents that now run on global racetracks created by temperature differences might suddenly halt or even reverse, according to Wallace Broecker of Columbia University -- leaving a frozen wasteland from London to Moscow. Don't laugh -- it happened 10,000 or 11,000 years ago.

Clouds. A recent report by the Marshall Institute states: "Clouds cover roughly half the area of the earth at any given time, shielding this area from the sun's rays. As a consequence, they have a cooling effect . . . {but} clouds also have a heating effect because, like greenhouse gases, they block the flow of heat to space from the earth's surface."

Either way, clouds could set up a feedback system, leaving lakes parched or overflowing -- or oscillating between the two -- as they did in Africa 10,000 years ago as the last glacial period ended. Either would alter CO2 production in the biosphere.

Soils and biological systems. If temperature belts shift, trees, crops and animals could die. Some would be replaced, but many scientists believe that plants will be unable to move as fast as temperature. Billings, for example, dismisses predictions that corn will grow in Siberia and Alaska: "In real life, you're not going to grow Illinois corn on these bleached and acid soils of the Canadian shield. It's highly unlikely over the next 20,000 years. You're just not going to change the soil overnight." Life in the Feedback Loop

Whether the various anomalies above are examples of runaway feedback reactions or merely a natural and temporary aberrations, they nevertheless point up the unknowns that surround global warming.

Ironically, partisans on both sides of the global-warming debate agree that too little is known. The difference is that one side sees that condition as a basis for business as usual; the other sees it as reason to curb air pollution and energy use.

Responding to criticisms that the massively complex models run by multimillion-dollar "supercomputers" ignore factors ranging from the possible cooling effect of cloud cover to the increased evaporation from oceans, John Firor of the National Center for Atmospheric Research in Boulder, Colo., says: "That's the problem -- there's no way that you can promise that you've thought of everything."

"The danger is that so many things are changing at once," says Irving Mintzer of the University of Maryland's Center for Global Change, a policy analysis institute. "The concentrations of trace gases are increasing while the global temperature is rising and the ozone layer is thinning. It's like playing football when the size of the field, the shape of the ball, the number of players and the method of scoring change with every play."

Because we don't understand the nature of all these interactions, says Wallace Broecker of Columbia University, humanity is playing "Russian roulette."

The man who first concluded the CFCs would destroy the ozone layer was F. Sherwood Rowland, also of UC/Irvine. He was also among the first to suggest the crucial role that temperature plays in the Antarctic ozone hole and to predict that similar losses would be found over the North Pole as well. He regards the CO2 data produced by Keeling and others as "significant -- and not very comforting."

Of course, Rowland is inclined to understatement.

Reflecting on the many recent atmospheric findings, Rowland recently recalled the day on which he concluded that chemicals were destroying the ozone shield. "There was no moment when I yelled 'Eureka,' " he says. "I just came home one night and told my wife, 'The work is going very well, but it looks like the end of the world.' "

Doing the Carbon Shuffle

CARBON, THE backbone of life, is the one chemical element common to trees, grass, muscle and every other thing that is or ever was alive, from Jell-O to Jack Daniels. There is as exactly as much carbon on Earth now as there was 5 billion years ago, but it's arranged differently today.

Before life, carbon was stored in rocks. More than 99.9 percent still is. The rest has been converted to a wide variety of compounds by biological action, starting when life first appeared, some 3 billion years ago.

Like money, this converted carbon circulates. Over time, a single carbon atom will "flux" through the air, land and water, existing as a tree trunk, sea shell, ocean sediment, a roving shark, several different bacteria and a trace gas. The two most important carbon-based trace gases from a global-warming perspective are carbon dioxide and methane.

Burning carbon in the presence of oxygen, whether it's by plants, animals, cars or power plants, produces CO2. In the absence of oxygen -- in the center of compost heaps, for example -- methane, known as natural gas, is generated. Both gases trap heat radiated from the Earth, although methane is about 20 times more powerful than carbon dioxide.

The amount of carbon dioxide in the air seems vanishingly small -- only about three parts in 10,000. But that tiny amount, together with other "greenhouse" gases, raise the Earth's temperature by about 33 Centigrade (60

F) above what it otherwise would be.

After 150,000 years of remaining relatively unchanged, the amount of carbon lingering in the air as CO2 began to skyrocket about 1850, climbing by 25 percent between then and now. Virtually all this increase comes from burning coal, oil and natural gas. North Americans, for example, emit about 30,000 pounds of CO2 each year.

So far, around half of this man-made increase has somehow been removed from the air -- absorbed by oceans, plants and soils according to processes not fully understood. Far less well understood: How long this vast and multiplex system will continue in its present manner.

Curtis Moore, who served for 11 yers as counsel to the Senate Committee on Environmental and Public Works, is an environmental analyst and writer.