THE ROAD up earth's most massive mountain -- Mauna Loa on the Big Island of Hawaii -- is a narrow, twisting strip of tar, laid by prison labor through raw, jagged lava fields. You can drive it -- with great caution and constant awe -- to a small cluster of white and blue huts standing more than two miles high i the clear, cool Pacific sky.

Here for 32 years the level of carbon dioxide in the earth's atmosphere has been recorded daily at the Mauna Loa Observatory by Charles David Keeling of the Scripps Institution of Oceanography and by scientists from the National Oceanic and Atmospheric Administration (NOAA). And for 32 years the level has risen, in a wavy curve of spring-fall variations (opposite), from 315 parts per million (ppm) in 1958 to more than 355 in mid-1990.

That steadily climbing CO2 level (air locked in glacial ice a century ago held only about 280 ppm -- 25 percent less) is an incontrovertible measure of what man and his machines have done to the atmosphere of the earth in scarcely one lifetime. Because of it, say many scientists who study the climate, our planet is bound to become warmer -- has already warmed -- as more and more energy from the sun is caught and held in the thin blanket of air around us.

"As Pogo put it, the enemy is us, " said Elmer Robinson, director of Mauna Loa Observatory, while we drove together up the desolate slope of the volcano. "By burning more and more fossil fuel -- gasoline, natural gas, coal, peat -- even ordinary firewood, we are putting into the air more of the gases that act much like a globe of glass around the planet. That's what's called the greenhouse effect.

"Up here we measure not only CO2," he went on, as he led me through a laboratory jammed with humming recorders and glowing computer terminals. "We also read methane, chlorofluorocarbons, nitrous oxide, and ozone, all of which add to the warming. We see and measure dust of various sizes in the troposphere, the weather layer of the atmosphere." Such data, recorded by NOAA here and at American Samoa in mid-Pacific, at Point Barrow in Alaska, and at the South Pole, form the hard evidence on which climatologists base widely varying and controversial visions of the future. Average temperature worldwide, by careful calculations, has gone up about half a degree Celsius -- one degree Fahrenheit -- since the late 1800s.

An unusual sight in San Francisco, smog hides the city only four or five days a year. Far more ominous: the invisible but relentless rise of carbon dioxide in the atmosphere.

Scientists on Hawaii's Mauna Loa, two miles above the Pacific Ocean, have recorded a steady increase in the global concentration of CO2 since 1958. The zigzag pattern reflects the seasonal growth and dieback of plants, which soak up prodigious amounts of CO2 during spring and summer. A century ago CO2 measured 280 parts per million, according human activities such as burning fossil fuels. Most believe the rising CO2 Level will lead to higher global temperatures and significant climate change.

In this century, the decade of the 1980s saw the six warmest years in weather records. Yet there are some researchers and statisticians who argue this apparent warming of the planet may be only a temporary blip, that natural warming periods have occurred before, without man's intervention, and that there is as yet no sure evidence of long-term change. All this was in my mind under the blaze of sunlight that beautiful winter day atop Mauna Loa. Through a small occulting telescope maintained there by the National Center for Atmospheric Research at Boulder, Colorado, I stared at the darkened face of the sun, ringed by its glowing, gauzy corona. From that blazing disk high in the Hawaiian sky comes the endless power that drives and rules all life on earth: its plant growth and the food chains of all its creatures; the winds, rains, and churning weather of the planet; the ocean currents, forests, prairies, and deserts.

Our home star in the heavens burns steadily, almost without variation. It is the "almost," coupled with what man's activities are doing to the atmosphere, that this story is about. It is becoming more and more apparent that the effects of the sun upon our planet are changing. Our one and only home may be in harm's way, and we can scarcely sense just what is happening -- or know what to do about it.

That burning energy of our sun works the miracle on earth called photosynthesis. Powered by sunlight, plants green with chlorophyll combine carbon dioxide from the air with water from soil or sea into energy-rich carbohydrates, releasing into the atmosphere the oxygen we need to breathe.

It is the same process by which primitive bacteria of ocean shallows, more than two billion years ago, first produced enough oxygen to permit other life on earth to develop. And it is the same process -- still imperfectly understood -- that grows the corn of Iowa, the grass of your lawn, the rain forests of Brazil, the floating plankton that sustains life in the seas. "Without carbon dioxide in the atmosphere, life as we know it would be impossible," Elmer Robinson had said on Mauna Loa. "We couldn't exist if it weren't for greenhouse warming."