Lester Brown, global watchdog, can cite enough looming catastrophes to spoil anyone's day: Water tables are falling, temperatures are rising, rain forests are shrinking.

Gordon Hempton, professional "sound tracker," faces a simpler problem: It's getting awfully hard these days to find 15 minutes of peace and quiet.

Each man, in his own way, is talking about the same thing. A lot more people live on the planet than ever before, and by and large we're a hungry, needy, noisy bunch.

Of all the changes the 20th century has seen, none is more far-reaching than the explosion of human population - the one trend to which everybody contributes.

One hundred years ago, 1.6 billion people lived on Earth. This year, world population will reach 6 billion.

How to keep all those people alive without ravaging the planet is a question Brown addresses daily as president of the Worldwatch Institute, an environmental research group based in Washington, D.C. Yet even he remains awed by humanity's talent for multiplication.

"There has been more population growth since 1950 than during the preceding 4 million years," Brown says.

While Brown's path to comprehension is paved with Big-Picture charts and graphs, Hempton has a more personal way of measuring how crowded the world has become.

He listens.

From his home in Port Angeles, Wash., Hempton treks to remote corners of the world with an expensive tape recorder in hand, seeking to capture nature's quiet symphony.

Trouble is, few places remain where human noise doesn't intrude. In rural glades of the southeastern United States, Hempton has tried in vain to escape the low drone of "monster flutes" - the smokestacks of coal-fired electric plants dotting the landscape. In Wyoming, his quest for quiet has been interrupted by the rhythmic booming of oil-well pumps. Even in the Southwest's lonely deserts, he finds no peace.

"If you listen in the middle of the night, the desert landscape is actually rumbling," he says. "A tremendous amount of sound is being pumped out from distant cities, highways, power transmission lines, industry and mining."

Fifteen years ago, Hempton documented 21 spots in Washington state where he could reliably capture 15 minutes of natural sounds uninterrupted by the likes of roaring jets, humming trucks and barking dogs. Now he finds only three.

He mourns the loss. When we can't escape noise, our senses start shutting down and life is not as sweet, Hempton believes.

And so, in his own quiet way, he reaches the crux of the population question: It's not whether 6 billion or 16 billion people can be crammed onto the planet. It's the quality of life those people enjoy, whatever their number.

Hempton craves solitude. Others want gasoline for their cars and electricity for their computers. Millions would settle for a daily loaf of bread or bowl of rice. Can the globe support us all in the manner to which we are accustomed?

Some perspective from the charts and graphs:

Population growth accelerated during most of this century. It took all of human history to reach a world population of 1 billion in 1804. It took 123 years to reach 2 billion in 1927, 33 years to reach 3 billion in 1960, 14 years to reach 4 billion in 1974; 13 years to reach 5 billion in 1987; and 12 years to reach 6 billion in October 1999.

The growth rate has started to slow, but world population still rises by 78 million each year, the U.N. Population Division says. That's like adding 1.5 million people, or a city the size of Philadelphia, every week.

All those people consume a lot of resources. In 1900, only a few thousand barrels of oil were used each day worldwide. Today, humanity uses 72 million barrels a day, Worldwatch says. Use of metals has risen from 20 million tons a year to 1.2 billion tons, the group says.

On average, people have never been healthier or wealthier, but the gap between rich and poor remains wide. Half of all American adults are overweight, yet elsewhere more than 13,000 young children die every day of malnutrition and related illnesses, the World Health Organization says.

Brown sees ozone depletion, global warming, overfishing and falling water tables as bills coming due from growth the Earth cannot sustain. He believes Americans and others living high on the hog should scale back their consumption to leave enough food and resources for others.

World grain production hovers just under 2 billion tons a year, Brown notes. How many mouths that can feed depends on how much is eaten directly vs. being fed to livestock, an equation that varies widely by nation.

"With 2 billion tons of grain, you can feed 10 billion Indians," Brown says. "Or you can feed 5 billion Italians. Or you can feed 2.5 billion Americans. If we're all eating like Americans, we need another planet, basically."

Some don't consider the century's near-quadrupling of population a problem.

"We should celebrate it," says Jerry Taylor, director of natural resource studies for the Cato Institute, a conservative think tank in Washington. "It's a product of improving human health, improving lifestyles and better nutrition. People are living longer."

While others worry that too large a population will exhaust natural resources, Taylor says human ingenuity is the true resource - and that will only increase with more people around.

"People aren't just mouths to feed. They're creators of art and generators of technology," he says.

People can also be surprisingly adaptable. Last October, U.N. demographers reduced their population-growth estimates by about 2 million a year, saying they hadn't anticipated how quickly women around the world would embrace the notion of having fewer children.

In the 1950s, the average woman gave birth five times during her lifetime. Today that global fertility rate is 2.7 births per woman and falling, the United Nations says.

Fifty years from now, there will be 8.9 billion people on Earth, according to the United Nations' mid level, "most likely" projection. Virtually all the gain will occur in the poorer nations.

Will resources be available, or life enjoyable, for that population, 50 percent greater than today's?

Gordon Hempton answers with a story. He was in the Everglades one day, working against the odds to make an uninterrupted recording of an Eastern meadowlark.

"Thirty seconds into the recording," he recalls, "sure enough, the roar of a jet came in."

Hempton let the tape roll, and now the recording is one of his favorites. The sweet song of the lark and coarse thunder of technology speak to him of nature's grace and humanity's striving - a duet affirming that even in a cramped and noisy world, there is music in the air.

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