If we are, as the U.S. Census Bureau and the recent Global 2000 report to President Carter have warned, to be all jammed together 20 years from now with our forests, fields and streams exhausted, then China today provides a glimpse at our future.

With only 5 percent of the world's arable land to feel 23 percent of the world's people, the Chinese have stretched their farms to the limit. Droughts are common. Forests are so depleted that Chinese movers argue about who gets to salvage the packing crates used for personal furniture shipments.

Yet perhaps the most disturbing thing, as crowded and poor as the Chinese are, is that they have adjusted to it, and even become a bit proud of their lives.

While most Americans would find life in China too cramped, living in limited space with a billion other people here is not quite the grim nightmare of the year 2000 that some environmentalists have warned of. That nearly a quarter of the human race can learn to live and such crowding suggests that lots more of us may find ourselves sliding into such lives without much of a fight, hoping for some galvanizing social disaster that never occurs.

Few expert economic forecasters would have predicted in 1949 that the Chinese, in the next three decades, could add another 400 million people to their population of 550 million without a major catastrophe, but that is what happened.

Life with a billion people has its annoyances. I have never got on a bus in China that had an empty seat. Mobs appear suddenly, like ants around a few spilled grains of sugar, whenever anything out of the ordinary -- say, a tourist loading his camera -- occurs on the street.

Nevertheless, it is possible for a society without many automobiles, like most nations expected to suffer great population growth in the next 20 years, to be crowded without inciting mass suicides.

Human beings, even a billion of them are quiet, soft machine that do not fill up a city the way bumper-to-bumper traffic on 14th Street does. People use bicycles here. There are hoards of them at rush hour, but they pass with only a faint tinkling of bells. A truck driver who hits one of them is in severe trouble.

By tradition, personal habit and 30 years of communist organization, the Chinese have succeed in removing or recycling many of the more un-attractive byproducts of overpopulation. Trash pickup is swift because it provides fuel, and jobs for unemployed youth. Lavatories are cleaned regularly because manure can fertilize crops. The masses themselves are not so visible because, in northern China, bungalows and housing compounds are surounded by walls. Beggars have returned to some cities, a sign of the current administrative laxity, but they are not very aggressive.

Peking residents who have lived in India speak with distaste of the crush and stench of humanity in New Delhi or Bombay, much better examples for prophets of doom in the year 200. "We pulled up to a theater once in Dehi and there must have been 20 men on the curb, urinating in the direction of our car" said an American woman here who much prefers Peking. An Indian diplomat living here said: "In Delhi I could feel the population exploding. I must say I have never felt that here."

Inside Chinese apartments and offices, of course, the crowding is oppressive, although life then slows down to avoid dangerous friction. Imagine your own office of factory with two or three times as many employes as it has now, but with the same amount of space and the same volume of work. The result would be many people sitting idle, with little to do. That happens here, but the Chinese are not necessarily unhappy with undemanding jobs.

The crowded, slow, somewhat relaxed city life is enough to keep Chinese from rushing to fill empty spaces. Elsewhere, the western half of the country is virtually deserted, but few people can be tempted to accept the vigorous frontier work there.

The Chinese are now, belatedly, trying to curb population growth with harsh penalties and forced abortions, a policy which, in India, may backfire. Nevertheless, China's economic planners are just as worried about the year 2000 as are planners in Washington, but they have to face the mixed signals of their own success in absorbing enormous numbers of new people. Thoughtful people wanting to protect dimishing resources must contend with an old Chinese saying:

"A person has one mouth to eat but also two hands to produce."