THROUGH THE elegant doorway of a Jackson Place townhouse, down the basement steps and across an obstacle course of broken chairs and half-empty supply boxes, Gerald O. Barney can be found in the former Xerox room of the Council on Environmental Quality. His location leaves him undaunted. Gerald Barney's domain is the world.

A man of 41 years, with a trim beard, black-rimmed glasses and a certain mild intensity, Barney is in charge of the Global 2000 study, an official U.S. government projection of the world's population, natural resources and environment through the end of the century. A physicist formerly with the Rockefeller Brothers Fund, he was hired to fulfill a grand pronouncement made in Jimmy Carter's environmental message two years ago.

At the start of a conversation, Barney clutches the text of the message, wrapped in a grass-green cover, and reads aloud:

"Environmental problems do not stop at national boundaries. In the past decade we and other nations have come to recognize the urgency of international efforts to protect our common environment . . . This study will serve as the foundation for our longer-term planning."

No ordinary promise, at least for Gerry Barney. It is something worth working nights and weekends for. Worth the frustration of trying what nobody apparently has tried before.

"Those two sentences can be incredibly important," he says. "The foundation for what our government does now is incredibly shaky."

The problem is that the U.S. government has people who study world energy supply, people who study world agricultural trends, people who study world economic forecasts. But they hardly ever talk to each other.

A few months ago, Barney got together the global planners and analyzers from a dozen agencies. "Not one had ever met any of the others!" he said. Even if Barney was not going to produce a 3-inch-thick study - which he will by this summer - he would count it as a major accomplishment that "now they all have each others' phone numbers."

The 21st century, it should be noted, is 21 years away. Should we be pessimistic or optimistic? The experts disagree. Since the recent slowdown in worldwide population growth, no one is sure how to set the clock on Connecticut Avenue that ticks off new humans by the second.

The U.S. Census Bureau has trumpeted the good news: 24 of the world's 50 most populous countries experienced a decline in birth rates over the last decade. The Population Reference Bureau reports that the world fertility rate dropped from 4.6 births per woman in 1968 to 4.1 in 1975.

But demographer Paul Ehrlich, who brought us "The Population Bomb," says there's no cause for complacency. "If India, with a 1970 population of 600 million, were to achieve a birth-control miracle and reach a net reproductive rate of 1 in the year 2000, its peak population size would be about 1.5 billion people . . . That is more than the present population of Africa, South America, North America, Oceania and Europe combined," he noted recently.

Doubling a population means doubling resources - food, energy, clean water, minerals, schools, housing - in a world where hunger, fuel shortages, disease, illiteracy and homelessness are the norm.

The idea behind Global 2000 is to take all the separate projections made by different U.S. agencies, put them together, iron out the inconsistencies and come up with a vision of which scarcities and problems will be serious by the year 2000.

Overall, Barney acknowledges, "The word in the year 2000 will look a great deal like it does today. But that superficial appearance is deceiving, because the cushion won't be there anymore. The stress will be more severe in every sector - energy, water, forests, food. It will work - but only if everything works right."

Meanwhile, Barney has some questions:

Are the people who project energy demand and supply talking to the people who decide agricultural export policy? "We have the most energy-intensive agricultural economy in the world," he says. "It's a source of foreign exchange, but it takes a lot of imported oil to make it work. Our whole approach suggests that we - and the world - can go to more fertilizers, more pesticides, more tractors, more irrigations. No one is thinking of alternatives that are less energy-demanding."

Are the scientists talking to the industrialists? "When you ask each nation how much coal they are going to burn and, therefore, how much carbon dioxide will there be, you find out the total amount exceeds the ability of the atmosphere to absorb it," he says.

"Those are the kinds of questions that need to be asked."

Barney's goal is to jolt the established agencies out of their "frozen assumptions." "The world really is highly connected. But is doesn't always come out looking that way when you organize it department by department."

Then he says, "We hope the study will lead to the reconsideration of some policies, and thus lead to quite a different future." What policies? He won't elaborate now.

But when the study is done this summer, and the flak has flown - he, at least, expects there will be a lot - Barney says, "I'm not sure I ever want to do a study again. Perhaps it's time to get out and do some things and stop studying them." CAPTION: Picture, Gerald Barney, By Douglas Chevalier - The Washington Post