Planet Earth's people

I don't worry about small things; something must be big to attract my anxiety. Like world population. Lester R. Brown, thoughtful head of Worldwatch Institute, in a study financed by the UN Population Fund has just noted that population on Planet Earth is approaching 5 billion. That's a lot of people; how are we all going to eat? Good question!

The Chinese are most drastic in trying to cut back. Each Chinese, it seems, subsists on scarcely one-tenth of a hectare (a hectare equals 2.47 acres). How can they do it? China has about a quarter of the world's population and is the first country to adopt the ''one-child family'' as a social goal. (It raises questions. One wonders, for example, what happens psychologically to a nation composed entirely of ''only'' children? The new study doesn't discuss that.)

The oil shortage gave the world a jolt, but for a while we thought that nuclear power would save us. Lester Brown surveys the new situation. The atom isn't going to save us, he thinks. His immediate conclusion: ''As the high cost of nuclear power became more apparent, it also became apparent that this energy source would never play more than a minor role in the world energy economy.'' Is this a premature judgment? Perhaps, but after reviewing the evidence his team concludes that ''the future is not promising'' from a fuel standpoint.

There's coal. But here there are pollution problems, acid rain, diminishing supplies, and, all the time, that expanding global population. On all continents there are currently ingenious schemes. Some nations try windmills. Brazil has gone in for eucalyptus trees, and their charcoal now helps supply 40 percent of its steel smelting. Brazil also has planted more land to sugar cane - over a million hectares of it now produce fuel alcohol. (It seems that a Brazilian car with ethanol requires 1.1 hectares of sugar cane to keep it going.) Or take South Korea where Mr. Brown approvingly notes that they are restoring denuded mountain hillsides as an energy source. The United States? Why here ''residential hot water needs for a typical family can be largely satisfied with 60 square feet of solar panels.''

''Moreover,'' he goes on, ''these panels can easily be placed on rooftops where they do not compete with other uses. If fully exploited this technology could satisfy much of the world's residential hot water needs without any important additional claims on land use.''

It all sounds quite futuristic. Yet nobody can study the careful statistics of fuel, land, and people without a shock. Is our transition to the future going to be skillful or haphazard? The world economy seems to be losing momentum; after an unprecedented advance averaging over 4 percent a year the World Bank now notes long-term trends the other way.

For example, economic growth fell behind population growth in 18 countries containing 121 million people recently. ''In the absence of more effective family planning programs,'' Lester Brown thinks, ''the ranks of these countries seem certain to swell during the eighties.''

The fact is that the expanding gap between increasing population and shrinking natural resources is about the biggest problem Planet Earth faces. This careful new study boiled down into 45 pages is really a quiet appeal for prolific countries to reduce birth rates. The problem is now worst in sub-Saharan Africa: ''There for the first time the World Bank is projecting a decline in per capita income for a major region of the world.''

Mr. Brown sums it all up: ''In an age of slower economic growth improvements in living standards may depend more on the skills of family planners than on those of economic planners.''