THE U.S. Census Bureau, summarizing global population trends over the last five years, offers further evidence of dramatic declines in population growth rates. They amount to what some population experts are calling a "demographic revolution."

Of the six major world areas, only one -- Africa -- experienced an increase in population growth rate during the five years from 1975 to 1979 as compared with the rate from 1965 to 1970. In Latin America, the rate fell by 11 percent over that decade. In Asia, North America and Europe, it dropped by about 25 percent, and in Oceania (Australia, New Zealand and the Pacific Islands) it fell by 45 percent.

Despite these dramatic drops, the kicker is that these are rates of growth , and these rates apply to ever larger population bases. So despite these unexpectedly large declines in fertility, the world's population is still growing very fast, and will probably continue to do so for about 100 years.

What do the current trends mean for eventual world population? If what is called "replacement level fertiliy?" (about two births per woman) can be achieved by the year 2000, global population will then number 6 billion. Since today's population is about 4.5 billion, that means adding nearly the equivalent of: the current populations of India and China. And since population growth has a built-in momentum after so-called replacement fertility is reached -- as large numbers of younger people reach childbearing age -- the world's population would not be expected to level off until it reached about 8.5 billion people late in the next century.

If, as most experts expect, the year 2020, not 2000, is realistic for this goal, population will then already number 8 billion and population will stabilize at 10.7 billion. Nine out of every 10 of these people would live in the developing world. Another 20 year's delay would mean a final world crowd of 13.5 billion -- triple today's number.

Time, then, is the key factor -- and the one that is seldom appreciated even by heads of state. As World Bank President Robert McNamara puts it, "The time lost in temporizing with population problems is simply irrecoverable. It can never be made up." Urgency, then, is what is called for. The recent good news of declining growth rates shows that there is hope for a livable world -- but only if greater population control efforts are made very soon.