The population explosion is not fizzling

Ever since the population ''explosion'' or ''bomb'' expression came into use, people have come to us with questions. What does population ''explosion'' really mean? Is it just another Doomsday prediction dispensed by empty-headed Cassandras? Is it real? Is it over?

Articles have appeared in the past year in major US dailies announcing the end of the population explosion and the averting of demographic collapse. But absolutely nothing has actually happened to warrant such statements.

The facts of population growth are elementary and have remained unchanged for decades; no ''new study'' has uncovered any startling new evidence of any kind. The current flurry of ''fizzle'' articles was caused by a 1982 report from the United Nations Fund for Population Activities (UNFPA) which noted the decline in birth rates in some third world countries. The report pointed out that if all countries' birth rates had remained at the rather high level of the 1950s, world population in the year 2000 would be about 7.5 billion (it is about 4.6 billion today).

That statement is true. It is also true that the most recently published UN projection for the year 2000 is 6.1 billion, not 7.5 billion. At this point it is not difficult to see how one can be misled into thinking that world population in 2000 is now expected to be 6.1 billion, down from the 7.5 billion of the ''earlier projection.'' But this is not the case.

The confusion stems from a misinterpretation of UN population projections, a series of projections issued at normally five-year intervals since 1958.

In order to project the future population size of the world's countries, one is faced with the rather knotty problem of guessing what the countries' future birth rates will be. This is no easy task. In most of today's third world countries women have anywhere from five to eight children each. It is, however, generally accepted that as these countries modernize, their fertility behavior will at some point resemble that of today's industrialized countries where women usually have two children or less.

But when will those birth rates fall? And when they fall, how rapid will the fall be? Will the decrease be swift at first and then lose momentum or vice versa? No one knows. It would be at least logical to assume that birth rates will exhibit different patterns in different countries depending upon their particular situation.

The UN evaluates each country and makes an educated guess as to just what the timing of its birth rate decline might be and projects populations with that assumption. Then, in order to indicate possible ranges of error in the future, the UN publishes three sets of projections: the ''high'' variant which assumes the slowest decline in birth rates, the ''medium'' variant, which assumes a somewhat faster pace, and the ''low'' variant, which assumes the quickest decline of all. This, of course, results in three possibilities for population in 2000 (the most recent range from the low to the high variant projection Being 5.8 (billion) to 6.3 billion). World population in 2000, barring any major calamities such as nuclear war, is expected to fall in that range.

It is a common (though not always totally approprmate) practice to ignore the high and low projections when discussing the future and simply cite the medium or ''average'' projection for 2000, 6.1 billion. In publishing its projections of world population since the late 1950s, the UN medium projection for 2000 has always hovered in the 6.1 (billion) to 6.5 billion range. Where, then, did the 7 .5 billion projection come from?

In 1963, the UN published a fourth illustrative projection along with the high, medium, and low series. This projection assumed that all birth rates would remain constant (e.g. ''constant fertility''). The projection was intended solely to show what world population would look like if all countries' birth rates remained at the level of the late 1950s, as opposed to what was actually expected.

The ''constant fertility'' projection of 7.5 billion was never meant to be a baseline against which subsequent actual trends were to be measured. It was fully expected that birth rates would begin to come down in some third world countries before 2000 while others remained high.

To date, the UN projections are ''tracking'' quite well overall. In 1963, the UN projected a 1980 population of 4.3 billion. When 1980 rolled around the UN estimated a population of 4.4 billion. Not bad considering the gaps in information in 1963.

The fact remains that the Earth entered this century with less than 2 billion population, will close the century with over 6 billion, and will probably reach somewhere between 8 (billion) and 14 billion when population stabilizes - that is, if the ecological system permits this to occur. There has been no change in the expectation for decades, and without radical shifts in values, little likelihood of change in the future.