In the face of overwhelming evidence that there is no way of fighting poverty in the Third World without more extensive family planning, the Reagan administration is cutting back its support of the most tested and experienced organizations in this field, condemning wide areas of the globe to ever bigger, ever more hungry populations.

Knowledgeable congressmen such as Sam Gejdenson (D-Conn.) suggest that the administration's penny-pinching will have the counterproductive result of accelerating illegal abortions in the less developed countries -- just what the White House presumably wants to avoid.

In a new book, "State of the World, 1985," Lester Brown and his Worldwatch Associates say the alternative to checking population by famine -- the present case in Ethiopia -- may be a one-child-per-family policy in 20 countries from Mexico to the Philippines if the birth control brakes aren't applied in other ways.

The ravages of famine aren't going to change until the affected countries are able to produce more of their own food. And even that can't happen until the United States joins in helping these countries to establish effective birth control programs.

Instead, the Reagan administration has cut off $17 million for the International Planned Parenthood Federation, because the IPPF allocates a dribble of funds -- less than one-half percent of the money it gets from non-American sources -- to clinics providing abortion services where they are legal in their own countries.

In addition, reacting to stories of infanticide and forced abortion in China published last month by The Washington Post, the administration has at least temporarily frozen $46 million for the United Nations Fund for Population Activities, even though none of this money was destined for China.

Agency for International Development Administrator M. Peter McPherson is being pressured by Sens. Jesse Helms (R-N.C.) and Gordon J. Humphrey (R-N.H.) to supply "detailed proof" that U.S. funds are not assisting Chinese population-control efforts, directly or indirectly.

Yet another blow to the financing of population control efforts came in the new budget for fiscal 1986, which cuts the AID's Family Assistance Planning funds to $250 million from $290 million in fiscal 1985.

If the Reagan administration can't be persuaded of the need for these funds on humanitarian grounds, or if it doesn't believe that excessive population growth sabotages economic development in the Third World, it at least should pay attention to the way in which the population explosion leads to political instability in the Third World, which, in turn, creates security problems for the United States.

According to sources at the Population Institute of Washington, a still-classified Central Intelligence Agency report lists many global flash points that could lead to wars in this century -- wars that have their roots in the unrestrained growth of population.

For example, the CIA report, titled "Population, Resources & Politics in the Third World: The Long View," predicts that Mexican-U.S. relations may be the most complex problem that the United States faces at the turn of the century because of migrant traffic across the border, and water and pollution problems.

The CIA says that the population explosion also may have enough of an impact on Turkey to destabilize NATO; lead Honduras and El Salvador into war; cause Vietnam to expand into underpopulated Laos and Kampuchia, perhaps bringing the Soviets and China to the brink of war; and create a variety of problems for Middle East allies of the United States, notably Israel and Egypt. (Egypt, Mexico, El Salvador and Vietnam are four countries listed as "outgrowing" their borders.)

Africa apart, this gives a sense of the range of potential security problems for the United States if world population soars from 5 billion now to 12 billion in the 21st century: Many experts wonder whether it will be possible to feed more than 8 billion.

Emergency aid to Ethiopia and other famine areas may alleviate human suffering temporarily (and assuage the guilt among those of us with full stomachs who sign the checks). But what escapes attention is that per capita food production in Africa has been declining by almost 1 percent a year since 1967.

When the TV networks caught up with the Ethiopian famine story toward the end of last year, the troubles were generally attributed to the drought. But as Brown observes, the drought was just the trigger -- the most recent calamity of a long-term deterioration.

Thus, the real need in Africa is for something more basic than "relief" packages -- especially control of the birth rate. Because this runs to an average of five children per family in some countries, the task ahead is grim.

In their book, Brown and his Worldwatch Institute staff read us the bottom line about Africa: The population explosion itself is changing the climate on that continent in a way that could lead to a crisis of historic proportions:

"The sheer number of people seeking to survive on arid, marginal land may be driving a self-reinforcing process of dessication -- literally drying out the continent."

What makes all this especially tragic is that the Western World, which now fears that the situation in Africa may not be retrievable, had plenty of advance notice. Recently, the Population Institute obtained declassification of a 1974 National Security Council document that predicted the Ethiopian famine, and warned of the critical need to do something quickly about population control.

This document (National Security Study Memorandum 200, Dec. 10, 1974) was prepared by National Security Adviser Brent Scowcroft for President Ford to summarize the implications of high population growth for American security and other interests abroad. According to Werner Fornos of the institute, it was reviewed and found current in 1976 by President Carter's national security adviser, Zbigniew Brzezinski.

"The most serious consequence of rapid population growth for the short and middle term is the possibility of massive famines in certain parts of the world, especially the poorest regions," the NSC report said.

"If future [population] numbers are to be kept within reasonable bounds, it is urgent that measures to reduce fertility be started and made effective in the 1970s and 1980s."

Although the report's recommendations for increasing American funding of population control efforts were adopted in a still-classified NSC policy memo dated Nov. 26, 1975, the follow-through has been weak: Despite the reality now of what was merely a projection 10 years ago, the United States has been niggardly in providing family-planning money.

The Scowcroft report said that, if we desire to see political stability in the Third World, it "will require that the president and the secretary of State treat the subject of population growth control as a matter of paramount importance." More than 10 years later, the advice, however sound, is being ignored.