WITHIN THE NEXT 45 years, if current trends continue, there would be more people in Mexico than there are in the United States. Similarly, Indonesia would have 1.78 billion people in a century, nearly half of the present world population.

These scenarios are unlikely to occur. Populations of that magnitude - added in such short periods of time - cannot be accomodated by the political or social structures of these countries. Political leader in some nations have begun to grasp this reality, and their countries have been making notable progress in holding down birth rates. But much more needs to be done if the world is not to be overwhelmed by the immense strains resulting from population problems.

World population growth, gathering momentum during the late 19th and early 20th Centuries, reached crisis proportions after World War II when death rates fell precipitously throughout the world while birth rates in developing countries continued at traditional high levels.

The peak rate of population growth of approximately 2 per cent was reached about 1965, when world population totaled roughly 3.3 billion, and the peak annual increase in numbers of people, roughly 70 million, occurred about 1970. Sometime in 1976, the world population passed the 4 billion mark, and 62 million have been added during the past year.

The addition each year of a population greater than West Germany, Great Britain or Mexico to the world total - mainly in the poorest continents and countries - inescapably has profound implications for the adequacy of food and fuel supplies, housing, education, health services and every social structure.

The implications of rapid population growth are most clearly seen with respect to food supplies. Although recent harvests indicate a food surplus in the United States this year, food production has barely matched population growth in the developing countries - creating a precarious balance which could mean disaster for the world's poor.

In the words of Norman Borlaug, father of the Green Revolution in world agriculture, "If a crop failure occurs in any of the large grain-producing areas of the world, tens of millions would die in an international disaster and little could be done to prevent it."

Future climatic conditions will greatly determine how well the world will be able to feed itself. Some experts, such as Reid Bryson of the University of Wisconsin, have concluded that the climate of the past 50 years has been unusually good and that we may face a return to the more normal - that is, poorer - conditions which existed for centuries before.

Although both the population growth rate and the annual increment of population have fallen since their peaks, the addition of more than 60 million people this year (170,000 new mouths each day) places great stress upon the world food system in the developing world - where, excluding China, 80 per cent of the population growth is occurring and where only 20 per cent of the world's food is being produced.

The ironic paradox of this autumn of 1977 is the super abundance of American grain harvests, exceeding storage capacities and piled on streets in the Midwest, while hundreds of millions suffer from severe malnutrition and starvation in Africa, Asia and Latin America.

And no longer can we, in good conscience, solve this U.S. grain surplus problem as we did in the 1950s and 1960s, by shipping our surpluses to the developing countries on concessional terms. Such action may appeal to American farmers and farm state legislators, but it can now be seen as often a long-term disservice to the developing countries.

Such "Food for Peace" aid to India, Pakistan and many other countries in the 1960s and 1980s enabled them to delay self-help actions to curb population increases and raise local food production - a delay which compounded and enlarged their basic problem of too rapid population growth.

UNICEF estimates at least a billion people - one quarter of the world's population - suffer from malnutrition. The U.N. Food and Agriculture Organization predicts that this number could increase by 50 per cent in 8 to 10 years, even without any major droughts or crop failures.

Obviously the relationship between population growth and food production is a dynamic one - with important food production gains from new seeds, more fertilizer and irrigation repeatedly being rapidly overtaken by burgeoning populations.

Vigorous action to increase agricultural production in developing countries through water management, road building, insect control, fertilizers and improved seeds and crop management will, of course, continue, but unless more than matched by actions to curb human fertility in those countries, these efforts to improve the human condition must necessarily fail.

At present it takes about .40 hectares per person to produce a minimally adequate diet. There are now only about .35 hectares of usable land per person in the world. Moreover, the total amount of prime agricultural land is shrinking due to soil erosion, climatic change and salination. These problems will probably increase as more marginal land is brought into production. Also, millions of acres are taken over each year by expanding needs for homes and other non-agricultural uses.

Yields on current acreage per family can be increased, perhaps doubled or tripled in developing countries, by using better seeds, double cropping and general modernization of agricultural techniques adapted to tropical conditions. But the major inputs needed to increase yields, such as fertilizer, fuel for irrigation and mechanization have increased dramatically in price, limiting their availability in most of the poorest countries.

In theory, consumption patterns could also be changed. Millions of tons of grains and legumes now fed to livestock every year and suitable for human use could be made available to feed the world's hungry. But even if people in affluent countries immediately cut meat consumption by 20 per cent, grain supplies for human use would increase by only 24 million tons or about 3.4 per cent of annual consumption. If population increases each year at the current rate, this one-time reduction in meat consumption would have little permanent impact on world per capita food supplies.

Some progress can, and indeed must, be made in all of these directions, especially in increasing per acre yield, but unless population growth is checked as well, none of these increases will be sufficient to avert disaster within the next generation. Burgeoning "Miseropoli"

WHILE FOOD is the most obvious need of burgeoning population, the whole spectrum of human needs is under stress. The imbalance of supply and demand is often most acute and apparent at the periphery of sprawling urban slums which surround all of the Third World's major cities. Today these shantytowns or, in Barbara Ward's worlds, "Miseropoli," account for 30 to 70 per cent of major city populations. Moreover, they are expanding faster than any other form of human settlement.The World Bank estimates shantytown growth rates to be as high as 20 per cent a year in many major cities - meaning a doubling of populations there in less than four years.

Housing shortages, traffic congestion, crime and pollution are urban ills suffered by rich and poor countries alike. But the accumulation of these problems is most acute in the developing countries, where city populations could quadruple in 20 years.

And as Enrique Penalosa, secretary general of the 1976 U.N. Habitat Conference, stated: "Answers won't come easily. Even right decisions carry penalties and we can ill afford mistakes. But there is one factor that can steel our resolve. We cannot go on as we are today."

The real question is time. If human settlements continue to expand so rapidly, the magnitude and cost of resolving these problems may well overwhelm human ingenuity and resources.

Whatever the other causes, rapid urban growth in developing countries continues to be fed by population growth. Unlike urbanization in developed countries, urbanization in developing countries produces no decline in the rural population. Thus, while urban population may double by the end of the century - from 1 to 2 billion - rural populations will also mushroom - from 2 to 3 billion - thereby supplying a continuous pool of new urban migrants. The Employment Gap

PUSHED OUT of rural areas by land shortages and lack of alternative economic opportunities, new urban immigrants add to surplus labor and unemployment problems in the cities. The growth of surplus labor and joblessness, while related directly to underdevelopment, is also obviously tied to population growth - with a lag time of about 15 years.

In many countries, unemployment is growing proportionately faster than employment. Between 1960 and 1975, economic growth in developing countries created over 150 million new jobs, but the labor force grew by almost 170 million. Jobless rates of 15 to 30 per cent (highest among youth) combined with underemployment rates of perhaps 35 per cent or more are turning some Third World countries into urban powder kegs. Measures to discourage migration into cities and greater emphasis on rural development may help, but eventually there must be a better balance between economic growth and population growth.

Half the population in the less developed world is under 19. In many poor countries, more than 40 per cent is under 15. In Nigeria and Peru, 45 per cent of the population is under 15; in Pakistan, 46 per cent. These young people are bound to swell the ranks of the unemployed since in most poor countries there already is significant unemployment.

In the United States, fertility has dropped substantially in the past decade. Yet population pressures from developing nations cause an estimated 800,000 illegal immigrants to be added to our population each year.U.S. population growth is itself still a cause for concern because we alone consume about one-third of the world's annual production of non-renewable resources and energy. Glimmers of Hope

THE MAGNITUDE and urgency of the world population problem were recognized by U.S. leaders in the 1950s and 1960s and led by key changes in U.S. policy and population program action beginning in 1965. By the end of the current fiscal year next Friday, the United States, through the population program of the Agency for International Development, will have provided $1 billion toward resolution of the world population crisis.

This $1 billion, representing 60 per cent of all international population program assistance during the past decade, has been the foremost source of contraceptives, training and assistance for family planning programs throughout the developing world. It also has spurred the population programs of many developing countries and of multilateral organizations, such as the U.N. Fund for Population Activities and the International Planned Parenthood Federation.

Hard lessons have been learned, and attempts to limit population growth are beginning to bear fruit. At least 40 of the world's most populous developing countries - nearly 80 per cent of the Third World population - are supporting some kind of family planning program as part of an official policy to reduce population growth. A handful now have population stabilization targets. Conversely, only about 15 of the world's countries (seven of them developed countries) provide no support at all for family planning programs.

In those countries where national leaders speak out, action is taking place, whether for the purpose of population stabilization or for the improvement of public health.

More and more national leaders have begun to speak forcefully about the need for such action. Mwai Kibaki, Kenya's minister of finance, addressing an outdoor meeting in the province of Vihiga, said, "Family planning is an integral part of development planning . . . you cannot hope to plan other forms of production if you cannot plan the production of the main resource, human beings."

Even in India, where popular resentment against coercive sterilization programs contributed to the fall of Prime Minister Indira Gandhi's government, the new leadership appears fully committed to stemming runaway population growth.

Parents around the world are becoming increasingly aware of the cost of raising children. "Even among farm families in less developed countries, children cost more to care for until they reach 15 than they contribute by their work," Thomas J. Espenshade, an economist at Florida State University, reports in "The Value and Cost of Children." Studies show "parents vastly underestimate the expenses involved in childbearing. Foreknowledge might affect fertility," he believes; "knowing what one child will cost them would well cause couples to pause before embarking on parenthood."

While the availability of family planning to the world's couples is still grossly inadequate, especially in rural areas, pilot programs have shown surprising acceptance of modern contraceptive methods when concentrated efforts are made. In Indonesia, where wholesalers have been distributing contraceptives to nearly 50,000 local retailers, and where government pressure for small families is strong, 1.5 million new contraceptive users were attracted in one year's time. In Egypt, several rural pilot projects served 30 per cent of the reproductive age couples in the area in just five months. In Colombia, family planning volunteer workers cooperating with the Coffee Growers Association attracted up to 50 per cent participation in the households they visited.

In a recent article on U.S. population program assistance overseas, Dr. R. T. Ravenholt, AID's population program director, wrote: "The most notable discovery of recent years is that poor and illiterate peasants use effective means of fertility control to approximately the same extent as literate urban residents when these means are made fully available in rural areas."

Another barrier slowly falling concerns the status of women. In most developing countries, women have fewer rights and opportunities than men, resulting in both less control of their reproductive lives and fewer alternatives to childbearing. Increased education for women and participation in a country's economic life would contribute toward smaller families.

Religion is frequently given as the immovable force preventing progress in adopting family planning. Yet, as the Rev. Rodney Shaw, president of the Population Institute, has pointed out, "there is no doctrine contained in the world's major religions that is so inflexible that it cannot be understood to favor responsible parenthood and birth control in the face of present human realities." Often the people are changing traditional social patterns even before their leadership. In the United States, middle-income Catholics use birth control to the same extent as Protestants; internationally, some of the major success stories are found in heavily Catholic Latin American nations such as Colombia and Costa Rica.

Global experience during the last decade has amply demonstrated that the problem in implementing family planning programs is not mainly with the people, but with the policymakers. If the political leaders of a country are truly dedicated to solving this problem, it can be done with remarkable speed, even in large, poor countries. Falling Birth Rates

WITH AN UNDERTAKING of such massive global dimensions, operating in a most sensitive area of human activity, it necessarily takes some years before the impact on fertility can be measured.

But sufficient time has now elapsed so that demographic changes in many countries can be related to program operations.

Birth rates are falling in almost all countries of Asia and Latin America, especially in nations with vigorous family planning programs. But in Africa, where such programs have lagged, little change in fertility has occurred during the past decade.

China, generally credited with having implemented the most powerful fertility control program during the past decade, has emphasized delay of marriage, full availability of all effective means of fertility control and intense social pressure for reduced child-bearing.

Among the countries assisted by AID, Korea, Indonesia, Thailand, Colombia and Tunisia are family planning leaders, with birth rates decreasing from 17 to 33 per cent during the last decade.

Many other countries receiving population program assistance from the United States and other donors have also achieved substantial declines in average completed family size: Costa Rica, from 6.8 children in 1965 to 3.9 children in 1975; Dominican Republic, from 7.4 to 4.8; Chile, from 4.9 to 3.3; Taiwan, from 6.0 to 3.0; Mauritius, from 6.0 to 3.5, and Sri Lanka, from 4.8 to 3.6.

Rapid fertility decreases in developing countries are becoming a general phenomenon in this decade, analagous to the rapid mortality decreases of the 1950s and 1960s. Data for 25 less developed countries indicate an average reduction of more than one birth per woman during a span of less than a decade. A dozen other countries have experienced a decline of more than 1.5 births per woman. Actions Required

FUTURE ACTIONS toward solution of the world population crisis should include intensified efforts by the United States, other developed countries, and the developing countries themselves. The impact of rapid population growth is no longer just a health or family planning matter. It requires concerted action at all levels of social concern.

In addition to making fertility control services and information fully available to all couples in developing countries, there is need to gear development assistance programs to this effort. For example, increased literacy and improved educational opportunities for all people - but especially for women who have received less than their fair share - will aid the population effort as well as being basic to human development in general.

Agrarian reform - giving people a more meaningful stake in their own production and a greater share of the benefits of society - can combine with population control to increase the net gain of each person in all human amenities.

In addition, there is a need in all countries for political leaders to make careful analyses of the impact any proposed action would have on population growth.

Many of the most crucial contributions toward a solution of population problems during the last decade were made by private individuals and organizations. Expanded action by individual donors, foundations and corporations, to the extent of at least $1 billion, is needed during the next decade. Multinational corporations, such as Exxon, with $3.1 billion net profits in 1976, and General Motors, with $1 billion, should find it in their own self-interest to contribute to this effort in a responsible and substantial way.

The dynamics of population growth require that, to achieve desired changes in the future, action must be taken now. A total of $10-12 billion in combined government and private funds during the next decade would accomplish more than hundreds of billions of dollars several decades later.

One reason for optimism is that it is greatly in the self-interest of individuals and families to determine the number and spacing of their offspring. For poor women, it is often a matter of life or death.

A peasant woman in Bangladesh, without family planning services, faces more than a 10 per cent chance of dying from complications of unlimited pregnancies. Surely she will choose to live and care for her existing children if given the opportunity.