World Bank president A. W. Clausen calls it a subject of ''vast importance and undeniable urgency.'' The subject is population, and the bank's 1984 World Development Report, scheduled for release today, focuses on the burgeoning population of the earth's developing countries as the primary impediment to their progress.
The earth's current population of 4.8 billion could more than double by the year 2150, to 11 billion people. After that, the report says, population should stabilize - but the explosive growth will cause severe strains in both the developing and the developed world.
''Opportunity,'' Mr. Clausen says, ''is on our side. But time is not.''
The World Bank report notes that at relatively minimal expense countries can reduce the population problem. But the momentum is so great that ''even if couples have fewer children, absolute annual increases will be close to or more than 80 million people a year in developing countries well into the next century.''
Still, the report is not altogether gloomy. It makes plain that ''these considerations should not obscure the central fact that the world's population growth rate is falling. The latter part of the 20th century has been a demographic watershed, the high point of several centuries of accelerating growth and the beginning of what demographers project to be a continuous decline , until world population stabilizes sometime in the 22nd century.''
''Though absolute numbers will continue to increase for several decades, the issue now is how quickly the rate of increase can be slowed down - and how individual countries (and the international community) are to cope with continued growth in the meantime,'' the report says.
One of the most important ways in which population growth can be slowed is by increased education - especially among the world's female population. The report points out that in all countries, women who have completed primary school have fewer children than women with no education.
Clausen, in a speech prepared for delivery today in Nairobi, Kenya, noted that the slowdown in population growth can be traced mostly to China, which has strictly cut average growth to 2.5 children per family. But in most other developing countries, families have at least 4 children, and in rural areas, 5 is the norm.
The problem continent is Africa.
In Africa most couples want more children. Although overall mortality rates in Africa are high, they are expected to decline. As they do, population will explode. ''We must,'' Clausen says, ''accept the likelihood that population growth will accelerate in Africa, because mortality still has far to fall and can be brought down fairly rapidly.''
He says the growing population retards economic progress in these ways:
* By forcing a choice between higher consumption now and the investment needed to bring higher consumption in the future: ''Every effort is thus required simply to maintain the (economic) status quo.''
* By threatening natural resources: ''Where populations are still highly dependent on agriculture, continuing large increases in population can contribute to overuse of limited natural resources, such as land, mortgaging the welfare of future generations.''
* By creating increasingly unmanageable urban economic and social problems: ''Cities in developing countries are growing to a size for which there is no prior experience anywhere.''
Where can progress be made? Where education is emphasized. Two examples are the crowded Asian countries of South Korea and Singapore. Though they are poor in resources, the high levels of education have promoted economic progress and have acted to slow population growth.
Poor parents decide to have numerous children for a variety of reasons, but the ''hidden hand of the market'' in many ways prompts these decisions: When wages are low, little income is lost while a mother cares for a child in infancy. When schooling is lacking, one cannot argue that having two or three educated children is better than a large uneducated brood. When the ''social safety net'' is inadequate, poor parents rely on their offspring to care for them in old age. And if childhood mortality is high, there is great incentive to have many babies to ensure survival of a few.
Because of these factors, Clausen says, governments should strive for improvements in health services, to lower childhood deaths; education, to raise hopes for children and broaden a woman's outlook; social security for the aged; goods and services that compete with childbearing; and family planning.
Clausen says that reducing population is not a panacea for development. But not to address the population problem adequately ''would permanently foreclose some long-run development options.''