The United Nations, an accepted authority on population levels and trends, estimates that the world population reached 5.3 billion in 1990, and is increasing annually by more than 90 million persons. The rate of increase, 1.7 percent per year, has fallen below the peak rate of 2 percent per year attained by 1970. However, absolute yearly increments are not expected to start declining until after the year 2000.
Estimates of world population before 1900 are based on fragmentary (不全的；不完整的) data, but scholars agree that average population growth approached approximately 0.002 percent per year. Growth was delayed and influenced by climate, food supply, disease, and war.
Starting in the 17th century, great advances in scientific knowledge, agriculture, industry, medicine, and social organization made possible substantial increases in population. Inanimate (无生命的) energy gradually replaced human and animal labor. People slowly acquired the knowledge and means to control disease. All continents shared in a five-time population increase over a 300-year period--from about 500 million in 1650 to 2.5 billion in 1950--but increases were most obvious in regions where new technologies were devised and applied.
Beginning about 1950, with improvements in water supplies, sewage (下水道里的污物) -disposal facilities, and transportation networks, agricultural yields increased, and deaths from infectious and parasitic diseases greatly declined. Life expectancy at birth in most developing countries increased from about 35-40 years in 1950 to 61 years by 1990.
According to the passage, what will happen in the next century