During the past twenty years, overall U.S. population growth has been moderate and steady. This growth has been due partly to health care improvements that have resulted in longer life expectancy. But, more important, it has been due to increases in net immigration, which has always been an important factor in U.S. population trends.

In contrast to the steady overall growth of the U.S. population, regional population growth has varied widely (Figure 6). Several major trends account for the redistribution of the U.S. population over the past forty years.

URBANIZATION

In 1987, the United States was far more metropolitan (77 percent) than it was in 1950 (56 percent). Large numbers of people moved from small towns and rural areas to large cities and their suburbs, creating areas of urban sprawl. The percentage of the U.S. population living in the larger metropolitan areas (over one million population) grew from 29 percent in 1950 to 50 percent in 1990.

SUNBELT GROWTH

Much of the U.S. population distribution in recent decades has shifted from the North-Central and Northeast industrial belt to the Sunbelt states, which enjoy a warmer climate. The Sunbelt includes the southern tier of states stretching from Florida to California. Nine of the twelve fastest-growing metropolitan areas in 1990 were located in Florida.

+5

0

-10

-5

Northeast Great Southeast Gulf of Pacific Atlantic Lakes Atlantic Mexico Coast

Source: CEQ 1992b

COASTAL GROWTH

Coastal populations have continued to increase, especially in the Southeast and Gulf regions. About 110 million people currently live in counties in the coastal areas of the United States. In 1988, the population density of coastal counties was more than four times greater than the overall U.S. average of 27 people per square kilometer. In over 20 percent of the nation's coastal counties, density exceeds 192 people per square kilometer, with the urban centers surrounding New York, San Francisco, Boston, and Philadelphia exceeding 3,846 people per square kilometer. Figure 7 compares the population growth rates of the five major U.S. coastal regions over the past twenty years.

Birth and Death Rates

The average life expectancy at birth in 1990 in the United States was 75.4 years. The average infant mortality rate in 1990 was 9.1 deaths per 1,000 live births. This is half the rate of twenty years ago, and only about onefifth the infant mortality rate of 1940.

Although the current 0.7 percent annual growth rate of the U.S. population is significant, it is far below that experienced during the 1946-64 "baby boom," when 75 million people were added to the U.S. demographic profile. During the 1950s, the average growth rate was slightly less than 2 percent per year. The median age of the U.S. population rose by about four years from 1972

to 1989: in 1989 the median age was 32.6 years―31.5 years for males and 33.8 years for females.

U.S. fertility has decreased significantly over the last thirty years. This change is frequently attributed to the greater participation of women in the labor force and the widespread use of effective contraception among both males and females. In 1957, the total fertility rate peaked at 3.7 births per woman. Today, American women are bearing an average of about two children, and this rate is projected to decline slightly over the coming years. However, the rate of natural increase (the excess of births over deaths) for the U.S. population has held steady at about 0.7 percent. The U.S. Census Bureau estimated in a January 1989 report that the U.S. population could increase from current levels to about 471 million by 2080.

Immigration/Emigration

Despite a low rate of natural increase, the U.S. population has grown by 22 percent from 1970 to 1991. Net immigration accounted for approximately 29 percent of the population growth in the 1980s. This compares with 11 percent during the 1950s, and perhaps 40 percent during the peak immigration years of 1900-10. During 1981-90, 38 percent of the immigrants who entered the United States were of Asian origin, 29 percent were of Latin American origin, 14 percent were of European origin, and 12 percent were of Caribbean origin. Immigration has increased steadily since the end of World War II, averaging about 450,000 annually in the 1970s and over 600,000 annually in the 1980s.

The natural and environmental resources of the United States are vast and varied. Its diverse climate zones, topography, and soils support many ecological communities and supply renewable resources for many human uses. The nature and distribution of these resources have played a critical role in the development of the U.S. economy and, therefore, have indirectly influenced the pattern of U.S. greenhouse gas emissions.

About 60 percent of the total U.S. land area is privately owned (70 percent, excluding Alaska). Although the private sector has played a primary role in developing and managing U.S. natural resources, federal, state, and local governments have also been important in managing and protecting these resources through regulation, economic incentives, and education, as well as by directly managing much of the other 40 percent, which includes forests, parks, wildlife preserves, resource development and range, recreational areas, and open space. Underlying these efforts have been private citizens demanding more environmental protection and management, seeking to be better informed about environmental risks, and influencing decisions in an open public process that is grounded in tradition and law.

Environmental organizations and other U.S. publicand private-interest groups frequently influence the development, implementation, and timing of state and federal programs. Groups active in environmental and resource management issues include long-established national conservation organizations and newer publicinterest groups, as well as business, labor, and other groups and associations.

Land Resources

The United States has a large and diverse land area of approximately 931 million hectares (2.3 billion acres). About 20 percent of the land is in crop land; 26 percent is in grassland, pasture, and range; and 29 percent is forested. About 12 percent of the land area is not classified (but includes wetlands, tundra, and urban areas), and the remaining 12 percent is classified under special purposes, including transportation routes, surface-water reservoirs, parks and wildlife refuges, and national defense (Figure 8).

FORESTS

Forestland has great potential to alter, and be altered by, the climate system. Forests not only act as sinks of greenhouse gases, but also are home to a diversity of biota on which climate change could have a significant impact. U.S. forests vary from the scrub forests of the arid interior West to the highly productive forests of the Pacific Coast and the Southeast. In 1987, forests occupied about one-third (296 million hectares, or 731 million acres) of the total U.S. land area. Of this total, twothirds were timberland, most of which (72 percent) is privately owned (Table 1).

By most measures, the productivity of U.S. timberland has been increasing. In part, this reflects management inputs over the past several decades. The United States currently grows more wood than it harvests, with a growth-to-harvest ratio of 1.37. This ratio reflects substantial new forest growth, although old-growth forests have continued to decline over the same period.

GRASSLANDS

Grasslands are relevant to climate change because of their potential to absorb significant quantities of greenhouse gases. In the United States, grasslands, including both range and pasture, accounted for about 324 mil

lion hectares (800 million acres) in 1987. Grassland is characterized as land on which the native vegetation in the climax or natural potential community is predominantly grass, grass-like plants, forbs, or shrubs suitable for grazing or browsing. Rangelands include native grasslands, savannas, alpine meadows, tundra, many wetlands, some deserts, and areas seeded to introduced species but managed like native rangeland.

The total area of pasture and range has declined by approximately 10 percent in the last decade. Some of the net decrease in range involved federal lands that were withdrawn for parks, wilderness, and similar areas or that were reclassified as unsuitable for grazing; ecologically, these lands remain grasslands. Among the reasons for decline in forested grazing land are fewer farms and less land in farms, changes in forest species, increases in stand density, and improvements in both livestock feeding and forest management practices.

In general, more than half of U.S. pasture and rangeland is classified as having poor to fair nutritional value for livestock grazing. These lands provide lowerquality forage, but good wildlife habitat and watershed protection. Sites having better-quality forage for livestock are usually also less prone to erosion problems. A site's condition can be improved by properly managing

1950s to the mid-1970s, wetlands in the lower 48 states were converted at the rate of 185,000 hectares (457,000 acres) a year. Estimates by the U.S. government in 1991 indicate that the rate of loss from the mid-1970s to the mid-1980s for the continental United States has slowed by 37 percent, to approximately 117,000 hectares (290,000 acres) a year (Table 2), although some recent studies suggest even slower rates of decline in the 1980s. Roughly 69 million hectares (170 million acres) of wetlands exist in Alaska. Many of these areas are publicly owned, although precise figures are not available. Total wetland losses in Alaska have been less than 1 percent, although in coastal areas losses have been higher.

AGRICULTURAL LAND

Agricultural land-and its productive capacity-is highly sensitive to the impacts of potential climate change. Currently, the United States enjoys a natural abundance of productive agricultural lands and a favorable climate for producing food, feed grains, and many other agricultural commodities. The area of U.S. crop land used has remained relatively stable (about 160 million hectares, or 395 million acres) for the last sixty years. However, the area of crop land harvested actually declined in response to improved crop yields-from 144 million hectares (356 million acres) in 1930 to 124 million hectares (306 million acres) in 1990.

The productivity of U.S. crop land has been markedly improved through the use of yield-increasing technologies, including drainage, irrigation, fertilizers, improved seeds and plant varieties, herbicides, and pesticides. Mechanization has also been an important influence.

Table 3 shows the price and yield statistics and the management practices applied to land dedicated to major U.S. agricultural crops. The United States today harvests about the same area as it did in 1910, although the population has grown by two and one-half times, and food exports are considerably larger. Another indication of the high productivity of U.S. crop land is the fact that the United States leads the world in donations of food to other nations. Between 1987 and 1989, the United States donated about 7 million metric tons of cereals, oils, and milk to other nations.

The number of farms in the United States has declined from 6.3 million in 1930 to 2.1 million in 1987. Meanwhile, the average number of hectares (acres) per farm has increased from 63 to 184 hectares (156 to 455 acres). The distribution of farms and the value of production by farm sales has changed even more drastically. In 1949, 6 percent of the farms (289,000) had farm sales exceeding $40,000 (based on 1980 prices) and were producing 40 percent of the total value of production or farm sales. In 1982, 29 percent of the farms (695,000) had sales exceeding