In 1992, concerned about worsening environmental conditions, delegates to the UN Conference on Environment and Development (UNCED) in Rio de Janeiro, Brazil, stressed the need for action. The Rio "Earth Summit" set specific goals for environmental improvements. Then in 1997 a Special Session of the UN General Assembly—popularly known as the "Rio Plus Five Conference"—met to assess progress toward these goals (9, 101, 127, 146, 223, 224, 232).

The conclusions were discouraging. In such sectors as land, freshwater, forests, biodiversity, and climate change, the 1997 UN assessment found that conditions either were no better than in 1992 or had worsened (223, 232). Despite lower poverty rates, the number of poor people had increased—in large part because of rapid population growth in developing countries, as well as uneven development, and increasing concentration of wealth (222, 223, 227).

Arable land. At the beginning of the 1990s, about 560 million hectares of cropland worldwide were degraded, of a total 1.5 billion hectares. At the end of the decade about 610 million hectares were degraded (265). Soils can become degraded rapidly when they are overworked and thus become more exposed to erosion.

Freshwater. Worldwide, the percentage of the population with access to clean freshwater increased during the 1990s. Nevertheless, due to rapid population growth, currently an estimated 1.2 billion people lack potable water—20% more than in 1990 (222, 227, 261, 265). Also, about 3 billion people lack adequate sanitation facilities compared with 2 billion in 1990 (227, 261).

Forests. Half the world's original forest cover—over 3 billion hectares—has been lost, largely during the past five decades (25). Deforestation has accelerated since 1990. For instance, tropical forests declined from 1.7 billion hectares in 1990 to 1.4 billion in 1999 (263, 265).

D. Hinrichsen

A solitary tree watches over a barren hillside. Around the world people are depleting forests, arable land, and water supplies. Can present needs be met while safeguarding resources for the future?

Globally, about 16 million hectares of forest, an area roughly the size of Nepal, are cut, bulldozed, or burned each year. In the Brazilian Amazon the annual deforestation rate has increased by about one-third since 1992 (268, 269).

Biodiversity. Human activities already have pushed many plant and animal species into extinction. While no one knows the exact number, there is wide agreement that the rate of extinction will accelerate as population growth and development put more pressure on prime habitats of other species (163, 164).

Pollution. Air pollution, already a serious problem in many cities, is becoming worse as urban populations grow and the number of motor vehicles rises. Water pollution is a serious problem almost everywhere (220). Biologist Peter Vitousek and colleagues have warned that human numbers and actions risk fundamentally disrupting nature's basic cycling of water, nitrogen, phosphorus, and carbon among the ecosystems. Largely by releasing carbon dioxide into the atmosphere and by destroying or altering biological resources, humanity is causing "rapid, novel, and substantial" changes to the environment (247).

Climate change. At the Rio Earth Summit in 1992, whether the global climate was changing was still a matter of debate. Since then, the evidence has mounted (see side-bar, Global Warming: Worrisom Signs). In 1990 atmospheric concentrations of carbon dioxide—the main climate—changing gas-were measured at about 355 parts per million (135). In 1997 concentrations were measured at about 364 parts per million (233). Since 1950 carbon dioxide emissions have increased fourfold (21).

Poverty. During the 1990s the number of people in poverty increased by about 1 billion. In 1990 about 2 billion people were subsisting on the equivalent of US$2 a day or less (222). By 2000 that number had risen to about 3 billion—half of the world's population (236).