World leaders meeting at the Earth Summit in Brazil are challenged by critical environmental threats to the atmosphere; the diversity of animals, birds, and plants; the soil; and the oceans and coastal areas. Atmosphere
Despite rising average temperatures worldwide in the last 50 years, experts are still debating whether the atmospheric accumulation of "greenhouse gases" such as carbon dioxide and methane is causing this global warming trend.
The "greenhouse effect" theory of global warming, supported by recent data from the United Nations Environment Programme, projects that over the next 50 to 100 years the additional heat trapped by these man-made gases could result in a severe decline in agricultural productivity in some regions, shifts in climate zones toward the poles, rising ocean levels and extensive flooding, and accelerated animal and bird extinctions.
But some scientists say the recent warm weather may simply be a natural fluctuation in the earth's long-term atmospheric cycles.
In addition, the earth's protective stratospheric ozone layer, which absorbs much of the sun's damaging ultraviolet radiation, is being depleted by the production and use of chlorofluorocarbons (CFCs) - refrigerant gases - and other compounds. Scientists say increased ultraviolet radiation could harm humans and animals, disrupt the marine food chain, and damage food crops. Recent efforts to control use of CFCs have not yet had an effect on ozone depletion, because these compounds take at least 10 years t o reach the stratosphere (12 to 31 miles altitude) and have a life of 50 to 100 years.
A second ozone problem is causing concern closer to home. Volatile organic compounds, also known as hydrocarbons, mix with nitrogen oxides in the presence of sunlight to create ozone, or smog, in the troposphere (ground-level atmosphere). The major sources of these pollutants are industrial and auto emissions, and other fuel combustion.
Data are not readily available to determine global trends in urban smog, but trends in emissions of nitrogen oxides show increases in nearly every city monitored.
Industrial pollutants are also responsible for the acid deposition, including acid rain, seen in the highly industrialized regions of North America, Europe, and China. Over time, both soils and surface waters can gradually acidify, disrupting chemical and biological processes.
Representatives of 143 nations on May 9 adopted a treaty to limit global emissions of carbon dioxide and other greenhouse gases. The document calls on industrial nations, which are responsible for most of the world's pollution, to aim at capping emissions at 1990 levels, although no timetable for achieving this goal was agreed upon. The treaty also sets up mechanisms to assess the threat of global warming and commits developed nations to partially fund emission-control measures in developing countries.
World leaders will sign the treaty after the last details are hammered out at the Earth Summit.