This computer generated color composite image, produced from data acquired from the Landsat 4 and 5 satellites, is a representation of deforestation in the Brazilian Amazon region from 1978 (top) and 1988 (bottom). The deforestation represented in these figures is confined exclusively to the forest strata and has been averaged into 10-by-10 mile cells. Source: NASA

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MONITORING THE EARTH

The increasing scale of human activities on the earth has brought with it increased risk of environmental damage on a global scale. Managing the earth in a responsible manner thus requires monitoring the atmosphere, the oceans, and critical terrestrial ecosystems, so that environmental degradation can be detected in time. Satellites, backed by aircraft and ground observations and by fundamental research on biogeophysical systems, are already playing a major role, and could play an even larger one in the future.

Satellite data helped to confirm the initial discovery of the Antarctic ozone hole and to show that degradation of the earth's protective ozone layer was a global phenomenon. Intensive field and laboratory research, coupled with aircraft and satellite data, soon demonstrated that the degradation was caused by human activities--the industrial chemicals known as chlorofluorocarbons (CFCs), which are degraded in the stratosphere to release chlorine and which, in turn, catalyzes the destruction of ozone. This research led to the signing of the Montreal Protocol and amendments, which committed nations to phase out production and use of CFCs. Subsequent satellite monitoring has shown continued declines in global ozone levels and the presence of high chlorine concentrations over the Arctic, possibly presaging the creation of an Arctic ozone hole as well. Such observations have led to accelerated deadlines for phasing out CFCs, with the result that global production and emissions of these chemicals are beginning to decline. What could have been a major global disaster, with sharply increased levels of ultraviolet radiation harmful to living creatures, is being averted.

Pictures from space have brought global attention in recent years to another environmental problem--the destruction of tropical forests. Astronauts have described seeing the plumes of smoke from space, and nighttime infrared images have shown the Amazon region lit with hundreds of fires. The loss of tropical forests not only threatens the ecosystems that harbor the largest portion of the world's species, but also increases the risk of global warming.

Satellite observations can not only call attention to environmental hazards, they can also help to assess the extent and character of the problem accurately. Recently, for example, careful analysis of satellite imagery from the Amazon region showed that the total area of forest loss over the past decade was less than had been initially thought. However, it also showed that the pattern of clearing and burning had increased the fragmentation of the forest, making an area two-and-a-half times that actually cleared vulnerable to loss of species through disruption of ecosystems. The analysis technique is applicable to other tropical forests as well, and research has already begun to re-examine past satellite images covering other tropical forest regions.

As human populations and industrial activity increase, so will pressure on our environment. Both fundamental research to better understand earth systems and increased monitoring using advanced satellites will be important to detect degradation in time and thus to help preserve the earth for future generations.