improved crops, irrigation, fertilizer, pest-control chemicals, and other tools of the Green Revolution had provided some answers to the global struggle for food. World population exceeded 4.5 billion in the early 1980s, but the worldwide per capita harvest of grain peaked in 1984. Now, in the early 1990s, the decline of global food stockpiles is causing concern again. There are many questions about how successful we will be in ensuring that the food supply will be adequate for the growing world population.

The continued rapid growth of cities raises questions about the loss of arable farmland to urban spread, about the effects of municipal wastes and industrial pollutants on land and water resources, and about competing demands on and overuse of water supplies. Irrigation can create arable land and increase food production in the short term, but it entails potential long-term damaging effects related to salt buildup and water-logged soils. Similarly, increased use of fertilizer can lead to runoff of mineral nutrients that can overload and alter the ecosystems that receive them. The continuing evolution of resistance to pesticides raises questions about the future effectiveness of current methods to prevent crop losses.

As developing and more densely populated countries improve their economy and standard of living, they seek more animal products in their diet, and that increases demands on the world's grain supplies for animal feeds. More widespread and intensive cultivation with more specialized and productive genetic strains is displacing traditional varieties of crops that had evolved and adapted to the challenges of particular ecosystems around the world. We do not know whether the more productive genetic strains will prove more vulnerable to environmental shifts and pressures or whether the biological and genetic diversity present in native varieties can be captured and preserved. Many potential environmental consequences must be understood and managed or blindly suffered as land, natural resources, and ecosystems are manipulated to meet the increasing need for feed and food to support continued population growth.

We Are Producing Global Change. For more than 99% of the time humans have inhabited the earth, their numbers and activities have been too small to affect more than their immediate, local environment. In the last few decades, however, humans have become a global force in modifying the atmosphere, oceans, land surfaces, and biota-a force that many believe is out of control. Human activities are measurably changing the composition of the atmosphere, adding gases (such as carbon dioxide and methane) that alter the radiative balance of the planet and adding other gases (which contain chlorine) that destroy the life-protecting ozone layer in the stratosphere. Human activities have destroyed vast tracts of tropical forests and

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