In the two minutes it will take you to read this story, the world will lose 300 acres of forest - almost half a square mile - to logging, farming, and development. In a full year the harvest will total 78 million acres, an area the size of Poland, or slightly bigger than Arizona.

Even as we begin to understand the necessity of forests to store the greenhouse gas carbon dioxide, we are accelerating their loss. And nowhere is our global schizophrenia toward tropical forests more apparent than in Costa Rica.

No country in the world has preserved more of its landscape. More than a quarter of this Central American country is protected in 22 national parks, biological preserves, and wildlife refuges, making ecological tourism its biggest industry.

Yet few countries have lost their forest base faster than Costa Rica; since 1950, the nation has lost half the virgin tropical forest that still covered 50 percent of its landscape. An exploding population, the export cattle market, and land clearing for coffee and banana plantations have all contributed to this loss. Costa Rica, only the size of West Virginia, tries to support 3 million people and two million cattle. The nation's cattle acreage ballooned from 12 percent of its land base in 1950 to more than 30 percent by 1990.

The loss of tree cover is as old as civilization. The richly watered ancient societies of Sumer and Ur are now barren Iraqi desert. Society collapsed on Easter Island when inhabitants ran out of trees. Mayan civilization fell in part because deforestation led to disastrous erosion. The Biblical "cedars of Lebanon" have been reduced to four small groves. A Mediterranean basin once thickly forested with oak, beech, and pine has been largely reduced to maquis scrub, and no more than 10 percent of the forests once found between Morocco and Afghanistan still exist.

India's Thar Desert was a jungle 2,000 years ago. The ancient forest that covered 75 percent of China now covers only 5 percent. Ireland was once thickly forested. The island of Madeira, named "isle of timber" by the Portuguese in 1420, was denuded by 1650. Haiti has less than 10 percent of its original forest. The examples go on and on.

The United States has cut most of its virgin forest outside Alaska. Replanting and natural reforestation of abandoned farmland has stabilized the total forest acreage, but the new trees are smaller, less diverse, and more susceptible to disease and insects.

Globally, the planet has lost at least 20 percent of its total forest cover (some estimates of depletion are as high as one third) and most of its virgin forest and grandest trees. Half the world's tropical forest, where up to 90 percent of terrestrial plant and animal species are found, has been logged.

This loss is crucial to climate change. Because life on earth is carbon-based, trees build themselves with the carbon dioxide that would otherwise heat our atmosphere. Forests act as a gigantic sponge, conserving water. They shade and moderate temperature, shelter organisms from wind and cold, and provide hiding places and food.

Tropical forests are particularly fragile because the high rainfall washes out nutrients, creating surprisingly poor soil. When cut for slash-and-burn agriculture, tropic forest sites are often exhausted of fertility in five years and can take centuries to recover.

Experts disagree on whether the rate of deforestation is slowing and on how fast abandoned areas recover. No one is certain, but global warming should actually produce a slight acceleration in plant growth because of added rainfall and more carbon dioxide in the air.

What experts do agree on is that conservation of forests is vital for biodiversity and climate stability, and here Costa Rica has some good news.

Starting in 1979, the nation began adopting a series of forest protection laws. In 1996 it passed Forest Law 7575, which recognizes the economic and biologic value of uncut forests and pays landowners to preserve them.

As a result, $57 million has been spent to preserve 544,000 acres of forest.

Deforestation has slowed sharply from the rate of loss in the 1970s and 1980s. Replanting and natural regeneration of abandoned pasture has produced re-growth equal to 75 percent of what has been deforested.

However, as with other such projects, the re-grown trees have neither the size nor the diversity of the forest that was lost. Nor do they shelter the same complex ecosystem. That means the uncut forests that remain, such as those of Monteverde Cloud Forest, are vital reservoirs of biological diversity and that they play a crucial role in our attempts to understand climate change.