Over the past 50 years, mankind has changed the natural environment of the planet faster and more extensively than at any other time in human history, according to the first comprehensive evaluation of the world's major ecosystems.

Attempts to meet growing global demand for food and other natural resources have resulted in a "substantial and largely irreversible" loss in the diversity of life on Earth, said the report, the Millennium Ecosystem Assessment, to be released Wednesday.

The report - an attempt to come to grips with the relationships between ecosystems and human well-being - was written by 1,360 experts from 95 countries and reviewed by 850 experts and government officials.

About 60 percent of the planet's "ecosystem services" - uses of the natural environment that benefit people such as freshwater for irrigation or ocean fishing - are being degraded or used unsustainably, the report said.

"Many of the changes under way to ecosystems are so intense that they are unprecedented," said University of Oregon marine biology professor Jane Lubchenco, a former president of the American Association for the Advance of Science and a contributor to the report. "We are really entering terra incognito here."

The report warns that dangerous environmental surprises - the sudden collapse of fisheries, the appearance of "dead zones" in coastal waters, outbreaks of new and reemerging diseases like SARS and regional shifts in climate - are increasingly likely.

"There is no simple fix to these problems since they arise from the interaction of many recognized challenges, including climate change, biodiversity loss and land degradation, each of which is complex to address in its own right," the report said.

This alarming picture "could grow significantly worse" during the first half of this century since most of the factors driving the degradation of ecosystems are continuing or growing in intensity, the report said.

Nevertheless, the report cites many policy and technological changes that could lessen or reverse environmental decline. However, many of the recommended changes - such as removal of certain agricultural subsidies, stronger limits on ocean fishing, better forest management practices, the development of markets for trading and pricing freshwater - are controversial and are generally not under way yet, the report noted.

U.N. Secretary General Kofi Annan, who launched the assessment four years ago, applauded the report as "an unprecedented contribution to our global mission for development, sustainability and peace."

"Only by understanding the environment and how it works can we make the necessary decisions to protect it," Annan said.

John Turner, assistant secretary of state for oceans and environment, said the report "contributes to the body of work on sustainability and ecosystem management."

"The United States continues to work hard at home and abroad through a wide range of initiatives that protect our natural resources and biodiversity, and promote sustainable development globally," Turner said.

Stanford University biology professor Hal Mooney, who co-chaired a panel that reviewed the report, likened the assessment to a business balance sheet that compares profit and loss.

"The message we want to say is that we're running down the account," Mooney said. "We're not balancing our budget and we have to turn our attention to what we're doing."

Other findings in the report:

- More land has been converted to cropland since 1945 than was cultivated in the 18th and 19th centuries combined. About 30 percent of the Earth's land area is devoted to some kind of agriculture.

- About a quarter of the world's coral reefs have been badly damaged or destroyed in the last several decades.

- The amount of water impounded behind dams has quadrupled since 1960. Six times more water is held in reservoirs than flows in natural rivers.

- More than half of all synthetic nitrogen fertilizer ever used on the planet has been used since 1985.

- Since 1750, atmospheric concentration of carbon dioxide has increased about 32 percent primarily due to the burning of fossil fuels and land use changes. About 60 percent of that increase has taken place since 1959.

- Between 10 percent and 30 percent of all mammal, bird and amphibian species are threatened with extinction.

The report was underwritten by the United Nations Environment Program, the World Bank, the U.N. Foundation, the David and Lucille Packard Foundation and the National Aeronautics and Space Administration, among other government and private organizations.