TOKYO — Growing populations and expanding economic activity have strained the planet's ecosystems over the past half century, a trend that threatens international efforts to combat poverty and disease, a U.N.-sponsored study of the Earth's health warned on Wednesday.

The four-year, US$24 million (euro18.57 million) study -- the largest-ever to show how people are changing their environment -- found that humans had depleted 60 percent of the world's grasslands, forests, farmlands, rivers and lakes.

Unless nations adopt more eco-friendly policies, increased human demands for food, clean water and fuels could speed the disappearance of forests, fish and fresh water reserves and lead to more frequent disease outbreaks over the next 50 years, it said.

"For some time, the changes have been good to us: Food output has increased," A.H. Zakri, director of the United Nations University Institute of Advanced Studies in Japan, said at a news conference. "The problem is these changes have been achieved at growing costs."

U.N. Secretary-General Kofi Annan stressed that the Millenium Ecosystem Assessment "tells us how we can change course," and urged nations to consider its recommendations.

Eliminating trade barriers and subsidies, protecting forests and coastal areas, promoting "green" technologies and lowering greenhouse gas emissions thought to contribute to global warming can all help to slow environmental degradation, Zakri said.

The study was compiled by 1,360 scientists from 95 nations who pored over 16,000 satellite photos from the U.S. National Aeronautics and Space Administration, and analyzed reams of statistics and scientific journals.

Their findings, announced in several cities worldwide, highlight the planet's problems at the end of the 20th century, as the human population reached 6 billion.

A fifth of coral reefs and a third of the mangrove forests have been destroyed in recent decades. The diversity of animal and plant species has fallen sharply, and a third of all species are at risk of extinction. Disease outbreaks, floods and fires have become more frequent. Levels of carbon dioxide -- a greenhouse gas -- in the atmosphere have surged, mostly in the past four decades.

The collapse of the cod industry in eastern Canada offers a cautionary tale of how poorly managed resources can ruin an economy. According to the study, after overfishing dramatically reduced the amount of cod in nearby waters in the 1990s, tens of thousands of people lost their jobs.

Conservation groups called on governments, businesses and individuals to heed the study's warnings.

"Ecosystems are capital assets. We don't include them on our balance sheets, but if we did the services they supply would dwarf everything else in value," said Taylor Ricketts, director of conservation science at World Wildlife Fund.

Zakri said sub-Saharan Africa, one of the world's poorest areas, represents one of the biggest challenges for policymakers.

"The millions of people there have the lowest levels of human well-being but they have only less than 10 percent of the world's water supply," he said.

As the desertlands expand, fewer people there will have access to food and water, making it more difficult for policymakers to raise living standards for those inhabitants, he said.

Zakri said that could hinder progress toward goals adopted at the U.N. Millennium Summit in Johannesburg, South Africa in September 2000: halving the proportion of people without access to clean water and basic sanitation by 2015 and improving the lives of 100 million slum dwellers by 2020.

Worldwide, some 1.1 billion people still lack access to safe drinking water, with 3 million to 4 million people dying each year from waterborne diseases, according to U.N. statistics.

The ecosystem assessment was designed by the U.N. Environment Program, the U.N. Development Program, the World Bank, the World Resources Institute, the Global Environment Facility and others. Governments, non-governmental organizations, foundations, academic institutions and the private sector also contributed their expertise.

Source: Associated Press