An unprecedented four-year study initiated by the United Nations has concluded the majority of the planet's ecosystems are being degraded in an unsustainable fashion.

At the first of nine news conferences being held in various cities around the world Wednesday, some of the 1,300 experts involved in the study warned of the damage that growing demand for resources is having on the Earth's life support systems.

The researchers, from 95 countries, concluded that 15 of 24 ecosystems are being damaged by such problems as human population growth, global warming and over-logging.

In the conclusions released Wednesday, the scientists warn the problems could get worse in the next 50 years if dramatic steps are not taken. Among the ecosystems studied were mangrove forests, rainforests and dry-land areas.

In Tokyo, United Nations Undersecretary Hans van Ginkel said the assessment reveals a consensus of the largest body of social and natural scientists ever assembled to examine the planet's ecosystems.

"It's not yet extreme, it's not exactly immediately collapse, but we better act before the collapse is there," he said. "And that may be very much a scientists' type of approach. But we have to make clear that the future of humankind is not based on simplistic pictures."

He called on Asian nations, home to 60 percent of the world's population, to take special note of the conclusions.

"Much of what it is in the report relates strongly to Asia," he added. "But, at the same time, it's not easy to come up with a one-fits-all solution because almost all the diversity of the world is present in that one continent, as well."

The study, known as the Millennium Ecosystem Assessment, has resulted in 2,000 pages of scientific data and technical volumes. The first two parts of the study have been released over the past few years.

There are concerns it could end up gathering dust on bookshelves unless there is a political and grassroots movement to act on its findings.

The report includes an optimistic note amid the gloomy forecasts. The assessment's co-chairman, Malaysian biologist A.H. Zakri, says much of the damage can be reversed if practices are changed.

"I would hope that individual governments would take action to take notice of the analysis and try to incorporate them into better policy formulation and management of ecosystems," said Mr. Zakri.

Mr. Zakri calls on people to speak up about imperiled ecosystems in their areas, to push policy makers to take concrete action.

Although its organizers hail the assessment as the first to focus on how ecosystem changes affect human well-being, they acknowledge no new research was undertaken and their mandate was not to present new findings.

Mr. Zakri, director of the U.N. University's Institute of Advanced Studies, said the starkest situation is the desertification of the world's drylands.

"That's the most urgent and that's the most vulnerable and that's the most disenfranchised of our brothers and sisters," he said. "On a scale of one to 10 I would give it the top mark."

The third part of the study will be released over the next year. It will include assessments of the Himalayan Hindu Kush, the Laguna Lake Basin in the Philippines, the Arafura and Timor Seas, western China region and Vietnam's Mekong wetlands, as well as several regions within India and Indonesia.