GENEVA (Reuters) - Humanity is heading for a sharp drop in living standards by the middle of the century unless it stops its massive depletion of the Earth's natural resources, according to a report issued Tuesday.

The main culprits are the rich powers -- the United States and Canada, 19 countries of Western Europe, and Japan -- said the study from the respected Swiss-based conservation body WWF-International.

"The U.S. government in particular seems completely insensitive to some of the consequences of what it is doing," WWF Director-General Claude Martin told a news conference.

Entitled "Living Planet Report 2002," the study said there was so much pressure on water supplies, forests, land and energy sources that within 150 years the planet's riches could be exhausted and temperatures pushed inexorably upwards.

At current exploitation rates and population trends, over 20 percent more natural resources were being used up every year than could be regenerated, meaning that by 2050 two Earths would be needed to cope with resource demands.

"It's like drawing down on a bank deposit much faster than it is being fed by interest payments. At some point, the money's going to run out," said one official of the Swiss-based WWF.

The document was issued in advance of next month's World Summit on Sustainable Development in Johannesburg, called just 10 years after the 1992 Rio de Janeiro Earth Summit agreed a plan to avert looming environmental and social disaster.

LESS WASTE

The report sets out WWF ideas on how the trend could be slowed down and reversed, including better use of technology to ensure economies were run with less waste, and ensuring fossil fuels were progressively abandoned.

It also said governments at the August 26-September 2 Johannesburg conference should promote education and health care to try to curb a population rise -- shorthand for birth control.

Such projects have been championed by United Nations agencies but are strongly resisted by a new coalition of Islamic states, the Vatican, and Christian fundamentalists in the United States who have the ear of President Bush.

Martin, a veteran Swiss environmentalist who has generally stayed aloof from "gloom and doom" predictions, said he was not optimistic that serious decisions would be taken.

"Governments are too often moved only by the political considerations of the moment, and are reluctant to look far into the future" he said.

"But delegates to the Summit should remember that the year 2050 is within the lifetime of most of our own children. Our over-consumption of natural resources today will affect the living standards not of an abstract future generation but of people we know and care about."

The study found that human economic activity had reduced by 35 percent the number of surviving animal and bird species -- as well as freshwater and marine fish which provide a major source of food for much of the world's people.

AFRICA USES LITTLE

WWF figures show while rich nations draw heavily on Earth's resources, people in poor African states eke out an existence without using all that is available to them within their national borders.

The report said the Earth had about 11.4 billion hectares of productive land and sea space -- 1.9 hectares (4.70 acres) for each of the planet's six billion people.

While the average U.S. citizen had 5.27 hectares of land and sea available in his country to meet his needs, he consumed the product of 9.7 hectares by using what was produced elsewhere to make up the gap.

And while the average African consumed natural resources from only 1.36 hectares out of the 1.55 available on the continent, the average West European used the product of 5.0 hectares -- 2.84 hectares more than the region has.