GENEVA (AP) -- Humanity's reliance on fossil fuels, the spread of cities, the destruction of natural habitats for farmland and over-exploitation of the oceans are destroying Earth's ability to sustain life, the environmental group WWF warned in a new report Thursday.
The biggest consumers of nonrenewable natural resources are the United Arab Emirates, the United States, Kuwait, Australia and Sweden, who leave the biggest "ecological footprint," the World Wildlife Fund said in its regular Living Planet Report.
Humans currently consume 20 percent more natural resources than the Earth can produce, the report said.
"We are spending nature's capital faster than it can regenerate," said WWF chief Claude Martin, releasing the 40-page study. "We are running up an ecological debt which we won't be able to pay off unless governments restore the balance between our consumption of natural resources and the Earth's ability to renew them."
But Fred Smith, president of the Washington-based Competitive Enterprise Institute and a former official of the U.S. Environmental Protection Agency during the Nixon and Ford administrations, said he was skeptical. In a telephone interview, Smith said the WWF view is "static" and fails to take into account the benefits many people get from resource use.
Use of fossil fuels such as coal, gas and oil increased by almost 700 percent between 1961 and 2001, the study said.
Burning fossil fuels -- in power plants and automobiles, for example -- releases carbon dioxide, which experts say contributes to global warming. The planet is unable to keep pace and absorb the emissions, WWF said.
Populations of land, freshwater and marine species fell on average by 40 percent between 1970 and 2000. The report cited urbanization, forest clearance, pollution, overfishing and the introduction by humans of nonnative animals, such as cats and rats, which often drive out indigenous species.
"The question is how the world's entire population can live with the resources of one planet," said Jonathan Loh, one of the report's authors.
The study, WWF's fifth since 1998, examined the "ecological footprint" of the planet's entire population.
Most of a person's footprint is caused by the space needed to absorb the waste from energy consumption, including carbon dioxide. WWF also measured the total area of cities, roads and other infrastructure and the space required to produce food and fiber -- for clothing, for example.
"We don't just live on local resources," so the footprint is not confined to the country where consumers live, said Mathis Wackernagel, head of the Global Footprint Network, which includes WWF.
For example, Western demand for Asia's palm oil and South America's soybeans has wrecked natural habitats in those regions, so the destruction is considered part of the footprint of importing nations. The same applies to Arab oil consumed in the United States.
The findings are similar to those in WWF's 2002 report, which covered the period up to 1999. But the latest study contains more detailed data stretching to 2001. It shows the situation has changed little in most countries and is now more worrying in fast-growing China and India.
The world's 6.1 billion people leave a collective footprint of 3.36 billion acres, 5.44 acres per person. To allow the Earth to regenerate, the average should be no more than 4.45 acres, said WWF.
The impact of an average North American is double that of a European, but seven times that of the average Asian or African.
Residents of the United Arab Emirates, who use air conditioning extensively, leave a 24.46-acre footprint, two-thirds caused by fossil fuel use. The average U.S. resident leaves a 23.47-acre footprint, also largely from fuel.
Swedes leave a 17.3-acre footprint, but most is caused by land use and the impact on other countries of its imports of food and clothing. Like its Nordic neighbors, the country has won praise from campaigners for cutting fossil fuel use.
The study also warned of increasing pressure on the planet's resources amid spiraling consumption in Asia.
The Competitive Enterprise Institute has been a frequent critic of what it calls "environmental alarmism" from organizations like WWF. Smith said the footprint idea is wrongheaded.
"It's sort of like saying, 'General Motors must be much more wasteful than the local laundromat because General Motors spends more resources.' Yes, but they are producing more product, too," he told the AP.
"The real question is not whether the United States is a wealthy place but rather whether it's producing more wealth than it's consuming. Obviously, we are. We're using a lot of the world's resources but we're producing far more of the world's resources."
Loh said governments, businesses and consumers should switch to energy-efficient technology, such as solar power.
"We can consume energy in a way that's harmful or in a way that's sustainable," he said. "The technologies are available to enable the world's population to live within the capacity of one planet."
High oil prices may help focus their minds.
"But it's not a question of how much oil is left," he said. "The question we should be asking is how much fossil fuel consumption the Earth can sustain. The Earth has a limited capacity."
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.