Human exploitation of the Earth's resources has pushed it "to the brink" and the planet has reached a point of crisis that can only be resolved with global action, Prince Charles warned Tuesday.

In a speech to the UN climate conference in Copenhagen, Britain's heir to the throne -- a committed environmentalist -- warned world leaders gathering here that they must agree a "comprehensive" deal on climate change.

"The grim reality is that our planet has reached a point of crisis," he said, according to an advance copy of his speech released by his office.

The largest and most important U.N. climate change conference in history opened Monday, with organizers warning diplomats from 192 nations that this could be the last best chance for a deal to protect the world from calamitous global warming.

At stake is a deal that aims to wean the world away from fossil fuels and other pollutants to greener sources of energy, and to transfer hundreds of billions of dollars from rich to poor countries every year over decades to help them adapt to climate change.

Scientists say without such an agreement, the Earth will face the consequences of ever-rising temperatures, leading to the extinction of plant and animal species, the flooding of coastal cities, more extreme weather events, drought and the spread of diseases.

Braking the rise in Earth's population would be a major help in the fight against global warming, according to an unprecedented UN report published on Wednesday that draws a link between demographic pressure and climate change.

"Slower population growth... would help build social resilience to climate change's impacts and would contribute to a reduction of greenhouse-gas emissions in the future," the UN Population Fund (UNFPA) says.

"It really is the first time that a United Nations agency has looked hard at the connections between population and climate change," lead researcher Bob Engelman, vice president for programmes at the green group Worldwatch Institute, told AFP.

"People are at the root of the problem and at the solution of it, and empowerment of women is the key."

The report, the 2009 State of World Population, paints a grim tableau of the peril of climate change and the likely impact on humans, in terms of floods, drought, storms and homelessness.

"Each person in a population will consume food and require housing, and ideally most will take advantage of transportation, which consumes energy, and may use fuel to heat homes and have access to electricity."

"The growth of population can contribute to freshwater scarcity or degradation of cropland, which may in turn exacerbate the impacts of climate change," says the report.

"So too can climate change make it more difficult for governments to alleviate poverty and achieve the Millennium Development Goals."

Yet the trouble with the chamber's petition, which it filed Tuesday with the EPA, is that it has little basis in precedent or law.

It's true that the EPA sometimes holds hearings before administrative law judges when the legality of its regulations is challenged, but the chamber wants it to hold such a hearing before any regulation has been approved, and the judge to rule not on a law but on the scientific basis for making a law. This has a whiff of a political stunt designed to fail -- and when it does, to give the chamber a pretext for accusing the Obama administration of not giving a fair hearing to scientific arguments that challenge mainstream climate-change theory.

Environmentalists can be dismayingly smug about climate change, sometimes claiming that the science is "settled" and there's nothing left to argue about.

The weight of scientific evidence suggests very strongly that the globe is getting warmer, that greenhouse gases emitted by humans are the cause and that the health and welfare of future generations are under serious threat.

A portion of the revenue from any U.S. system capping carbon emissions must go toward softening the impact of higher energy prices on consumers, a White House official said on Wednesday.

Joseph Aldy, special assistant to the president for energy and the environment, said building a clean energy economy will not be easy.

"There will be those who are going to be vulnerable as we make this transition and ... we need to actually target the allowance value and revenues to those households, communities, and businesses," Aldy said at an Energy Information Administration forum.

Obama's proposal would use most of the revenue generated from the sell of carbon permits for tax breaks, offsetting costs for consumers.

The draft legislation, which will be considered by the House Energy and Commerce Committee in coming weeks along with other panels, marks the latest attempt by Congress to bring the United States into a global effort to curtail emissions of carbon dioxide and other pollutants.

Many scientists think the growing amounts of pollutants are contributing to extreme weather, melting polar ice and threats to humans, animals and plant species.

"This legislation will create millions of clean energy jobs, put America on the path to energy independence, and cut global warming pollution," said House Energy and Commerce Committee Chairman Henry Waxman.

But the proposal circulated on Capitol Hill by Waxman and Representative Edward Markey would establish a new "cap and trade" regime for reducing emissions of carbon dioxide and other harmful greenhouse gases separately from the Clean Air Act.

Among the proposals aimed at protecting domestic industries are "rebates" to help them compete with overseas companies and up to 2 billion metric tons annually in "offsets" they could claim along with buying permits that push them to emit fewer greenhouse gases.

In Thursday's speech, the Prince will warn that a failure to act in the next eight years will have catastrophic effects for the planet.

In the country that is home to the world's largest rainforest, Prince Charles will urge world unity to combat deforestation in the run-up to the UN conference on climate change in Copenhagen in December this year.

He will say there is not necessarily a clash between the needs of big business and the environment. He will argue that being green can be good for businesses and can create jobs.

The Prince will say that tackling deforestation in the 3.5 billion acres of rainforest on the planet is a key priority.

Scheduled shifts in Earth's orbit should plunge the planet into an enduring Ice Age thousands of years from now but the event will probably be averted because of man-made greenhouse gases, scientists said Wednesday.

They cautioned, though, that this news is not an argument in favour of global warming, which is driving imminent and potentially far-reaching damage to the climate system.

Earth has experienced long periods of extreme cold over the billions of years of its history.

These climate swings have natural causes, believed to be rooted particularly in changes in Earth's orbit and axis that, while minute, have a powerful effect on how much solar heat falls on the planet.

They found dramatic swings in climate, including changes when Earth flipped from one state to the other in a relatively short time, said one of the authors, geoscientist Thomas Crowley of the University of Edinburgh, Scotland.

According to the model, published in the British journal Nature by Crowley and physicist William Hyde of Toronto University, Canada, the next "bifurcation" would normally be due between 10,000 and 100,000 years from now.

The chill would induce a long, stable period of glaciation in the mid-latitudes, smothering Europe, Asia and North America to about 45-50 degrees latitude with a thick sheet of ice.

However, there is now so much CO2 in the air, as a result of fossil-fuel burning and deforestation, that this adds a heat-trapping greenhouse effect that will offset the cooling impacts of orbital shift, said Crowley.

In September, a scientific research consortium called the Global Carbon Project (GCP) said that atmospheric concentrations of CO2 reached 383 parts per million (ppm) in 2007, or 37 percent above pre-industrial levels.

Present concentrations are "the highest during the last 650,000 years and probably during the last 20 million years," the report said.

Crowley cautioned those who would seize on the new study to say "'carbon dioxide is now good, it prevents us from walking the plank into this deep glaciation'."

"We don't want to give people that impression," he said. "(...) You can't use this argument to justify [man-made] global warming."

Left unchecked, climate change could inflict widespread drought and flooding by the end of the century, translating into hunger, homelessness and other stresses for millions of people.

Just as John F. Kennedy set his sights on the moon, Al Gore is challenging the nation to produce every kilowatt of electricity through wind, sun and other Earth-friendly energy sources within 10 years, an audacious goal he hopes the next president will embrace.

Gore said he fully understands the magnitude of the challenge.

"This is an investment that will pay itself back many times over," Gore said. "It's an expensive investment but not compared to the rising cost of continuing to invest in fossil fuels."

He likened his challenge to Kennedy's pledge in May 1961 to land a man on the moon by the end of the decade.

To meet his 10-year goal, Gore said nuclear energy output would continue at current levels while the nation dramatically increases its use of solar, wind, geothermal and so-called clean coal energy.

Exactly 20 years after warning America about global warming, a top NASA scientist said the situation has gotten so bad that the world's only hope is drastic action.

James Hansen told Congress on Monday that the world has long passed the "dangerous level" for greenhouse gases in the atmosphere and needs to get back to 1988 levels. He said Earth's atmosphere can only stay this loaded with man-made carbon dioxide for a couple more decades without changes such as mass extinction, ecosystem collapse and dramatic sea level rises.

"We're toast if we don't get on a very different path," Hansen, director of the Goddard Institute of Space Sciences who is sometimes called the godfather of global warming science, told The Associated Press. "This is the last chance."

Burning fossil fuels like coal is the chief cause of man-made greenhouse gases. Hansen said the Earth's atmosphere has got to get back to a level of 350 parts of carbon dioxide per million. Last month, it was 10% higher: 386.7 parts per million.

Hansen, echoing work by other scientists, said that in five to 10 years, the Arctic will be free of sea ice in the summer.

But Rep. Ed Markey, D-Mass., committee chairman, said, "Dr. Hansen was right. Twenty years later, we recognize him as a climate prophet."

Humans are widely believed to have arrived in North America from Asia over a land-bridge between Alaska and Siberia during a warmer period. A variety of dates has been proposed and some are in dispute.