Many damaging effects of climate change are already basically irreversible, researchers declared Monday, warning that even if carbon emissions can somehow be halted, temperatures around the globe will remain high until at least the year 3000.

"People have imagined that if we stopped emitting carbon dioxide the climate would go back to normal in 100 years, 200 years; that's not true," climate researcher Susan Solomon said in a teleconference.

Solomon, of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., is lead author of an international team's paper reporting irreversible damage from climate change, being published in Tuesday's edition of Proceedings of the National Academy of Sciences.

She defines "irreversible" as change that would remain for 1,000 years even if humans stopped adding carbon to the atmosphere immediately.

The findings were announced as U.S. President Barack Obama ordered reviews that could lead to greater fuel efficiency and cleaner air, saying the Earth's future depends on cutting air pollution.

Said Solomon, "Climate change is slow, but it is unstoppable" — all the more reason to act quickly, so the long-term situation doesn't get even worse.

Alan Robock, of the Center for Environmental Prediction at Rutgers University, agreed with the report's assessment.

Changes in rainfall

"It's not like air pollution where if we turn off a smokestack, in a few days the air is clear," said Robock, who was not part of Solomon's research team. "It means we have to try even harder to reduce emissions," he said in a telephone interview.

Solomon's report "is quite important, not alarmist, and very important for the current debates on climate policy," added Jonathan Overpeck, a climate researcher at the University of Arizona.

In her paper, Solomon, a leader of the International Panel on Climate Change and one of the world's best known researchers on the subject, noted that temperatures around the globe have risen and changes in rainfall patterns have been observed in areas around the Mediterranean, southern Africa and southwestern North America.

Warmer climate also is causing expansion of the ocean, and that is expected to increase with the melting of ice on Greenland and Antarctica, the researchers said.

"I don't think that the very long time scale of the persistence of these effects has been understood," Solomon said.

Global warming has been slowed by the ocean, Solomon said, because water absorbs a lot of energy to warm up. But that good effect will not only wane over time, the ocean will also help keep the planet warmer by giving off its accumulated heat to the air.

Climate change has been driven by gases in the atmosphere that trap heat from solar radiation and raise the planet's temperature — the "greenhouse effect." Carbon dioxide has been the most important of those gases because it remains in the air for hundreds of years. While other gases are responsible for nearly half of the warming, they degrade more rapidly, Solomon said.

'Real concern'

Before the industrial revolution, the air contained about 280 parts per million of carbon dioxide. That has risen to 385 ppm today, and politicians and scientists have debated at what level it could be stabilized.

Solomon's paper concludes that if CO2 is allowed to peak at 450-600 parts per million, the results would include persistent decreases in dry-season rainfall that are comparable to the 1930s North American Dust Bowl in zones including southern Europe, northern Africa, southwestern North America, southern Africa and western Australia.

Gerald Meehl, a senior scientist at the National Center for Atmospheric Research, said, "The real concern is that the longer we wait to do something, the higher the level of irreversible climate change to which we'll have to adapt." Meehl was not part of Solomon's research team.

While scientists have been aware of the long-term aspects of climate change, the new report highlights and provides more specifics on them, said Kevin Trenberth, head of climate analysis at the centre.

"This aspect is one that is poorly appreciated by policymakers and the general public and it is real," said Trenberth, who was not part of the research group.

"The temperature changes and the sea level changes are, if anything, underestimated and quite conservative, especially for sea level," he said.