International scientists and officials hailed a UN report Friday that said human activity was "very likely" the cause of global warming and that higher temperatures and rising sea levels would continue for centuries, regardless of reductions in greenhouse gas emissions.

"It is critical that we look at this report … as a moment where the focus of attention will shift from whether climate change is linked to human activity, whether the science is sufficient, to what on earth are we going to do about it," said Achim Steiner, the executive director of the UN Environment Program.

U.S. government scientist Susan Solomon says 'there can be no question' that human activity is the cause of increased greenhouse gases.

"The public should not sit back and say 'There's nothing we can do,'" Steiner said. "Anyone who would continue to risk inaction on the basis of the evidence presented here will one day in the history books be considered irresponsible."

The 21-page summary of the panel's findings released Friday said climate changes are "very likely" caused by human activity, a phrase that translates to a more than 90 per cent certainty that global warming is caused by the burning of fossil fuels.

A top U.S. government scientist, Susan Solomon, said "there can be no question that the increase in greenhouse gases are dominated by human activities."

It was the strongest conclusion to date, making it virtually impossible to blame natural forces for global warming, according to the authors.

"We have said the warming is unequivocal," said Ken Denman, a senior research scientist at the Canadian Centre for Climate Modelling and Analysis and one of the authors of the study.

"When physical scientists say something, they don't like to say "certain," but I think that means certain," he told CBC News.

The Intergovernmental Panel on Climate Change — consisting of hundreds of scientists and representatives of 113 governments — said global observations of air and ocean temperatures, widespread melting of ice sheets, rises in sea level as well as regional changes in precipitation, wind patterns and extreme weather all point to a shift in the world's climate.

People 'can make a difference'

The report said global warming and a rising sea level will continue for centuries, even if greenhouse gas emissions are slowed or reduced.

But the severity of these change is still in our hands to control, said John Fyfe, who is also a research scientist at Environment Canada's climate modelling centre.

"We're locked into a temperature increase of about 0.5 degrees by 2025 regardless of what we do, but the increases start to diverge depending on the levels of emissions when you look a hundred years from now," said Fyfe, one of four Canadian scientists involved in the report who spoke in Ottawa Friday morning.

"So what we do now can make a difference," he said.

Temperature increases of 1.8 to 4 C by 2100

The panel predicted average temperature increases of 1.8 to four degrees by the year 2100. That was a wider range than in the 2001 report.

On sea levels, the report projects a rise of 17.8 centimetres to 58.4 centimetres by the end of the century. An additional rise between 9.9 to 19.8 centimetres is possible if the recent, surprising melting of polar ice sheets continues.

Canada would be among the countries to see disproportionately high temperatures changes — perhaps enough to keep the Arctic ice-free in summer.

Canadian Environment Minister John Baird said after the report's release that global warning required "real action," while Prime Minister Stephen Harper acknowledged it was an "enormous" problem, but said any solutions would be long term.

Need to stop using economy as bottom line: prof

David Suzuki, well-known environmentalist and host of the CBC's The Nature of Things, said from Halifax on Friday the evidence has been "a long time in coming."

'I would say that we are in a state of crisis, that it's the equivalent of a hundred Pearl Harbors going off at once in the environment.'—David Suzuki, environmentalist

"Scientists have been conservative and cautious, now I think they've thrown that caution out," he said.

"I would say that we are in a state of crisis, that it's the equivalent of a hundred Pearl Harbors going off at once in the environment."

Peter Brown, a professor of philosophy at the McGill School of Environment, said he hopes the sobering news will lead to a change not just in carbon emissions, but in how the world economy functions.

"What the report shows me is a very deep paradigm shift is needed," he told CBC News Online.

"We need to move away from a model where economic growth is the only indicator of success."

The Intergovernmental Panel on Climate Change, created by the UN in 1988, releases its assessments every five or six years, and is used as a major source of information for government policy makers.

The summary is the first of four major reports expected this year. The second report on April 6 will detail the impacts of global warming and focus on vulnerable regions, while the third report on May 4 will look at ways policy makers and scientists might mitigate climate change.

A final report released on November 17 will synthesize the work of the previous three reports.

With files from the Associated Press