'It's later than you think'

Climate change report pins blame on human race

The years of scientific toil, the thousands of journal articles and countless e-mails, the scores of meetings, the painstaking revisions - they all boiled down to two critical words in the highly anticipated Intergovernmental Panel on Climate Change report issued in Paris on Friday:

Very likely.

"The warming of the climate system is unequivocal," the report says, and human-generated, heat-trapping gases are "very likely" responsible for it. We humans are, by extension, very likely to cause future warming. Our impact is so great that, even had we halted our collective driving, heating, electrifying and fertilizing in 2000, the world would continue to warm for at least 20 years anyway as the oceans release heat they've stored up. And civilization rolls on.

"Very likely" is a concrete term in "Climate Change 2007: The Physical Science Basis," a 1,600-page draft report with a 21-page summary for policy makers that was released early Friday in Paris. It means there's a 90 percent chance that humans are heating the planet.

In the last major IPCC report in 2001, the word was simply "likely," or a 66 percent chance that humans are responsible.

"It is a very strong statement that global warming is also human warming," said Richard Somerville, a Scripps Institution of Oceanography scientist and one of 152 lead authors of the report, 10 of whom work in Boulder.

Susan Solomon, a senior scientist with the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, was co-leader of the five-year international effort to create the report.

"There's no doubt that for policy people, the difference between 'likely' and 'very likely' due to human activity is something that's being noticed," Solomon said. "What we're saying is, 'It's later than you think it is,' and that's just a fact that policy people are really paying attention to."

In language agreed upon by 300 delegates from 113 countries in a group line-edit Solomon led, the summary listed higher air and ocean temperatures, rising sea levels, glacial and Arctic thawing, heat waves and drought and more-powerful hurricanes among global warming's proven impacts.

Supercomputers all over the world spent years churning through 23 computer models to determine that, by 2100, there's a two-in-three chance that average global temperatures will be 3.6 to 8.1 degrees warmer.

The report said global human emissions of fossil-fuel carbon rose from 6.4 billion tons a year in the 1990s to 7.2 billion tons in 2005, a heft equivalent to a block of concrete spanning more than 1,000 football fields and as tall as the Empire State Building.

Pieter Tans, a NOAA Earth System Research Lab senior scientist whose group monitors global carbon-dioxide, says the number is higher yet now - more than 8 billion tons a year, probably. He said the global biosphere's net absorption is perhaps a billion tons per year.

"One out of eight - it's nice, but it's not a lot," Tans said. "What it implies, and I think the world is already acting - is that if you do not tackle CO2 emissions, you're not doing anything substantial."

Warming would continue even with monumental action.

Scientists ran climate models that assumed humanity managed to cut back carbon emissions more than 80 percent, stabilizing them at 2000 levels, said Gerald Meehl, co-leader of a team of 14 authors on the report's chapter on climate-change predictions. They found warming of about 0.2 degrees per decade to continue for 20 years.

Such "committed warming" is from the heat already pumped into the system and absorbed in oceans in the 20th century, Meehl said. The oceans eventually give it back. The pace of warming roughly doubles in more realistic scenarios, he said.

Linda Mearns, a National Center for Atmospheric Research scientist who was a lead author on the chapter on regional climate change, said the tropics could become drier and the Rocky Mountain region might expect shorter snow seasons and temperatures on average 7 degrees warmer. The drying heat would offset any precipitation increase, she said.

Uncertainties remain. Estimates of future sea-level rise - the worst case modeled being about 2 feet by 2100 - continue to be a big question, said Reto Knutti, an NCAR visiting scientist who worked on the climate modeling results. He said scientists still don't understand exactly how to connect temperature with melting of the sort happening in Greenland, which lost 220 million cubic kilometers in 2005, according to NASA's Jet Propulsion Laboratory in Pasadena, Calif.

And there is at least a bit of good news: Climate models all found that the "ocean conveyor belt" that generates the Gulf Stream and differentiates Europe's climate from that of northern Canada shows no signs of shutting down in the coming century.

In general, Solomon said she was "very, very pleased" with the report.

"I think it will be remembered as the report that took the issue beyond warming into lots of other areas," Solomon said. "I honestly think the facts spoke for themselves."

The facts have not spoken in such a way in a long time.

"We are, in a sense, doing things that perhaps have not happened in 650,000 years, based on the scientific evidence that's going to be placed before you," IPCC Chair Rajendra Pachauri said at the report's release.