Humans, warming tied

Chilling appraisal from Boulder scientists in study

Jim Erickson, Rocky Mountain News

Saturday, January 20, 2007

A long-awaited report by an international scientific team will provide the strongest evidence to date that humans are changing the planet's climate by pumping heat-trapping gases into the air, according to Boulder scientists involved in the study.

On Feb. 2 in Paris, the Intergovernmental Panel on Climate Change will summarize the key findings from its latest assessment of global climate change.

The report will discuss observed changes - retreating mountain glaciers, melting polar ice sheets, rising sea levels, and shrinking summertime arctic sea ice, for example - as well as projections for the future.

The projections are based on 23 computer climate models operated by 16 research groups worldwide.

All 23 models agree that the planet will continue to warm as levels of carbon dioxide and other heat-trapping "greenhouse" gases rise in the coming decades, said William Collins of the National Center for Atmospheric Research in Boulder.

"This report will provide the most compelling evidence to date that climate is changing and that mankind is responsible for that change," said Collins, a lead author of the report's climate projections chapter.

"Children being born at this point in the 21st century will experience - we believe, under certain projections - significant climate change," he said. "I personally hope that this report stimulates people to take action to slow our influence on climate."

The report will be issued as momentum to limit U.S. greenhouse gas emissions builds on several fronts. Just this month:

• Congressional Democrats announced four bills, with more expected, to control carbon dioxide emissions.

• House Speaker Nancy Pelosi said she wants to create a special committee on climate.

• Leading scientists joined evangelical pastors to declare their intention to fight the causes of climate change, as well as public confusion on the subject.

• Ten major U.S.-based corporations - including General Electric, DuPont and Alcoa - joined leading environmental groups to call for a firm nationwide limit on carbon dioxide emissions.

And next week, in his State of the Union address, President Bush will lay out his policy on global warming. But the plan will not include mandatory emissions caps, according to press secretary Tony Snow.

Evidence in greater detail

Collins was one of several Boulder scientists who contributed to the latest IPCC report, Climate Change 2007: The Physical Science Basis.

Boulder atmospheric scientist Susan Solomon is one of two co-chairs of the team that produced the first of four volumes in this year's update.

The IPCC's last major update, in 2001, said the planet warmed 1 degree Fahrenheit over the past century and is likely to warm another 2.5 to 10.4 degrees by 2100.

In the 2007 update, that temperature range will be narrowed a bit, largely because of improvements in the climate models, NCAR's Kevin Trenberth said.

"I think probably the low value (in the 2001 report) and also the high value came from models that probably had mistakes in them," Trenberth said Friday.

"The confidence in those numbers was probably not that good, and they probably never should have been used in the way in which they were used," said Trenberth, who worked on the atmospheric observations chapter in the upcoming IPCC report.

"And I think you will find, then, that the numbers are probably narrowed," he said.

The 2001 report also stated that most of the warming over the past 50 years was likely because of buildup of greenhouse gases, largely from burning fossil fuels.

"That's a pretty strong smoking-gun statement that directly ties human activity to climate change," said NCAR's Gerald Meehl, another leading participant in the upcoming report.

"And I think we've got more evidence that supports that conclusion in greater detail now," Meehl said. "And when you really connect a cause and effect like that, I think it's a pretty powerful kind of argument."

Meehl said the upcoming report shares a sense of urgency.

"The longer you wait, the worse it gets," he said. "So really, if you're going to do something about this problem, the sooner the better."

Meehl and other IPCC authors are barred from discussing details until the report's release. But it assesses studies recently published in peer-reviewed journals. Some major themes have emerged in recent research, Meehl said.

A role in extreme weather

One topic that's received a lot of attention is extreme weather.

In an October study titled Going to Extremes, Meehl and his co-authors concluded that extreme weather - heat waves, droughts and heavy rains, for example - will likely become more frequent and more intense in coming decades. Dry spells could lengthen significantly across the western United States, southern Europe and other areas.

"I can't say what's going to be in the next report, but that's the type of research that's assessed for the IPCC," Meehl said.

Trenberth said a section in his chapter explores possible links between intense hurricanes and global warming. The drought plaguing the West for much of this decade also is discussed.

Another topic examined in recently published climate studies is "climate-change commitment." Because heat-trapping carbon dioxide has a lifetime of about a century, gases pumped into the air today will continue trapping heat far into the future.

That heat will warm the air, land surfaces and oceans. As the heat creeps deeper into the ocean, the warmed water expands, resulting in sea-level rise.

Change will last centuries

The concept of climate-change commitment has been around for about 20 years. What's new is that some of the latest, most sophisticated climate models now confirm the dire predictions of earlier, cruder simulations.

In a 2005 report in the journal Science, NCAR researcher Tom Wigley said that even if greenhouse gas levels could be magically stabilized today, sea levels would rise 10 to 20 inches per century for the next 400 years or more, imperiling coastal regions.

Because of carbon dioxide's long lifetime, actions taken today to reduce emissions "mainly benefit the next generation and the generation after that," Trenberth said.

"That's one of the things which I'm not sure is fully realized," he said. "This is a long-time-scale problem, and that's why you really want to get ahead of it.

"The other side of that is that it means we've got to live with climate change, and that means we should plan for it."

How many researchers does it take . . .

450: Number of lead authors for four-volume climate assessment

800: Number of contributing authors

2,500: Number of scientists reviewing findings

Feb. 2: Release date for summary of the first volume

ericksonj@RockyMountainNews.com or 303-954-5129. The New York Times and the Associated Press contributed to this report.