Climate: A peek at the next IPCC report

By DAN WHIPPLE  |  Jan. 24, 2005 at 11:35 AM
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BOULDER, Colo., Jan. 24 (UPI) -- The world of climate science is not renowned for self-deprecating humor, but Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, recently described progress made on the fourth assessment report for Intergovernmental Panel on Climate Change, which is due out in 2007.

The panel's fourth report will be acronymed AR4, a break from tradition. The second assessment report was called SAR and the third was short-handed as TAR. Logically, Meehl said, the fourth would have been FAR, but that would mean the fifth assessment report would have the same acronym.

"Someone suggested that we can call that one 'Too FAR,'" Meehl said.

Not exactly George Carlin, and the AR4 does not promise to be a laugh a minute, but the next IPCC report does figure to feature a few shifts in emphasis, including attempts to quantify the uncertainty about climate shifts, provide a better estimate of climate sensitivity, report more multi-model results and get a better handle on the extremes.

The change likely to cause the sharpest intake of breath is the revision of the measurement of "climate sensitivity." This is the change in temperature measured by models from an equilibrium state that then respond to forcings and give a range of results for possible warming.

The numbers that result from the climate sensitivity calculation are widely quoted as the predicted warming the globe will experience under the current regime of increasing greenhouse gases in the atmosphere. According to the TAR, the numbers fall between +2 degrees and +8 degrees Fahrenheit (+1.1 degree and +4.5 degrees Celsius).

The median response is about 6.3 degrees F (3.5 degrees C), but the method of calculating that in the past has been based on climate models that use what climatologists call a slab ocean and do not fully mix the exchange of heat from the deep ocean.

"The numbers haven't changed in a long time, and people say, 'Well haven't you learned anything?'" Meehl told UPI's Climate.

The upgraded calculation of climate sensitivity will use coupled atmosphere-ocean global climate models to come up with a range and median, thereby refining the figures and giving a narrower range and lower median value.

Based on 17 of these AOGCMs, as they are known, the equilibrium range now is expected to be 3.6 degrees to 7.9 degrees F (2 degrees to 4.4 degrees C), with a mean value of 5.6 degrees F (3.1 degrees C).

Meehl said many people who remain skeptical about the result of the climate science have charged the scientists with fudging the sensitivity figures to get the result they would like to see. As a lead author on the climate science section of the IPCC report, he said he has spoken to most of the modeling groups and no one is aware of any such shenanigans.

"There is a feeling in some quarters that people are faking with their forcings," Meehl said. "That doesn't happen here. I don't even know how you'd do it. There are so many non-linear relationships that it's impossible to know which parameters to adjust to get the outcome you wanted. All you can do is say that this is the best we can do with the current state of the knowledge."

There remain considerable uncertainties, particularly the effect of aerosols -- dust and other particles -- in the big climate picture. Aerosols play several roles in weather and climate, such as cloud formation and solar reflectivity. Some of these roles tend to heat the atmosphere and some tend to cool it.

"In terms of the chemistry, for the future, both of those sources (black carbon and organic carbon aerosols) are more important than we had previously thought, since they tend to stick around longer," Lynn Russell, associate professor at the Scripps Institution of Oceanography at the University of California, San Diego, told Climate late last year.

Russell and colleagues reported in a paper published in the journal Science last week that aerosols may produce a slightly greater cooling effect than climate models previously estimated.

NCAR runs two climate models: the older PCM model and the newly released CCSM3. The older model does not attempt to calculate aerosol inputs, while CCSM3 does -- although Russell's work implies the modelers used to adjust the values. The uncertainty is large, however.

"The deeper you get into aerosols, the worse it gets," Meehl said.

Meehl said most models are showing a slowdown in the overturning circulation of the North Atlantic, but not enough to overcome the warming trends in Europe.

"We're putting so many things in the atmosphere, and it is heating up so much, that anything that would cool Europe is overwhelmed by greenhouse warming. For the next 100 to 200 years, no model indicates a cooling of Europe."

AR4 will present a "snapshot of current knowledge in climate and climate change. An IPCC assessment is policy relevant, but not policy prescriptive," Meehl said.

The IPCC is an organization of 190 governments around the world interested in learning the latest about climate change. "It's not our job on the science side to tell them what to do," he said.


Climate is a weekly series examining the potential human impact on global climate change, by veteran environmental reporter Dan Whipple. E-mail:

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