The measurement, called climate sensitivity, indicates how surface temperatures respond depending on how much greenhouse gas is pumped into the atmosphere by humans. The usual measurement baseline is a doubling of CO2. In other words, scientists ask the question: If we double carbon dioxide in the atmosphere from pre-industrial levels, how much will the global average temperature warm?

It is not exactly the same thing as predicting the total amount of expected warming, because climate sensitivity is expressed as a range. In technical terms, it is an equilibrium response to an atmospheric forcing, all other things being equal. In the real world, all other things are never equal, but with climate computer model s, you can run these kinds of experiments.

For policymakers, establishing sensitivity is important for setting future targets for CO2 emissions. If you know approximately how much you heat up the globe with each ton of CO2 you emit, you can set a target for future temperatures and then restrict CO2 emissions to meet that target. Equally obvious, the inferences are very rough, but you have to start somewhere.

Refining these estimates of climate sensitivity has become something of a cottage industry. For example, the Intergovernmental Panel on Climate Change's third assessment report, published in 2001, concluded climate sensitivity was between 2.7 degrees and 8.1 degrees Fahrenheit (1.5 degrees and 4.5 degrees Celsius).

Therefore, if you double CO2 in the atmosphere and leave everything else alone, the global temperature will increase somewhere within that range.

Recent and albeit preliminary work by Gerald Meehl of the Na tional Center for Atmospheric Research - conducted for the next IPCC report, which is due in 2007, and based on improved coupled ocean-atmosphere models - narrows the range somewhat, to between 3.6 and 7.9 degrees F (2 degrees and 4.4 degrees C), with a median expectation of 5.6 degrees F (3.1 degrees C).

These are fairly large numbers - the European Union's official policy is to hold warming to 3.6 degrees F (2 degrees C) above pre-industrial temperatures - but they are nothing compared to the recent results from an Oxford University study. Its results agree with the lower sensitivity estimates of about 3.6 degrees F, but finds a scorching 19.8 degrees F (11 degrees C) on the upper end.

The study, by Oxford physicist David Stainforth and 15 others, ran simulations on the idling time of thousand of individual computers, using their climateprediction.net Web site much like the SETI@Home project used computer downtime to search - so far unsuccessfully - for intelligent life elsewhere in the galaxy.

In their paper, published in the Jan. 27 issue of the British journal Nature, the authors wrote, "The range of sensitivities across different versions of the same model is more than twice that found in the (global climate models) used in the IPCC Third Assessment Report."

Stainforth and colleagues suggest, furthermore, that there is no reason to reject even the very high-end or very low-end sensitivities as possible outcomes. They do not assign probabilities to any of these outcomes, though most scientist who look at sensitivity think the extreme highs, at least, are unlikely.

Thomas Crowley, a professor of earth and ocean sciences at Duke University, told UPI's Climate: "It is really a reporting of the behavior of ocean-atmosphere models. I'm not disputing their results. There is some marginal probability of that happening. Where my skepticism comes in is that if you were looking at other types of information, comparing against actual observations, you could constrain the model a little better."

Crowley has looked at paleoclimate data, for instance, and said his preliminary unpublished research indicates it might be possible to narrow these limits from other sources. The problem is the data are "noisy," he said, so it may not be possible to reject the extremes entirely.

"It's a perfectly legitimate study to do," he added. "Everybody likes doing this; 'What do you think it really is?' My opinion is that it is closer to 2 or 3 degrees for a doubling of CO2."

Meehl concurred. "You can't get much lower than 1 degree C for a doubling of CO2," he told Climate. "On the other extreme, there is a long tail, and less probability. In a probabilistic view of climate sensitivity, these high-end values are still there, but a very low probability."

Meehl said he would l ike to discuss these issues in terms of probability, because people have gotten used to things discussed that way, such as the probability of rain on the television weather report, or the chances of picking up an inside straight in the World Series of Poker.

"We are trying to move to this probability mentality," he said, "trying to get away from just a range. The median value may behave more like a probability."

Now, more of the discussion centers on targets, he said. "What if you wanted to target some temperature increase you thought was dangerous to the climate system. Then you could have an emissions target, and the ultimate equilibrium does become more relevant."

The steering committee for a meeting titled "Avoiding Dangerous Climate Change" at Exeter in England last week addressed exactly this issue. The committee wrote: "Limiting climate change to 2 degrees C (3.6 degrees F) implies stabilizing the atmospheric conce ntration of all greenhouse gases. The CO2 concentration must not exceed 500 (parts per million by volume), if the climate sensitivity is 2.5 degrees C (4.5 degrees F). Global emissions would need to peak in 2020 and decline to 3.1 (gigatons of carbon per year) by 2095."

The committee also recommended major investment now "in both mitigation and adaptation. The first is essential to minimize future impacts and the latter is essential to cope with impacts (that) cannot be avoided in the near-to-medium term."

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

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