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Climate change a 'questionable truth'

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“Nobody can really tell you what the probabilities are,” says Carl Wunsch, a leading climate and oceans expert at the Massachusetts Institute of Technology. “The probability of another metre of sea-level rise in the next 50 years isn't zero, but it isn't 90 per cent, either. And if you pinned me down to tell you what it really is, I couldn't do that.”

Robert Mendelsohn is an environmental scientist at Yale who specializes in modelling the regional impacts of climate change. “The drought issues are one of the great uncertainties,” he says. “We know precipitation will increase, but we don't know exactly where.”

The very great uncertainty of long-term climate impacts is a point that often gets lost in the debate. The scenarios range from mild to severe, but it's the extreme ones that get the ink.

On top of that, many scientists say the average global surface temperature (which is the most popular way to talk about global warming) doesn't tell us very much at all about what's going to happen in any given region of the globe. “It's almost useless for what people care about, which is their growing season and how they live,” says Roger Pielke Sr., professor of climatology at Colorado State University and the state's official climatologist.

Dr. Pielke is steeped in decades of climate research. And he points out that carbon-dioxide emissions are just one of many man-made impacts on the climate. Land-use changes are another. “There are a lot of things that humans are doing to the climate beyond CO{-2}, and we don't understand them,” he says.

We also don't know enough to say with any degree of certainty whether taking action X will produce result Y. “Say you spend a trillion dollars to limit CO{-2} emissions. Will you be able to limit the sea-level rise, or droughts?” Prof. Wunsch says. “People are asking questions of the science that science can't answer.” (For the record, he believes that rising seas could cause “an awful lot of damage to an awful lot of people.”)

Science can, and must, inform policy decisions. But science by itself can't tell us what to do. What to do is a matter of great (and understandable) dispute, and making policy to deal with climate change is fraught with all kinds of real-world complications. Figuring out smart policies to adopt is hard, and implementing them is even harder. It's all very well to sign on to the Kyoto Protocol. But if nobody complies, then what's the point?

All this doesn't mean we should throw up our hands and give up. The challenge is to figure out what actions are feasible and give us a reasonable insurance policy against disaster.

At this point, most people say: Well, obviously we've got to start cutting greenhouse-gas emissions as fast as possible. Surely that's the way to make a difference.

This is one of the most common misunderstandings in the global-warming debate. To explain why, I called on Roger Pielke Jr., a leading climate-policy expert (and, coincidentally, Roger Sr.'s son.)

He supplied this analogy: Imagine that you are filling up a bathtub. Right now, it is 380 inches deep. You are filling up the tub at a rate that adds seven inches every year. The drain at the bottom lets out the equivalent of two inches a year.

Under these conditions, the bathtub will keep filling up at a rate of five inches a year. If we cut the rate of filling by, say, 20 per cent, to 5.6 inches per year, the bathtub will keep filling up by 3.6 inches a year. Cuts in the rate of filling do not mean that the level in the tub will go down. In fact, if you cut the rate of filling to zero, it will still take quite a while for the tub to drain down to, say, 280 inches.

Now, substitute CO{-2} concentrations for tub depth and emissions for water, and you have a rough analogy to the climate system. Carbon cuts will have an impact — but not for many, many years, because they represent only a tiny fraction of the total CO{-2} that's already in the atmosphere.

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