Uncertainties in Climate Forecasts: Causes, Magnitudes and Policy Implications

author: Stephen Schneider, Department of Biology, Stanford University
published: March 28, 2013,   recorded: April 2007,   views: 2319

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As one of the lead researchers in the ongoing Intergovernmental Panel on Climate Change (IPCC), which informs the UN Framework Convention on Climate Change, Stephen Schneider has labored mightily to map out the appropriate purview for scientists in climate change discussions. He reminds policy makers that scientists can’t decree what constitutes a perilous increase in global temperatures. “It’s not a scientist’s judgment to decide what’s dangerous.” He must lay out precise terminology for these discussions, and provide the models on which the various (mostly dismal) forecasts are based.

At the heart of the global discussion lie the contentious issues of what kind of rise in temperature constitutes “dangerous anthropogenic interference,” (DAI, in U.N. shorthand) and over what kind of time frame. Schneider notes that there is no threshold below which we’re OK, and above which we’re not. “We’ve already passed the threshold when some species are driven to extinction.” Also, the “damages that will occur are highly differential: melting Arctic sea ice will probably save the fishing industry $50 million a year in having shorter routes, but it will wreck the culture of the Inuits, established over 5,000 years, or destroy the polar bear ecosystem. How do you weigh those in comparable monetary metrics?”

In successive passes over the last decade or so, Schneider and other scientists have attempted to provide increasingly sophisticated answers to these questions, not only running thousands of climate models but looking at the risk of drastic climate change as a function of alternative policy choices. If the world doubles the amount of carbon dioxide in the atmosphere over pre-industrial levels, say to 550 parts per million, what will temperatures be by 2100, and how will that impact different regions, economies, societies? The latest IPPC assessment shows that we can expect a temperature increase of anywhere from 1.1-6.4 degrees C. Decision makers, who are essentially risk managers, have difficulty accepting such an uncertain forecast, says Schneider, but the best that can be accomplished is to provide the likelihoods of different temperature ranges in varying scenarios.

One of the most useful devices Schneider has found for illustrating this way of thinking is MIT’s Greenhouse Gamble roulette wheel, which depicts possible temperature increases should the world decide to spend the trillions of dollars necessary to stabilize carbon emissions at 550 ppm -- or pursue business as usual. With a rational global policy in place, the most likely outcome is an increase of no more than 1.5 degrees C. With no policy, expect a 2- 2.5 degree increase. “Which world would you rather be on?” asks Schneider.

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