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How close are we to catastrophic climate change?



As you may have noticed, scientists remain convinced that humans are altering the global climate with an excess of greenhouse gas emissions—soot, methane and the ever-present carbon dioxide we pump out from our lungs and coal-burning power plants. The question is: how bad is said climate change going to get?

After all, concentrations in the atmosphere are going up by roughly two parts per million per year—now hovering at roughly 387 ppm, and climbing. But it may be that the Earth's climate is resilient and can withstand a lot more CO2 and other greenhouse gases before flipping to some altered state. Or it may be that various positive feedbacks—light-reflecting white Arctic ice melting away in favor of warmth-absorbing, dark Arctic waters, et al.—underway have already doomed us to a much warmer planet.

In a bid to answer that question, a group of scientists decided to do what I do…interview the experts in a spirit of objectivity. Engineer M. Granger Morgan of Carnegie Mellon University, climate scientist Kirsten Zickfeld of the University of Victoria in British Columbia and physicist David Frame of the University of Oxford in England interviewed 14 "leading climate scientists" about three possible climate scenarios to ascertain what might happen depending on how much heat greenhouse gases end up adding. The experts ranged from Oxford physicist Myles Allen to climate scientist Tom Wigley, who retired from the National Center for Atmospheric Research in Boulder, Colo.—and is a repeat of a similar study conducted by Morgan in the mid-1990s.

The goal was to round up the most senior climate experts and gauge their opinions on what is most likely to happen under three scenarios: a high degree of warming, a moderate amount of warming and relatively little warming—as well as to judge when, if ever, the global climate might reach a "tipping point" into a completely altered state, one that might be less amenable to human civilization.

One main point from the result published in the June 28 Proceedings of the National Academy of Sciences: What was most uncertain to all 14 experts was clouds—specifically, whether clouds would exacerbate climate change by trapping more heat or ameliorate it by reflecting more sunlight. Regardless of the unknown effects of clouds, 13 of the 14 judged the odds better than even that if the extra heat trapped by greenhouse gases peaked and leveled off at seven watts per meter-squared by 2200—we would see an entirely new climate. In fact, nine of the experts judged the probability of such a "basic state change" in the atmosphere to be at least 90 percent, or more. That corresponds to a warming of as much as 12.5 degrees Celsius—a worst case scenario.

Fortunately, the human-induced extra heat at present hovers at about 1.2 watts per meter squared. That's about half what the greenhouse gases already in the atmosphere are trapping; we're gaining some counterbalancing cooling from other pollution, such as sulfur dioxide, and those mysterious clouds. All told, Earth's average temperature has warmed by 0.75 degree Celsius, so far, as a result of that extra trapped heat.

The worst news? The interviewed experts don't expect to be any more able to understand clouds and the other uncertainties by 2030—even if funding for such research were tripled in the next 20 years. That matches real world results, since the experts interviewed back in the 1990s were just as uncertain about clouds and the like as when re-interviewed in the 2000s. Or, as climatologist Stephen Schneider of Stanford University, one of the experts interviewed this time and last, told me last year: "We don't know much more than we did in 1975" about climate sensitivity.

Fortunately, as investor Vinod Khosla is fond of saying (about himself and others): "Experts are usually wrong" when it comes to forecasting the future. Let's hope he's right about that at least, in this case.

The views expressed are those of the author and are not necessarily those of Scientific American.

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