FOR THE MELTING ARCTIC, FUZZY FORECASTS ARE THE BEST WE CAN DO...

Climate warming makes an ice-free summer Arctic Ocean all but inevitable. But some groups are risking their credibility to predict when this will happen with far more accuracy than is possible given current science. Photo by imageBROKER/Alamy Stock Photo

 

People keep claiming to know when the Arctic will become ice-free. They don’t...

One of the most symbolic shifts in the modern story of climate change will come sometime in the next century, when the summertime Arctic Ocean, illuminated by the midnight sun, becomes completely free of sea ice. But when exactly that will happen is anybody’s guess.

According to recent research, it’s impossible, given current knowledge, to predict when the Arctic will first become ice-free; any estimate is only accurate to a 20-year window. But that hasn’t stopped a range of people and organizations, from activist groups to scientists, from making much more specific—and dire—predictions.

“Some people want to know whether they are going to have a white Christmas this year. We don’t expect that now, in September, anyone is going to be able to tell you whether you will have a white Christmas in Washington, DC,” says Alexandra Jahn, an atmospheric and oceanic scientist at the University of Colorado, and lead author of the new study. “It’s similar with the climate. We can predict that sea ice is going to decline as long as we continue emitting carbon dioxide into the atmosphere. But when it will be gone—that is like trying to predict a white Christmas, now.”

Yet people keep doing—or trying to do—just that. In 2008, for instance, Canadian researchers forecast the Arctic would be ice-free by 2015. Then, in June 2015, University of Cambridge physicist Peter Wadhams submitted a forecast that the Arctic would be effectively ice-free by that September. It wasn’t. In April of this year, scientists at a geoscience conference in Vienna, Austria, predicted the Arctic’s summer sea ice would shrink below the record low observed in 2012. But it was a colder-than-usual summer, so that didn’t happen, either. In August, Wadhams told the Guardian the Arctic will likely be ice-free as early as next summer.

Earth’s climate is a chaotic, complex system in which small perturbations can result in big changes. Scientists’ best estimates of how the climate will evolve come from complex computer models, which use sophisticated equations to represent a wide range of climate phenomena, and are run on supercomputers. But computer simulations aren’t able to account for all possible scenarios, especially far into the future, says Jahn.

To overcome models’ individual quirks, they are occasionally combined into a single, multiple-model analysis. One such analysis is the recent Coupled Model Intercomparison Project Phase 5. That analysis calculated that the Arctic will be ice-free within a 100-year window.

Previously, researchers have used statistical methods to narrow that range to a few decades. Jahn says her study is the first to ask how much narrower that projection can realistically get. She came up with 20 years, based on the variability built into the models, and into the real climate. When the Arctic is almost ice-free, a certain date will be easier to predict, the way a Christmas snow forecast is more accurate on December 22 than it is on September 10, she says.

“It’s not a failure of the model; it’s an inherent feature of the system,” Jahn says. “Twenty years is the lower bound of this prediction uncertainty, so we can kind of stop trying to predict the actual year.”

Scientists risk losing credibility when they make predictions that turn out to be wrong, according to several climate scientists interviewed for this article. People notice bold claims, and they notice when those claims don’t pan out, says Dirk Notz, a climate researcher at the Max Planck Institute for Meteorology.

“As a climate scientist, I think the only thing we have that we can give to the general public is our credibility of being independent observers of what the future of our Earth will look like,” says Notz. “If scientists make claims that everybody will easily see to be wrong, or exaggerating, this will undermine our credibility and make it harder to get across the science we do have.”

Ultimately, however, the onset of an ice-free summer Arctic is more of a social tipping point than a scientific one. Ships will be able to traverse the Arctic, and polar bears will lose their habitat, long before the ice fully fades. The fascination with that milestone may lie in the way humans experience climate change: short-term trends are much more obvious, and much more relatable, than the changes taking place across decades. 

“The Arctic has become this special place where we are seeing climate change happen, and people are kind of fascinated by that,” Jahn says. “In our lifetimes, we are going to see Arctic sea ice go away.”

Published September 15, 2016
original story HERE
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David Pike, Editor