When judging risks, we often go wrong by using what’s called the availability heuristic: we gauge a danger according to how many examples of it are readily available in our minds. Thus we overestimate the odds of dying in a terrorist attack or a plane crash because we’ve seen such dramatic deaths so often on television; we underestimate the risks of dying from a stroke because we don’t have so many vivid images readily available.

Slow warming doesn’t make for memorable images on television or in people’s minds, so activists, journalists and scientists have looked to hurricanes, wild fires and starving polar bears instead. They have used these images to start an “availability cascade,” a term coined by Timur Kuran, professor of economics and political science at Duke University, and Cass R. Sunstein, a law professor at the University of Chicago.

The availability cascade is a self-perpetuating process: the more attention a danger gets, the more worried people become, leading to more news coverage and more fear. Once the images of Sept. 11 made terrorism seem a major threat, the press and the police lavished attention on potential new attacks and supposed plots. After Three Mile Island and “The China Syndrome,” minor malfunctions at nuclear power plants suddenly became newsworthy.

“Many people concerned about climate change,” Dr. Sunstein says, “want to create an availability cascade by fixing an incident in people’s minds. Hurricane Katrina is just an early example; there will be others. I don’t doubt that climate change is real and that it presents a serious threat, but there’s a danger that any ‘consensus’ on particular events or specific findings is, in part, a cascade.”

Image Credit... Amelia Bauer

Once a cascade is under way, it becomes tough to sort out risks because experts become reluctant to dispute the popular wisdom, and are ignored if they do. Now that the melting Arctic has become the symbol of global warming, there’s not much interest in hearing other explanations of why the ice is melting  or why the globe’s other pole isn’t melting, too.

Global warming has an impact on both polar regions, but they’re also strongly influenced by regional weather patterns and ocean currents. Two studies by NASA and university scientists last year concluded that much of the recent melting of Arctic sea ice was related to a cyclical change in ocean currents and winds, but those studies got relatively little attention  and were certainly no match for the images of struggling polar bears so popular with availability entrepreneurs.