David Spratt writes:

Would you live in a building, cross a bridge, or trust a dam wall if there were a 10 percent chance of it collapsing? Or five percent? Or one percent? Of course not! In civil engineering, acceptable probabilities of failure generally range from one-in-10,000 to one-in-10-million.

So why, when it comes to climate action, are policies like carbon budgets accepted when they have success rates of just 50 to 66 percent? That’s hardly better than a coin toss.

Policy-relevant scientific publications, such as those produced by the Intergovernmental Panel on Climate Change, focus on the probabilities—the most likely outcomes. But, according to atmospheric physicist and climatologist Hans Joachim Schellnhuber, “calculating probabilities makes little sense in the most critical instances” because “when the issue is the survival of civilization is at stake, conventional means of analysis may become useless.”

Have scientists and policy makers given too much weight to middle-of-the-road probabilities, instead of plausible-worst possibilities? If so, it’s an appalling gamble with risk. Humanity could end up the loser.

Understanding risk. A key approach to climate “risk” is estimating how much damage will be caused by a climate event, and how likely that event is to occur. These two key factors combine to give the size of the risk in dollar terms by multiplying the probability (likelihood) of an event happening by the monetary damage (impact) caused should it happen. The higher those figures, the greater the risk. Policy makers faced with a number of choices should rationally pick the option with the lowest overall monetary risk.

In everyday life, events may have a relatively high probability—for example, light rainfall—but cause little damage; these are low risk events. But very heavy storms made more intense by climate warming can result in widespread flooding and loss of property and life, and are considered a high risk because the potential damage is so large, even though these storms happen rarely. It is the high-end possibilities that matter most. [Continue reading…]