My friends in the nuclear weapons community have, over the years, helped me understand the tools for thinking about low-probability, high-consequence events, like a warhead accidentally going off. You really don’t want that to happen, so even though averaged across all possible futures the average badness might be relatively low, it’s worth spending some time and energy thinking through ways of reducing the probability of the high-consequence event.
But if the chance of the truly horrific event is somewhere betwen 57 and 95 percent over the next four decades? Holy crap. If you’ve seen that coming and haven’t already started doing an awful lot to try to drive that number down, your system for solving societal problems is seriously fucked up.
57 to 95 percent is the estimated probability of the levees in California’s San Joaquin-Sacramento Delta failing between now and 2050, according to a new report that Rober Service talks about in yesterday’s Science magazine (subscription). Put another way, there’s at least a 57 percent chance that the system that delivers water to half of all Californians will stop delivering water to half of all Californians:
By 2050, the chance of widespread levee failures is as high as 95%, due to runoff from the northern Sierras, which is predicted to be more concentrated in the late winter and early spring, and the increasing risk of earthquake, according to a report last summer by the Public Policy Institute of California (PPIC). If that occurs, salt water from the San Francisco Bay would rush in to fill the voids, dramatically increasing the salinity of water in the delta, possibly making it undrinkable. Adding sea-level rise to the equation–as climate models predict–brings the date of levee failures closer. “It will happen,” says Ellen Hanek, a PPIC economist in San Francisco.
update: Of possible interest to those who follow discussions of journalism and science, when I first wrote this post, I took Service’s word for the 95 percent figure he used in his article without taking careful note of the “as high as” qualifier he used. When I reread the post and noticed the qualifier, I went and found the original PPIC report and found that they actually say this:
[T]he best possible failure probability under rapid rates of sea level rise, high seismic risk and increasing inflows is approximated by the island with least current combined risk of failure from flooding and earthquakes at 2 percent per year, for a cumulative probability of failure of 57 percent by 2050. For the high end of risk, we used the current estimated combined flood and earthquake annual failure probability of the western Delta islands of approximately 7 percent per year, or a cumulative probability of failure of 95 percent by 2050.
So 57 and 95 percent reasonably bracket the risk as estimated by PPIC, and don’t materially change the “holy crap” tone of my reaction. (And I haven’t read the whole report, just found the section that explained their risk numbers in brief.)