Climate change and the lessons of Hurricane Sandy Even as Sandy underwent its bizarre metamorphosis from hurricane to winter storm, the question arose in many inquiring minds (at least those not beholden to a solemn oath of climate-change denial): Was this historic storm a symptom of global warming? Climate science has two ready answers: Absolutely! And, of course not! On the one hand, a warming globe makes megastorms more probable, while on the other, it is impossible to pin a global warming sticker on Sandy because the circumstances that turned it into a monster could have been mere coincidence. There is, however, another way of looking at Sandy that might resolve this debate, and also help frame what we really should be worried about when it comes to global warming: An infrastructure created to defend against historical measures of worst-case natural threats was completely overpowered by this storm. New York City's defenses were inadequate, and coastal defenses failed over a swath of hundreds of miles. Around the nation, such mismatches have been repeated ever more frequently in recent years. This summer, barge owners discovered that dredging in the Mississippi River, predicated on the history of the river's ups and downs, left it too shallow for commercial traffic because of the intense Midwestern drought. And, famously, levees in New Orleans that were largely through the process of being improved even as Hurricane Katrina struck in 2005 were still breached in 50 places. Then, seven years to the day after Katrina struck, Plaquemines Parish was drowned by Hurricane Isaac in flooding residents described as worse than Katrina's. It's true that factors other than megastorms - loss of flood plains, subsidence and neglect - can exacerbate a failure, but the number of failures of all types of defenses has been stunning. PHOTO ESSAY: Seven states, seven warning signs of global warming Such failures are telling us that something new is afoot. Our levees, dredging protocols and, in New York City, subway tunnel designs and improvements incorporate society's best guess of what it takes to protect against the worst nature might throw at us. Such defenses are expensive, so a city or agency won't spend more than it deems necessary. But the consequences of underestimating are also so enormous - consider the billions that will be spent restoring Manhattan's infrastructure and ruined neighborhoods alone - that we routinely construct them to withstand 100- or even 500-year events, estimates based on probability calculations and history of rare, extreme disasters. Yet these days such events seem to occur annually. This is borne out by statistics. Among the many records set by Sandy, one was for the highest wave ever recorded in New York Harbor: 32.5 feet. That eclipsed the previous record wave of 26 feet. When was the earlier record set? Just last year, courtesy of Hurricane Irene. Another message from Sandy is the reminder that climate change is camouflaged. It arrives as familiar weather events and after slowly accumulating changes. Sandy was unusual in many ways, but it is also easy to dismiss its significance because it started out as a hurricane and hurricanes have always marched up the Atlantic coast, even as late as November. As for the surge that inundated beach towns and city streets, it came on top of a sea level that has been rising slowly, on average less than one-tenth of an inch per year, though the pace has been accelerating in recent decades. The oceans are now roughly 9 inches higher than they were 140 years ago, and, for the most part, our sea defenses have not kept pace. Perhaps the most important message from Sandy is that it underscores the enormous price of underestimating the threat of climate change. Damage increases exponentially even if preparations are only slightly wrong. In trying to protect Grand Forks, N.D., from a spring flood in 1997, the city used sandbags to defend against a high-water mark of 52 feet, comfortably above the 49-foot crest predicted by the National Weather Service but, unfortunately, below the 54-foot crest that occurred on April 21. It was only 10% higher than what was expected, but the damage was many hundred times greater than if the protections had not been breached; 50,000 homes suffered damage. At some point the consensus among climate scientists might convince even those now in denial that they ignore the role of global warming in extreme weather events at the nation's peril. In the meantime, Sandy's trampling of the Northeast's defenses against the weather, as well as scores of other major infrastructure failures in the face of extreme floods, heat, drought and winds in the United States and around the world, tell us that climate change is already here. Eugene Linden is the author of "Winds of Change: Climate, Weather and the Destruction of Civilizations," which won the 2007 Grantham Prize Special Award of Merit.