Rosenzweig’s group has calculated that today’s one-in-100-years New York flood would, in the 2080s, be considered a one-in-40-years or perhaps a one-in-four-years event. The order-of-magnitude difference is simply the result of variations between models. To calculate a more useful range of probabilities, Rosenzweig is currently combining global models with regional ones. By “nesting” models of smaller regional areas in the global grid boxes, she hopes to increase the resolution of climate-change predictions to 10 to 15 kilometers. In six-hour time increments, a global model introduces a fresh batch of climate variables into the regional models, which then make local calculations.
The project is ongoing; so far, efforts to validate such nested regional models against actual temperature measurements have shown the predictions to be off by 1 ºC or more, an unacceptable margin of error. Still, Rosenzweig expects that regional models will become more precise with further work. And as they do, one of their uses will be to better predict storm surges by accounting for changes in local wind patterns. “The large majority of climate impact studies have been done with the GCMs,” Rosenzweig says, referring to global climate models. “We are just now beginning to do more with the RCMs [regional climate models], and they are very much in research mode. Sea-level rise is the number one vulnerability, and we need better information for the agencies. It’s critical for their planning.”
To be sure, global sea-level projections are still a matter of debate: the IPCC pegged the 21st-century increase at between 18 and 38 centimeters under a scenario that assumed lower greenhouse-gas emissions and between 26 and 59 centimeters with higher emissions. This uncertainty makes perfect storm-surge predictions impossible. But the lack of information about local climate change remains a stumbling block that prevents New York City–and every other coastal area–from developing the detailed information it can act on. “You don’t always protect against the worst case, because you would bankrupt the city,” says Rohit Aggarwala, director of long-term planning and sustainability under New York’s mayor, Michael Bloomberg. “How urgent is it to invest in multibillion-dollar projects? Knowing that over the whole Atlantic seaboard there will be x sea-level change and x change in violent storms doesn’t necessarily help New York City make different decisions than Miami or Halifax.” On the other hand, he notes, if New York were to operate on incorrectly optimistic information and delay the most ambitious storm-surge barriers too long, the consequences could be disastrous.
New York City authorities have already gotten some specific warnings from Rosenzweig’s group, which made a study of how the city’s water-supply and sewage-treatment infrastructure could be affected by rising sea levels. For example, a pump station north of the city on the Hudson River–built to draw emergency fresh water during times of drought–will eventually require expensive new filtration systems as rising seas push salinated water to within range of the intake areas.