Over the last few decades, a warming Atlantic Ocean has produced a number of very powerful hurricanes, some of which retained strength much further north than usual. Fortunately for the US, however, few of them made landfall on the continental US, leading to talk of a "hurricane hiatus" that came to a decisive close this year.

A new analysis of New York City's hurricane risk suggests that a similar fate might be in store for the city. While warming waters will produce more powerful storms in the future, climate models suggest that they'll generally track further offshore of the city, a combination that ends up cancelling itself out. Unfortunately, due to rising oceans, the risk of flooding will keep going up, with what was once a 500-year flood occurring every 25 years by mid-century.

Out to sea

As with many studies, this one started with two emissions pathways defined by the Intergovernmental Panel on Climate Change. One of these is RCP 4.5, in which the rise in atmospheric carbon dioxide finally stops toward the end of this century. The second is RCP 8.5, which represents a "business as usual" approach to emissions, bringing a continued rise in atmospheric CO 2 . These were used to drive three climate models that tracked two different periods: the remainder of this century, and a period extending from the present to the year 2300. Approximately 12,000 storms were tracked per century under these models. 1970-2005 was used as a baseline comparison period.

The first analysis focused on storm surges that would strike New York City, as measured by a tide gauge at the southern tip of Manhattan. For context, Hurricane Sandy caused a storm surge of 2.8m above the expected tide at this site.

The analysis indicates that storms will get bigger in the future, with an increase in the area that experiences the highest winds. The maximum winds speed also goes up, while the low pressure at the eye of the storm drops. All of these factors should increase storm surges. Yet in the absence of sea level rise, the southern tip of Manhattan doesn't see much in the way of trends in storm surge events. What's happening?

To find out, the authors divided up the region into a grid and tracked which grid squares storms entered. The analysis showed that there's a drop in storm frequency more or less centered on top of New York City. A corresponding increase is present well out to sea. This doesn't mean New York doesn't see any storms; instead, it sees a slight decrease in frequency, which more or less offsets the increase in storm intensity.

Relatively good news for New York City, but perhaps not such great news for elsewhere. The authors note, "Stronger storms with shifted tracks could lead to more direct or severe tropical cyclone impacts in other coastal regions, such as New England or northwestern Europe—an issue that merits further study."

Rising tides

But all of that is keeping sea levels right where they are currently. In reality, sea levels have been rising at an accelerating pace over the course of this century. To handle that increase going forward, the researchers used a probabilistic model that took into account ice sheet and glacier losses, expansion due to ocean warming, and storage of water in reservoirs.

This model produced some very sobering numbers under the business-as-usual emissions scenario: roughly a meter of sea level rise by the end of the century (with uncertainties from 0.5 to 1.4 meters) and 3.2 meters by 2300 (1.5 up to 5.7 meters). And that's assuming Antarctic ice sheets stay stable throughout this time—something that's not guaranteed. If these ice sheets become destabilized, the upper bound on sea level rise goes up to 15.7 meters.

All of which makes flooding dramatically worse. By the end of this century, a 500-year flood goes from about Sandy-sized up to four to five meters above preindustrial sea levels under business-as-usual emissions. By 2300, they max out at over 15 meters.

Put differently, before industrial times, a 500-year flood was about 2.25 meters above mean sea level. Thanks to sea level rise, we can expect similar water levels within 25 years in the present. By the middle of this century under any emissions scenario, we'll be seeing those water levels every five years. In other words, oceans will rise to the point where a relatively minor storm could cause flooding that would once have been considered unprecedented.

All of which is to say that New York City is in for a hard time of it even if, as projected, more hurricanes pass a safe distance offshore. And while a lot of work has been put into making the city's infrastructure more robust in the case of flooding, there's been little talk of how to deal with the sorts of ocean levels we're likely to be facing by the end of this century.

PNAS, 2017. DOI: 10.1073/pnas.1703568114 (About DOIs).