Rising hazard of storm-surge flooding.

The 2017 Atlantic hurricane season is one for the history books. It has blown a number of records out of the water. Harvey dumped more rain on the United States than any previous hurricane. Irma maintained the highest category 5 longer than any storm anywhere in the world. September 2017 has accumulated themost cyclone energy of any month on record in the Atlantic. Last, but not least, if early estimates of damages hold up, three of the five costliest storms in US history will have occurred this year: Harvey, Irma, and Maria (1–3). The other two are Katrina and Sandy, which flooded New Orleans in 2005 and New York in 2012 (Fig. 1), respectively. A new study in PNAS by Garner et al. (4) tackles a critical and highly topical question: How will coastal flood risk change in the future on a warming Earth? They approach this question in a case study for New York, but most coastal cities in the world will be facing similar issues in the coming decades and, indeed, centuries. Global warming affects the coastal flood hazard (by sea water, not rainfall) in two main ways. The first is through rising sea levels. The second is through changes in storm activity affecting the statistics of storm surges. How these factors combine depends strongly on local conditions. For New York City, Garner et al. (4) find that sea-level rise dominates a massive increase in flood risk, while changes in storm surge height are expected to be minimal. Let us first gather some basic facts about these two ingredients. Global sea levels are rising as a result of global warming. They have risen by ∼20 cm since the late 19th century, and the rise is accelerating in response to warming. Since satellite records began in 1993 the rate of rise is ∼3 cm per decade and is also accelerating (5). The main reasons are that the ocean waters expand as they heat up, and continental ice melts and adds water to the oceans. The latter increase in oceanmass was responsible for roughly half the global sea-level rise at the beginning of the satellite record but has risen since to about 70%, mainly due to an acceleration of mass loss from the Greenland Ice Sheet (5). The two remaining large ice sheets on Earth are also the main cause of concern for future longterm sea-level rise: Greenland holds enough ice to raise global sea levels by 7 m and Antarctica by 58 m. Both ice sheets are subject to possible instability when critical thresholds are crossed (6). For Greenland this is due to the positive ice-elevation feedback and for Antarctica it is due to the marine ice sheet instability mechanism. The slow mass loss from these ice sheets will continue for thousands of years in a warmer world and reach at least 2 m per degree of global warming (7, 8). We should remember that at the end of the last Ice Age, between 15,000 and 5,000 y before the present, two-thirds of the glacial ice sheets were lost in response to ∼5 °C of global warming, resulting in 120 m of global sea-level rise. When it comes to the local sea-level rise, which is responsible for changes in local flood risk, additional climate factors are at play: changing ocean currents, the gravity fingerprint of shrinking ice sheets, and changes in prevailing winds. Factors unrelated to modern climate change can also be important on some coasts, like vertical land motions due to plate Fig. 1. Map of New York City flooding resulting from hurricane Sandy, October 29, 2012. Dotted red lines show proposed future storm barriers. Image courtesy of Federal Emergency Management Agency, National Institute for Coastal & Harbor Infrastructure.