Rising to the challenge of surging seas

W aking up to the sound of crashing waves, ocean views and sandy beaches remains, for many, the dream. However, according to a series of recent reports and studies on rising sea levels, sleepless nights could be in store for beachfront real-estate owners and residents of coastal cities the world over. Earlier this year, the United States’ National Oceanic and Atmospheric Administration (NOAA) released a technical report on sea-level rise that included a new extreme scenario. The revised upper estimate warns that US coastlines, currently populated by 12 million residents, could experience a sealevel increase of 2.5m by the year 2100 if no mitigating action is taken1. The extreme projection in part reflects new research on the stability of the Antarctic ice sheet, which, while uncertain, indicates that the southern ice cap could alone contribute as much as 1m to global sea levels by the end of the century2. While this scenario remains highly unlikely, it is not the only study to markedly increase end-of-the-century sea-level rise estimates3,4. These revisions significantly increase projections provided by the last Intergovernmental Panel on Climate Change (IPCC) assessment report, in some cases more than doubling the upper bound5. With the next IPCC report not due until 2021, policy makers are commissioning their own reports in an effort to keep abreast of rapid scientific developments and the associated uncertainties6. The timing of decisive action could be crucial in ensuring that coastal defences are adequate. However, it remains difficult, and expensive, to prepare for the worst when future projections continue to change. Furthermore, the effects of rising sea levels are more than physical in nature and are likely to propagate far from the coast. Rapid economic growth, urban development and migration are expected to increase the number of people living in the low-elevation coastal zone to 1.4 billion by the year 2060 (ref. 7). With sea levels rising three times faster than they were 25 years ago8 and recent reports suggesting that an increase as small as 0.2m could double the risk of coastal storm-related flooding9, the potential impact is likely to become increasingly severe. Under an extreme sea-level rise scenario, one study forecasts that as many as 13.1 million US citizens could become climate change refugees, triggering a wave of mass migration away from the coastal zone, resulting in a heavy socioeconomic burden on the infrastructure of inland cities10. The cost of sea-level defence is, however, high. A 2014 assessment estimated that annual investments in the range of $12-71 billion would be required to protect against the threat11, while inaction could total $1 trillion per year by 2050 for 136 of the world’s largest coastal cities12. With population growth roughly twice the US average and a ribbon of expensive real-estate occupying its low-lying coast, Florida—the state for which NOAA’s new extreme scenario is most worrying—is well aware of the multifaceted nature of the sea-level rise problem. Submerged highways, flooded basements and backedup sewage systems are an increasingly common occurrence for residents of the OPEN