Sea-level rise is causing extremely high water levels to occur more frequently, which is associated with increased risk of flooding. As the Earth continues to warm from climate change, it can be difficult to determine exactly when sea levels will rise high enough to warrant upgrades in coastal protection, in part because levels of protection from floods vary globally. While sea-level rise and flooding increasingly threaten coastal areas around the world, it becomes crucial to implement measures for coastal adaptation in a timely manner. But how long do we have to adapt our coasts to withstand such increases in flooding? The answer is not always clear given the differences between coastal communities.

A new study by a team of international researchers from Princeton University, Utrecht University, Deltares, NIOZ, and others devised a new method to calculate when we can expect a certain increase in flood probability in a specific area. The calculations show that within 30 years, the estimated probability of flooding will be 10 times higher in more than a quarter of the nearly 500 locations studied. The increase is most rapid in central America, southern Europe, South Africa, and parts of Asia and Australia. 

To counteract the increased risk of flooding associated with climate change and sea-level rise, coastal defenses need to be implemented and upgraded in a timely manner. Planned relocation -- the process of orderly retreat from the coast -- is already becoming necessary in some areas. However, the planning and implementation process for coastal adaptation takes time. For instance, it took 45 years following the North Sea Flood of 1953 to realize the Delta Works in the Netherlands despite already-existing plans for coastal mitigation. Completion of the Thames Barrier protecting London took 29 years. The researchers’ method of calculating flood probability can provide policymakers with an estimate of how much time is left to put coastal protection measures in place in their communities. This critical information can inform policymakers in taking necessary action to reduce the impact of coastal flooding.

“Our study shows that sea-level rise is shortening the runway for delivering coastal adaptation projects,” said D.J. Rasmussen, coauthor of the study and a former postdoctoral researcher at Princeton’s Center for Policy Research on Energy and the Environment. “This is not good news given how long it has taken solutions to get implemented in the past,” Rasmussen said.

If countries continue to emit high levels of greenhouse gasses, the risks to coastal communities will accelerate, necessitating measures for coastal adaptation. Consequently, the time left to adapt will become shorter and shorter. That’s why the researchers stress the importance of making flexible adaptation plans and detecting when additional measures may need to be implemented faster. 

“As disastrous events stack closer together in space and time, some countries could get to the point of never recovering fully, never catching up, spending their time fixing damage from the last event instead of adding protection against the next one,” said Michael Oppenheimer, coauthor and director of the Center for Policy Research on Energy and the Environment at the Princeton School of Public and International Affairs.

The timeline by which coastal protection measures ought to be taken depends on how much of an increase in the probability of flooding policymakers find unacceptable. By estimating the probability of flooding and the timelines by which these probabilities increase, the researchers offer policymakers the opportunity to make informed decisions during the implementation of coastal protection. This research can help protect coastal communities in the face of accelerating climate change and sea-level rise.

The paper, “The Timing of Decreasing Coastal Flood Protection Due to Sea-Level Rise,” was published on March 23, 2023 in Nature Climate Change. The authors are Tim Hermans of NIOZ, Delft University of Technology, and Utrecht University; Victor Malagón-Santos of NIOZ; Caroline Katsman of Delft University of Technology; Robert Jane of the University of Central Florida; D.J. Rasmussen and Michael Oppenheimer of Princeton University; Marjolijin Haasnoot of Delft University of Technology and Utrecht University; Gregory Garner of GRO Intelligence; Robert Kopp of Rutgers University; and Aimée Slangen of NIOZ. This study was supported by the European Union’s Horizon 2020 research and innovation program, and by the National Science Foundation as part of the Megalopolitan Coastal Transformation Hub (MACH) under NSF award ICER-2103754. The opinions, findings, and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This story was adapted by C-PREE from a press release prepared by Utrecht University, together with NIOZ and Deltares.