The Haulover Marine Center parking lot is flooded Nov. 14, 2016, in North Miami. According to a new report, a little more than a foot of sea-level rise could case a 25-fold increase in the frequency of damaging floods in coastal U.S. cities. (Joe Raedle/Getty Images)

A new report, released by the National Oceanic and Atmospheric Administration on the last day of Barack Obama’s presidency, presents a series of updated estimates for future sea-level rise, both in the United States and worldwide. It suggests that, under extreme future climate change, global sea levels could rise by more than eight feet by the end of the century — one of the highest estimates yet to be presented in a federal report... 

The report also contains a series of regional estimates, suggesting that many parts of the United States will experience sea-level rise at a rate well above the global average. And with little more than a foot of sea-level rise, many coastal cities could see a 25-fold increase in the frequency of damaging floods. How soon this could happen will depend on the severity of future global warming.  

“When we apply these scenarios, it’s giving communities a better sense of what the future might hold with continued sea-level rise so they can plan accordingly and have better insights and make smart decisions about how they want to plan for the future,” said William Sweet, a NOAA oceanographer and one of the report’s authors.

Prior public reports have focused on only global sea-level-rise estimates, Sweet noted. The new report aimed to both update these global estimates and provide regional assessments as well, so local governments can have the best available information when making decisions about how to protect coastal communities.  

A 2012 NOAA report on global sea-level rise included four possible climate scenarios, each involving different degrees of ocean warming and glacier melting around the world: one scenario with high sea-level rise, one with low sea-level rise and two intermediate scenarios. The report suggested that the most extreme scenario could result in 6.6 feet of global mean sea-level rise by the year 2100. At a minimum, it suggested 0.7 feet of sea-level rise by that time.

The new report includes six possible climate scenarios, and it updates both the highest and lowest sea-level-rise estimates. It suggests that in the most extreme scenario, global mean sea levels could rise 8.2 feet by the year 2100. And in the lowest scenario, sea levels may rise by about one foot by the end of the century.

The authors decided on these updates by consulting a number of recent published studies on the subject. The increase in the upper extreme estimate from 6.6 feet to 8.2 feet was partly based on an improved understanding of the physical processes affecting the world’s major ice sheets, Sweet noted.

He pointed to several recent studies modeling the response of the Antarctic ice sheet to climate change. Such studies have helped shed new light on the increasing instability of the ice sheets in areas like West Antarctica and the conditions that could lead them to collapse in the future. The updated research is “suggestive that these higher [sea-level rise] outcomes are more probable than we once thought they were,” Sweet said.  

It’s important to note that the highest estimate represents a worst-case scenario, which has a low probability of occurring even under a business-as-usual climate trajectory. The estimates are intended to help policymakers make decisions about how to protect communities in the unlikely event that such a scenario should occur in the future.  

The new report also includes a series of regional projections, which the authors developed using a model informed by their global sea-level-rise estimates. The model was able to account for a wide variety of processes known to affect regional sea levels, said Robert Kopp, an associate professor of earth and planetary sciences at Rutgers University and another of the report’s authors.

In many places, the motion of the land plays a big role in rising water levels. In parts of Louisiana, for instance, the ground is actually sinking — this is a process called “subsidence,” which can be exacerbated by a variety of human activities, including the extraction of groundwater. Changes in ocean currents over time can also affect the way water is distributed around the world. And recent research has suggested that melting glaciers can also affect the Earth’s gravitational field — and even its rotation — in ways that can influence the distribution of water around the globe.  

Because gravity is proportional to mass — in other words, objects with greater mass exert a greater gravitational pull — large amounts of ice loss in places like Greenland or Antarctica can result in a decreased gravitational pull between the land mass and the ocean. When this happens, ocean water moves away to other parts of the globe. The result is that “when you melt ice sheets, they cause greater-than-average sea-level rise far away from the ice sheet,” Kopp said.  

The takeaway from all these physical processes is that sea levels rise at different rates in different parts of the world. And knowing what to expect in a specific location is key for policymakers trying to make decisions about how to plan for their own communities.

Along much of the East Coast and the Gulf of Mexico, for instance, sea-level rise could be greater than the global average in just about all possible climate scenarios. And under the higher scenarios, the same will be true for nearly all U.S. coasts except for Alaska.  

The authors also investigated the future of flooding along U.S. coastlines. Specifically, they focused on five-year flood events — these are damaging floods that occur at a frequency of about once every five years in any location. In other words, these are events that have about a 20 percent chance of occurring in any given year. The authors decided to investigate what it would take to increase the frequency of these events 25-fold — from once every five years to five times every year.

An analysis of 90 U.S. cities suggested that such an increase in damaging floods could occur by 2030 in most locations under an intermediate-high sea-level rise scenario and by 2080 under a low scenario. In general, the report suggests it would take just shy of 14 inches of sea-level rise for this to happen in any given location.  

Overall, the report’s findings will be used to help inform the fourth installment of the National Climate Assessment, a federal report released every four years by the U.S. Global Change Research Program. The next assessment is due out in 2018. In the meantime, the authors suggest that the findings could be used by local governments in efforts to plan for future climate change.  

“As our understanding based upon the observations and the models improves, we want to make sure that the types of guidance that communities receive is most current as possible,” Sweet said. “I think it’s important for people to know that sea level’s not rising like it would in a bathtub. At NOAA, we want to supply this environmental intelligence so folks really have this ability to be proactive in their decision-making with the best data at their disposal.”

By Chelsea Harvey

January 23, 2017

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