That means, because of rising temperatures, an extreme sea level event that up until now has been expected to occur once every 100 years is expected to occur every year, or even more frequently, by the end of this century. The study appeared today in the journal Nature Climate Change. While the researchers say there is uncertainty—as always—about future climate, the most likely path is that these increased instances of extreme sea levels will occur even with a global temperature increase of 1.5 or 2 degrees Celsius compared to pre-industrial temperatures. Scientists consider these temperatures the lower end of possible global warming. And the changes are likely to come sooner than the end of the century: at many locations, projections indicate that the frequency of extreme sea level events could increase 100-fold as early as 2070.

Mapping effects, all over the world

Professor Roshanka Ranasinghe of Deltares and IHE Delft, the Netherlands, co-designed this world-first study together with lead author Dr. Claudia Tebaldi of the US Department of Energy’s Pacific Northwest National Laboratory. The study brought together an international team of researchers from the USA, the Netherlands, Italy and Australia who have led previous large studies of extreme sea levels and the effects of global warming on sea level rise. The team pooled their data and introduced a novel synthesis method, treating the alternative estimates as expert voters, to map out likely effects of temperature increases ranging from 1.5 to 5 degrees C compared to pre-industrial times, a temperature range unprecedented in previous studies.

The scientists found, not unexpectedly, that the effects of rising seas on extreme sea level event frequency would be felt most acutely in the tropics and generally at lower latitudes. Locations likely to be affected most include the Southern Hemisphere, areas along the Mediterranean Sea and the Arabian Peninsula, the southern half of North America’s Pacific Coast, and areas including Hawaii, the Caribbean, the Philippines and Indonesia.

Regions that will be less affected include the higher latitudes, the northern Pacific Coast of North America, and the Pacific Coast of Asia.

“The study presents a more complete picture than before by combining several data sets and looking at a wide range of warming levels in very fine spatial detail,” said Prof. Ranasinghe “Our results show that every fraction of additional warming significantly speeds up the transformation of once-per-century extreme sea level events to once-per-year events. For example, with a trajectory that reaches 1.5°C by 2100, this 100-fold increase in frequency is projected to happen between 2070 and 2080 at most locations in the tropics and sub-tropics; for warmer trajectories reaching up to 3.0°C by 2100, this 100-fold increase will occur one decade earlier, between 2060 and 2070. The highest global warming levels considered in our study—4.0°C and 5.0°C—projects that most of these locations will experience a 100-fold increase in frequency even earlier, between 2050 and 2060.”

IPCC report

The new study mirrors the assertion of the recent Intergovernmental Panel on Climate Change report, which stated that extreme sea level events would become much more frequent and widespread worldwide by the end of the century due to global warming.

The best- and worst-case scenarios put forth by the study vary due to uncertainties that the study authors represented in remarkable detail. In one scenario, at the pessimistic end, 99 percent of locations studied will experience a 100-fold increase in extreme events by 2100 at 1.5 C of warming. In another, at the optimistic end, about 70 percent of locations don’t see much of a change even with a temperature increase of 5 degrees C.

The authors call for more study to understand more precisely how the changes will affect particular communities. They point out that the physical changes that their study describes will have varying impacts at local scales, depending on several factors, including local geomorphological features, how vulnerable the site is to rising waters and how prepared a community is for change.

In addition to Dr. Tebaldi and Prof. Ranasinghe, other authors of the paper include Michalis Vousdoukas of the European Joint Research Centre in Italy; D.J. Rasmussen of Princeton University; Ben Vega-Westhoff and Ryan Sriver of the University of Illinois at Urbana-Champaign; Ebru Kirezci of the University of Melbourne in Australia; Robert E. Kopp of Rutgers University; and Lorenzo Mentaschi of the University of Bologna in Italy.

The study was funded by the U.S. Environmental Protection Agency and DOE’s Office of Science.

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