Life-sustaining. Beautiful. Destructive. Ocean waves are all of those—they transport nutrients that nourish marine life, but they also damage ships and batter coastlines, triggering erosion that can send homes and roads tumbling. Now, scientists have used climate models to show that some parts of the globe, including the Southern Ocean, are likely to experience larger waves by the end of the 21st century. That’s potentially bad news for coastal residents, the researchers suggest, because large waves could generate increased flooding and erosion.
Generated by Wind
Ocean waves are kicked up by the wind, and warmer conditions promote stronger winds. Larger waves might therefore be a hallmark of our planet’s future, Alberto Meucci, an oceanographer at the University of Melbourne in Australia, and his colleagues hypothesized. They set out to test that idea using climate models.
The researchers focused on a metric known as significant wave height. Defined as the average of the highest one third of waves, it’s a commonly used parameter in oceanography. (Waves more than twice the significant wave height are called rogue waves. These watery monsters, which can top 25 meters, were dismissed as sailors’ tall tales for years before they were finally recorded using modern instruments.)
Many Climate Models
Meucci and his collaborators relied on two very different greenhouse gas emission scenarios. One, Representative Concentration Pathway (RCP) 8.5 (the “business-as-usual” scenario), which assumes that carbon emissions will remain largely uncurbed, yields 8.5 watts per square meter of additional warming by the end of the 21st century (about 4.3°C). The other, RCP 4.5, assumes that some emissions mitigation policies are enacted and yields about 2.4°C of warming by 2100.
The researchers input both scenarios into seven different climate models. Meucci and his colleagues used multiple climate models to beat down statistical uncertainties. “The point of using seven global climate models instead of a single model was to be able to reduce the uncertainties connected with the representation of extremes,” said Meucci.
The novelty of this study is its use of multiple climate models, said Laure Zanna, a physical oceanographer at New York University not involved in the study. “If you want to be able to predict changes in extremes, having enough data to do so is important.”
Meucci and his collaborators fed the surface wind outputs of the climate models into wave generation models. The scientists determined the 1-in-100-year significant wave height—corresponding to a wave that has a 1% probability of occurring each year—for both the RCP 8.5 and RCP 4.5 scenarios for two time periods: 1979–2005 and 2081–2100.
The scientists found an increase in the 1-in-100-year significant wave height at the end of the 21st century in the Southern Ocean for both greenhouse gas emission scenarios. The largest difference, roughly 15%, was observed in the Southern Ocean for the RCP 8.5 scenario.
Larger waves might spell bad news for this region, which is already routinely battered by waves topping 20 meters, the team concluded. Large swells could very well roll up on the coasts of South Africa, South America, and Australia, said Meucci, where they might contribute to flooding and coastal erosion. Their impact could also be more far-reaching, he said. “Changes may be felt up to the North Pacific.”
But not all ocean basins will experience larger waves, the team found. In portions of the North Atlantic, wave height might even decrease by the end of the 21st century, Meucci and his collaborators noted.
The researchers next investigated changes in the frequency of extreme waves. Again, the Southern Ocean stood out. That basin was more likely to experience large waves at the end of the 21st century compared with 1979–2005, the team found.
These findings are believable, but it’s important to pinpoint what’s driving the changes in the Southern Ocean, said New York University’s Zanna. “An understanding of the physical mechanisms would help strengthen even further their results.”
These results were published in June in Science Advances.
In the future, Meucci and his colleagues plan to conduct similar analyses using a new ocean wave model with finer resolution. They’re also looking forward to using a revised suite of global climate models (Coupled Model Intercomparison Project (CMIP) 6) that includes new versions of greenhouse gas emission scenarios.
—Katherine Kornei (@KatherineKornei), Science Writer