Massive waves poured over English sea defenses during the cold and stormy winter of 2013-2014. These waves were the most powerful scientists have ever recorded on Europe's Atlantic coastline.
The storms that generated these waves may have been unusually intense, but they were part of a trend that is putting coastal communities at greater risk. In a study published this week in Geophysical Research Letters, scientists found that wave heights have risen everywhere along the west coast of Europe — from Scotland down to coastlines north of Portugal — over nearly the last 70 years.
Taller waves can wash over traditional defenses against storms, and flood communities that lie behind these sea walls.
Just a small rise in wave heights mean substantially stronger waves, Gerd Masselink, a professor of coastal geomorphology at Plymouth University's School of Biological and Marine Sciences and coauthor of the study, said in an interview. Any increase in wave height results in a doubling of wave energy, he said.
During the intense winter storms of 2013 and 2014, wave heights were about half a meter above the historical average of 2 meters, meaning there was a 20 percent increase in wave height, but a far greater increase in strength.
"If you have a coastline and are throwing, on average, 40 percent more energy at it — that's a big difference," said Masselink.
But, Masselink cautions, winters are inherently fickle, and average wave heights vacillate each year, "jumping up and down like a yo-yo," he said. So, the increasing wave height trend — based on a combination of satellite, ocean buoy, and computer modeling information — is not nearly as prominent as the rise in global average surface temperatures, which have been on an unambiguous upward course for decades.
"Winter weather is hugely variable," said Masselink. "But it's certainly been that the more extreme winters have been occurring more frequently."
Scientists can't yet conclude that increasing wave heights are stoked by climate change.
"There, we have problem," said Masselink. "The consensus among climatologists is lacking."
The primary cause of increased wave heights in this region, Masselink said, is a weather phenomenon called the North Atlantic Oscillation, or NAO.
The NAO is a pattern of atmospheric pressure over the North Atlantic. When the NAO is in a "negative phase," it is more favorable for powerful snowstorms in the eastern U.S. and continental Europe. On the other hand, when it is in a positive phase it tends to limit the number of intense storms in the Eastern U.S., but can make Western Europe more vulnerable.
During the positive phase of the NAO, there is a pronounced pressure difference between Spain and Iceland, which yields strong winds over the North Atlantic Ocean and, in turn, high waves. Such conditions seem to be occurring more frequently than they used to, Masselink said.
But the increasing frequency of the positive phase of the NAO might simply be due to natural climate variations, as the study only measured around seven decades of data — not enough time to say it isn't just a natural climatic swing. The NAO can vary on timescales of several days to weeks, making it difficult to tease out statistically significant trends.
"It could be an expression of climate change, and also it could not," said Masselink.
Still, the increasing trend in wave heights doesn't bode well for seawalls, and the people and property they protect. A pummeling storm won't cause the structures to completely topple, said Masselink, but it will cause more flooding as the waves breach the tops of the sea defenses.
The waves will also continue to erode cliffs and beaches. "Most of the dunes seem like they’re in a permanent state of decline," Masselink said.
“So any increases in wave heights, and greater frequency of extreme storms, are going to have a major impact on thousands of communities along the Atlantic coastlines of Western Europe," Bruno Castelle, senior scientist at the French National Centre for Scientific Research and coauthor of the study, said in a statement.
Today's coastal defenses may be equipped to handle a one-hundred year storm. But powerful storms that once rarely struck might now arrive once a decade, if the trend continues.
And more powerful storms with taller waves are all exacerbated by sea level rise, which is currently amplifying as world's thawing ice sheets — notably those in western Antarctica and Greenland — melt previously locked-up water into the seas.
"This is the new now," said Masselink.