21/08/2019

Warming Climate To Stir Up More Damaging Waves As 'Rare Event' Nears: Research

Sydney Morning Herald - Peter Hannam

Waves are forecast to become larger and more powerful and to shift direction if the climate continues to warm at its current rate, with southern Australia among the regions to be hardest hit globally, new research says.
The study, led by scientists from Griffith University and published in Nature Climate Change on Tuesday, comes as meteorologists forecast a "quite rare event" later this week as a potentially damaging swell hits eastern Australia.
A monster wave in January 2015 that washed a woman off Nobbys breakwall in Newcastle. Credit: Cordelia Troy
Using about 150 model simulations, the researchers found about half the world's coastline was "at risk from wave climate change" by the final two decades of this century if greenhouse gas emissions remained at their current "business-as-usual trajectory".
The wave changes, driven primarily by strengthening winds "might potentially exacerbate or even exceed in some coastal regions, impacts of future sea-level rise", the paper said.
Widespread ocean regions can expect annual mean significant wave height -  the average difference between trough to crest of the highest third of waves - to increase between 5 and 15 per cent compared with a 1979-2004 baseline.
The mean period between waves was projected to increase by a similar range of 5 to 15 per cent, implying increased forces to pound beaches and infrastructure when the waves reach the coast.
Wave direction would also shift between 5 and 15 degrees, potentially shifting sand and hammering some areas now typically sheltered by headlands and other coastal formations.
"For Australia, the south coast and Victoria especially are really going to be impacted by the Southern Ocean changes," Joao Morim, a PhD candidate at Griffith University and lead author of the paper, said.
The expected impact from swells for Australia's coastline would be driven by an intensification and poleward shift of storm activity.
The west coast of South America was another region facing more damaging waves while the North Atlantic would most likely see a reduction because of a decrease mid-latitude storm activity in that basin, Mr Morim said.
Surfers ride big waves along the Sydney coastline in November 2015. Credit: Nick Moir
However, should emissions be limited sufficiently to contain global warming to within the Paris climate target of 2 degrees compared with pre-industrial levels, the projected wave changes "are not standing out from the natural variability", Mr Morim said. His work was part of a six-year project also involving the CSIRO.
Mitchell Harley, a lecturer at the University of NSW's School of Civil and Environmental Engineering who was not among the paper's authors, said the work was a "nice step forward and presents a nice summary of what we do know and importantly what we don't know".
Dr Harley said his interest was that waves would change over the next decade or so at a more local level.
"What this paper shows is there is still a lot of uncertainty in these projections," he said.
"Storms are changing, the wind fields are changing around the world, and that's going to have inevitable consequences on how our waves are striking the coast and consequently how our coastline is going to respond," he said, adding that satellites had identified an increase of about 3 per cent in wave energy over the past half century.
Even small changes in the direction of waves "can cause large-scale reorientations" of the coastline, as sand gets scoured from some beaches and dumped on others, Dr Harley said. Clarkes Beach near Byron Bay was one recent example where dunes had been eroded by shifting conditions.
Beach erosion has lately been a pressing issue for parts of Perth. Credit: WA Today
Beaches in Australia south of about the Sunshine Coast tended to be shaped primarily by waves while those further north are dominated by tropical cyclones and tidal changes, he said.
Coastal residents might get to experience some relatively severe wave activity this week, with significant wave heights expected to reach about six metres along parts of the NSW coast on Thursday.
Jordan Notara, a forecaster at the Bureau of Meteorology, described the predicted swell as "quite a rare event" for August, and said waves could be damaging. The bureau has also issued a warning about the surf conditions.
Dr Harley said this week's swell would be coming from "the dead south", a typical direction for such big waves.
In the future, though, direction changes of the waves in extreme events - as predicted by the research paper - "really causes huge effects on the wave-exposure of the coastline", he said.

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