Ocean waves becoming more threatening to erode coastlines due to climate change
Sea level rise isn’t the only way climate change will devastate the coast. Our research, published today, found it is also making waves more powerful, particularly in the Southern Hemisphere.
We plotted the trajectory of these stronger waves and found the coasts of South Australia and Western Australia, Pacific and Caribbean Islands, East Indonesia and Japan, and South Africa are already experiencing more powerful waves because of global warming.
This will compound the effects of sea level rise, putting low-lying island nations in the Pacific — such as Tuvalu, Kiribati and the Marshall Islands — in further danger, and changing how we manage coasts worldwide.
But it’s not too late to stop the worst effects — that is, if we drastically and urgently cut greenhouse gas emissions.
But our focus was on how warming oceans boost wave power. We looked at wave conditions over the past 35 years, and found global wave power has increased since at least the 1980s, mostly concentrated in the Southern Hemisphere, as more energy is being pumped into the oceans in the form of heat.
And a more energetic ocean means larger wave heights and more erosive energy potential for coastlines in some parts of the world than before.
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So why is this happening?
Ocean waves are generated by winds blowing along the ocean surface. And when the ocean absorbs heat, the sea surface warms, encouraging the warm air over the top of it to rise (this is called convection). This helps spin up atmospheric circulation and winds.
In other words, we come to a cascade of impacts: warmer sea surface temperatures bring about stronger winds, which alter global ocean wave conditions.
Our research shows, in some parts of the world’s oceans, wave power is increasing because of stronger wind energy and the shift of westerly winds towards the poles. This is most noticeable in the tropical regions of the Atlantic and Pacific Oceans, and the subtropical regions of the Indian Ocean.
But not all changes in wave conditions are driven by ocean warming from human-caused climate change. Some areas of the world’s oceans are still more influenced by natural climate variability — such as El Niño and La Niña — than long-term ocean warming.
In general, it appears changes to wave conditions towards the equator are more driven by ocean warming from human-caused climate change, whereas changes to waves towards the poles remain more impacted by natural climate variability.
How this could erode the coasts
While the response of coastlines to climate change is a complex interplay of many processes, waves remain the principal driver of change along many of the world’s open, sandy coastlines.
So how might coastlines respond to getting hit by more powerful waves? It generally depends on how much sand there is, and how, exactly, wave power increases.
For example, if there’s an increase in wave height, this may cause increased erosion. But if the waves become longer (a lengthening of the wave period), then this may have the opposite effect, by transporting sand from deeper water to help the coast keep pace with sea level rise.
It’s not too late
It’s not surprising for us to find the fingerprints of greenhouse warming in ocean waves and, consequentially, along our coastlines. Our study looked only at historical wave conditions and how these are already being impacted by climate change.
But if warming continues in line with current trends over the coming century, we can expect to see more significant changes in wave conditions along the world’s coasts than uncovered in our backward-looking research.