In a warmer world, rising sea levels could cause many coastlines, beaches, and reef islands to be destroyed. Since pre-industrial times, the Earth has warmed.
Some coastlines and even low-lying coral reef islands are growing rather than being destroyed by sea levels. This is happening on some beaches in Australia, along with coastlines in Asia and Africa.
This goes against the general understanding of how climate change impacts the coast and has led to confusion that has been sown into public discourse by climate change deniers. What is going on?
Historical aerial photographs and satellite records were used to examine the phenomenon. The amount of sand, rocks, and other debris moving into and out of the beach over time is a key factor in the observed growth of coastlines.
Our results show how complex the coast is, and that there is a need for greater understanding of local coastal changes, even down to individual beaches, when making coastal management plans.
To understand this phenomenon, we need to understand the budgets. When more sand comes to the beach than leaves, it's called a positive sediment budget. When more sand leaves than arrives, the budget is negative.
Growth on the coast is driven by a positive budget and beaches expand further into the ocean.
Sea level rise erodes sand from the beach and places it on the coast. Sand can be lost from the beach and the shoreline can retreat inland.
If sea levels are rising, why are some beaches getting bigger?
The answer is that for growing beaches the positive budget has a greater impact than the sea level rise. The amount of sand coming to the coast is greater than the amount lost to sea level rise.
We used an aerial photograph record from the 1930s to present to investigate change on the coast of the state. We looked at shoreline change using the satellite record.
Every beach we looked at was growing despite global sea levels rising.
Africa and Southeast Asia were found to be growing when we looked at coastal changes on a global scale. The coast has net positive sediment budgets.
There are two things that may explain it. Extra sand arrives in natural settings from either the continental shelf or the river. Human intervention in the form of coastal development drives growth.
Over time, the natural input of a nearby river is likely to have contributed to the growth of Bucasia Beach. Coolangatta Beach in the Gold Coast has grown due to human intervention that placed additional sand on the beach.
The coast of China has grown due to human development. South America has grown due to large or fast rivers transporting enormous quantities of silt to the coast.
The results show that human intervention can be more important than a small rise in sea level in driving coastal change.
Sea level rise driven erosion is a real risk in the future. What happens when the rate of sea level rise continues to accelerate?
If global emissions continue, the Intergovernmental Panel on Climate Change predicts sea levels will reach 1.01 meters higher by the year 2200.
Sea level rise is getting faster. During 1901-1971, 1.9mm per year was found, during 1971-2006, and 3.7mm per year was found.
The increase in sea level may lead to a loss of sand to the beach. This could cause erosion on the beaches.
It isn't seen as evidence that sea level rise doesn't drive coastal erosion. It's not clear if the coast is free from future erosion risk.
Even if there is enough silt to maintain growth on the coast, storms can cause hazardous erosion.
When we seek to understand and mitigate the future impact of sea level rise on the coast, we should also ask when coastal erosion becomes hazardous.
When human infrastructure or livelihoods are at risk, coastal erosion is only a problem.
Sea level rise in the future is just as important as the decisions we make on the coast.
Most of the coast of Australia is not developed and the positive budget on many beaches will limit future erosion.
The risk of future hazardous erosion under climate change is low if we continue to leave them alone. We will increase our climate risk if we place people and infrastructure too close to the shoreline.
Daniel Harris, Dylan Cowley, and Yongjing Mao are PhD candidates.
This article is free to use under a Creative Commons license. The original article is worth a read.
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