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A warmer world could see rising sea levels making many beaches, coastlines and reef islands uninhabitable or even disappear altogether. The 1.09℃ Earth has warmedSince pre-industrial times, sea levels have already risen by 20 centimetres.
Surprisingly, however research shows that there are some coastlines. Low-lying coral islands can also be found in the reefsAre actually GrowingRising sea levels are causing erosion rather than preventing it. This is happening at beaches in Queensland, New South Wales, and along with coastlines in Asia or Africa.
This is contrary to the general understanding of climate change’s impact on the coast. It has also led to confusion that has been intentionally sown into public discourse in part by climate change denier. So what’s going on?
We used coastal changes to examine the phenomenon. Historical aerial photographsAnd Satellite records. We found the observed growth of coastlines is largely linked to the “coastal sediment budget” – the amount of sand, rocks and other sediment moving into and out of the beach over time.
Our results show how complex and dynamic the coast can be. This highlights the need for better understanding of local coastal change.When planning coastal management plans, it is important to consider all aspects, including individual beaches.
Understanding sediment budgets
We need to understand sediment budgets in order to make sense of this phenomenon. A “positive” sediment budget is when more sand comes to the beach than leaves. A “negative” budget the opposite, when more sand leaves than arrives.
Over time a positive sediment budget drives growth on the coast – and beaches expand further into the ocean.
Sea level rise on the other hand erodes the sand from beaches and places it elsewhere along the coast. This can lead to a loss of sand from the beach – and the shoreline retreats inland.
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So, even though sea levels are rising all over the globe, why are some beaches growing larger?
The positive sediment budget is currently more important than erosion due to sea level rise for beaches that are growing. The amount of sand that reaches the coast is greater than what is lost to sea rise.
Author provided Background images courtesy of QImagery.
Beaches in Queensland
We investigated change on the coast of Queensland at 15 beaches stretching from north of Cooktown to Coolangatta, using the aerial photograph record from the 1930’s to present. We also examined global shoreline change using the satellite record from 1984.
Despite the fact global sea levels rose by 20 centimetres, every beach we checked in Queensland was still growing.
We found that large areas of entire continents such as Africa and Southeast Asia were also growing when we looked at coastal change on a global level. This suggests that net positive sediment balances along the coast are quite common.
Two factors could explain why it happens. In natural settings, more sand comes from deeper sediment on the continental shelves or from rivers. Human intervention in the form coast development also drives coastal growth.
Author provided Background images courtesy of QImagery.
Bucasia Beach in Queensland, for instance, has grown because of the natural inputs of sediment over time. This is likely to have been a result of a nearby river. Coolangatta Beach, on the Gold Coast, has seen its erosion reversed by human intervention.
At a global scale, parts of China’s coast have grown due to human development on the coast. Other regions, like Suriname in South America, have grown because of large or fast rivers transporting huge quantities of sediment to its coast.
Author provided. Author provided
These results indicate that sediment budgets as well as human intervention can drive coastal change more than a relatively small increase in sea levels.
However, this doesn’t mean sea level rise driven erosion isn’t a real risk in the future. Instead, we should ask ourselves: What happens if sea level rise accelerates, as is forecast?
What does this mean?
The Intergovernmental Panel on Climate Change forecastsSea levels will rise up to 1.01m (relatively the 1995-2014 level) by 2100, if global emissions are not reduced.
What’s more, sea level rise is getting faster. It rose by 1.3 mm per annum between 1901-1971, 1.9mm per annum during 1971-2006 and 3.7mm per a year between 2006-2018, according the IPCC.
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An increase in sea level could cause beach sediment loss that can’t be offset by current positive sediment budgets. This could lead to erosion on beaches that are currently growing.
So it’s important coastlines presently growing aren’t seen as evidence that sea level rise does not drive coastal erosion. These coasts are not immune to future erosion risk.
Even if there’s enough sediment to maintain growth on the coast, hazardous erosion and inundation due to storms and cyclones can still occur.
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We need to ask the question: When does coastal erosion become dangerous?
Coastal erosion can be a natural process by itself and is not a problem if human infrastructure or livelihoods is at risk.
The sediment budget and decisions we make on the coast – where we build, where we intervene, and where we don’t – are just as critical as sea level rise in the future.
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Most of Australia’s coast is undeveloped and the positive sediment budget on many beaches will limit future erosion.
We can keep them safe and reduce the likelihood of future dangerous erosion due to climate change. We will increase climate risk if we move infrastructure and people too close the shoreline and disrupt coastal sediment banks.
