(ORDO NEWS) — In a warmer world, rising sea levels could render many coastlines, beaches and reef islands uninhabitable or destroy them entirely. The warming of the Earth by 1.09℃ since pre-industrial times has already led to sea level rise by 20 centimeters.
But it is curious that, according to research, some coastlines and even low-lying islands of coral reefs are not being destroyed, but are growing under conditions of rising sea levels. It occurs on some beaches in Queensland and New South Wales in Australia, as well as on the coasts of Asia and Africa.
This runs counter to the common understanding of how climate change impacts coasts and has led to confusion that has been, in part, deliberately planted in the public discourse by climate change deniers. So what’s going on?
To study this phenomenon, we examined coastline changes using historical aerial photographs and satellite recordings. We have found that the observed growth of shorelines is largely related to the “shore sediment budget” – the amount of sand, rocks and other sediments moving in and out of the beach over time.
Our results show how dynamic and complex the coastline is, highlighting the need for a deeper understanding of local shoreline changes, down to individual beaches, when developing coastline management plans.
Understanding Sedimentary Budgets
To understand the meaning of this phenomenon, we first need to understand deposit budgets. A “positive” sediment budget is when more sand enters the beach than leaves. A “negative” budget is the opposite, when more sand leaves than comes in.
Over time, a positive sediment budget stimulates coastal growth and beaches expand further into the ocean.
Rising sea levels, on the other hand, wash sand off the beach and place it elsewhere on the coast. This can lead to the loss of sand from the beach – and the coastline recedes deep into the ocean.
So, if sea levels are rising all over the planet, why are some beaches still getting bigger?
The answer is that for growing beaches, a positive sediment budget currently has a greater impact than erosion from rising sea levels. In other words, the amount of sand entering the coast is greater than the amount of sand lost due to sea level rise.
Beaches in Queensland
We examined changes along the coast of Queensland on 15 beaches stretching from the northern part of Cooktown to Coolangatta using aerial photographic data from the 1930s to the present. We have also explored coastline change on a global scale with satellite data since 1984.
Despite the fact that during this time the level of the world’s seas rose by 20 centimeters, every beach that we explored in Queensland increased.
When we looked at coastline changes on a global scale, we found that large parts of entire continents such as Africa and Southeast Asia are also on the rise. This suggests that a net positive sediment budget on the coast is a common occurrence.
This can be explained by two reasons. Under natural conditions, additional sand is likely to come either from deeper sediments located on the continental shelf or from rivers. Human intervention, in the form of coastal development, also stimulates the growth of the coastal zone.
For example, in Queensland, Bucasia Beach has grown due to a natural influx of sediment over time, probably from a nearby river. Meanwhile, Coolangatta Beach on the Gold Coast has grown thanks to human intervention, which has placed extra sand on the beach to soften and reverse erosion trends.
On a global scale, part of China’s coast has grown thanks to the development of human potential on the coast. Other regions such as Suriname, South America have grown because of the large and fast rivers that bring a huge amount of sediment material to the coast.
These results indicate that sediment budgets and human intervention may be much larger contributors to coastal zone change than relatively small sea level rise.
However, this does not mean that erosion caused by sea level rise is not a real risk in the future. On the contrary, we should ask: what will happen if, as predicted, the rate of sea level rise continues to accelerate?
What does this mean for the future?
According to the forecasts of the Intergovernmental Panel on Climate Change (IPCC), if global emissions do not stop, then by 2100 the sea level will rise by 1.01 meters (relative to the level of 1995-2014).
Moreover, sea level rise is accelerating. The IPCC found that in 1901-1971 it increased by 1.3 millimeters per year, in 1971-2006 – by 1.9 mm per year, and in 2006-2018 – by 3.7 mm per year.
This rise in sea level could result in a loss of sediment material on the beach that would not be able to offset the current positive sediment budget. This can cause erosion on the beaches that are currently growing.
Therefore, it is important that currently rising coastlines are not taken as evidence that rising sea levels are not causing coastal erosion. Nor should it be assumed that such banks are free from the risk of future erosion.
Even if there is enough sediment to support coastal growth, dangerous erosion and flooding from storms and cyclones can still occur.
As we try to understand and mitigate the future impact of sea level rise on the coast, we must also ask the question: when does coastal erosion become dangerous?
Coastal erosion is a natural process in itself and is only a problem when human infrastructure or livelihoods are threatened.
The sediment budget and the decisions we make on the coast – where we build, where we intervene and where we don’t – matter as much as sea level rise in the future.
Much of Australia’s coastline is undeveloped and a positive sediment budget on many beaches will limit future erosion.
If we continue to leave them alone, there will be little risk of future dangerous erosion in the face of climate change. However, if we place people and infrastructure too close to the coastline and disrupt the coastal sediment budget, we increase our future climate risk.
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