Sea levels are rising, but some beaches are becoming larger. This is why:

Sea levels are rising, but some beaches are becoming larger. This is why: ...

Rising sea levels may make many coastlines, beaches, and reef islands unhabitable, or cause them to die altogether. The 1.09 Earth has been warming since pre-industrial times has already increased seas by 20 centimeters.

In the face of rising sea levels, however, research suggests that certain coastlines and even low-lying coral reef islands are actually increasing rather than eroding. This is especially happening on some beaches in Queensland and New South Wales in Australia, along with coastlines in Asia and Africa.

This goes against the general view of how climate change affects the coast and has resulted in a conflict that has been deliberately sown into public discourse by climate change negoters. So what''s going on?

Our research into coastal changes is mainly related to the "coastal sediment budget," which includes the amount of sand, rocks, and other sediment that travel into and out of the beach over time.

When it comes to coastal management, our results show how dynamic and complex the coast is, highlighting the need for a greater understanding of local coastal changes, even down to individual beaches.

Understanding sediment budgets

To understand this phenomenon, we must first consider sediment budgets. A "positive" sediment budget is when more sand arrives on the beach than leaves. A "negative" budget is the opposite, when more sand leaves than arrives.

Over the years, a robust sediment budget has driven the coast''s growth, and beaches have expanded further into the ocean.

On the other hand, sea level rises, erodes sand from the beach and places it elsewhere on the coast. This can result in a loss of sand from the beach and the shoreline retreats inland.

Why are some beaches still getting bigger as sea levels across the globe?

The answer is that for growing beaches, the positive sediment budget has a greater impact than erosion due to sea level rise. In other words, the amount of sand coming to the coast is greater than the amount lost to sea level rise.

Beaches in Queensland

We investigated ocean changes from north of Cooktown to Coolangatta on the coast of Queensland, using the aerial photographic record from the 1930s to present. We also investigated the ocean change worldwide using the satellite record since 1984.

Despite the rising of 20 centimeters in this time, every beach we investigated in Queensland was increasing.

When we looked at coastal changes on at a global scale, we found that large parts of entire continents, such as Africa and Southeast Asia, were also growing. This suggests that net positive sediment budgets on the coast are common.

It may be explained by two things: in natural situations, additional sand will be likely to come from either deeper sediment on the continental shelf or from rivers. Human intervention, in the form of coastal development, also drives coastal growth.

Bucasia Beach has grown due to the natural contribution of sediment over time, likely from a nearby river. Similarly, Coolangatta Beach in the Gold Coast has also grown due to human intervention that placed additional sand on the beach to mitigate and reverse trends of erosion.

Human development on China''s coast has grown at a global scale. Other areas, such as Suriname and South America, have benefited from extensive or rapid flights transferring huge amounts of sediment to the coast.

These findings show that sediment budgets and human intervention may be much greater contributors to coastal change than a relatively modest rise in sea levels.

As predicted, sea level rise does not mean erosion will pose a real threat in the future. Instead, we should contemplate how dramatic the rate of rise is?

What does this mean for the future?

If global emissions continue, the Intergovernmental Panel on Climate Change (IPCC) forecasts sea levels to reach 1.01 meters higher (relative to the level from 1995 to 2014).

More importantly, the sea level rise is increasing faster in 1901-1971. The IPCC found it increased by 1.3 millimeters per year between 1971 and 2006, and 3.7mm per year during 2006-2018.

This increase in sea level might result in a loss of sediment to the beach that current favorable sediment budgets can no longer compensate. This could result in erosion in beaches presently growing.

The coastlines that are currently growing aren''t seen as evidence that the sea level rise does not prevent coastal erosion. Nor that such coasts are free from future erosion risk.

Even if there''s enough sediment to maintain growth on the coast, dangerous erosion and inundation can still occur due to storms and cyclones.

When we attempt to comprehend and mitigate the future effects of the sea level rise on the coast, we should also ask: when does coastal erosion become hazardous?

Coastal erosion is by itself a natural process and is only a problem when human infrastructure or livelihoods are at jeopardy.

The sediment budget and decisions we make on the coast where we build, where we intervene, and where we don''t work are just as important as the rise of sea level in the future.

The majority of Australia''s coast is undeveloped, and the excellent sediment budget on many beaches will limit future erosion.

If we continue to leave them alone, the danger of future harmful erosion under climate change is low. However, if we bring people and infrastructure too close to the shoreline and disrupt coastal sediment budgets, we will reduce our future climate danger.

Daniel Harris, a senior lecturer in geography at the University of Queensland; Dylan Cowley, PhD Candidate at the University of Queensland; and Yongjing Mao, PhD Candidate at the University of Queensland.

This article has been republished from The Conversation under a Creative Commons license.

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