Coastal ecosystems like tidal marshes, mangroves, and coral reefs have long been vital to humans for various purposes, from fisheries to storm protection. A recent study by an international team of scientists offers insights into the potential fate of these ecosystems as climate change drives sea levels higher.

Although such ecosystems thrive on a bit of sea level rise, there is a critical threshold past which they may start to slowly collapse. The study found that 5 mm to 7 mm of sea level rise per year may be enough to outpace these ecosystem’s abilities to adapt. We’re already close to 4 mm per year on average, with some regions well above this rate.

The intersection of sea level rise and climate change

Since the 1900s, the global mean sea level has risen by 0.20 meters, with rates reaching 3.7 millimeters annually between 2006 and 2018. But that’s just on average — sea level rise is not uniform around the world, and some regions experience significantly more sea level rise than the mean. If we take the US for instance, sea levels around the sunny West Coast have risen by only 3 inches since 1950 because the land is actually rising due to shifting tectonic plates, offsetting some of the expected sea level rise. But on the Gulf Coast, the water levels have risen by as much as 15 inches due to sinking land.

The Intergovernmental Panel on Climate Change’s Sixth Assessment Report predicts further sea level rise by 2100, with estimates ranging from 0.44 to 0.81 meters under global warming scenarios of 1.5°C and 5°C, respectively. The scenarios represent a mean sea level rise rate of 4.3 mm and 11.7 mm per year, respectively.

How will all these vital coastal ecosystems fare in the face of such fast-paced sea level rise? That’s an important question that the researchers set out to answer, led by Neil Saintilan, professor of environmental studies at Macquarie University in Sydney.

“The question was at what rate of sea level rise do the really important coastal ecosystems like tidal marshes, mangroves, and coral reef island systems start to get into trouble. In other words, the point at which they start to fall behind that rate of sea level rise,” Saintilan told ZME Science.

The tipping point

Some coastal ecosystems can be quite resilient and adaptable. Coral reefs, for instance, can vertically expand as new coral formations build upon one another over centuries, even millennia. Similarly, mangroves and tidal marshes employ sediment trapping and vertical root system growth to keep pace with rising waters.

Presently, these ecosystems seem to be maintaining a stable equilibrium with sea level rise across various locations worldwide — and that’s certainly good news. The main concern thus lies in determining the upper limit of this adaption, the threshold past which these ecosystems no longer thrive but start to literally drown.

Sea levels have always waxed and waned throughout our planet’s geological history. What’s different this time is the unprecedented rate at which such changes are happening as a result of global warming melting glaciers and higher temperatures expanding the volume of the oceans. Nevertheless, there is much we can learn from other sharp seal-level rise events from thousands of years ago.

To unravel this mystery, two main sources of information were employed. Firstly, researchers studied how these ecosystems responded during periods of higher sea levels in the past. The melting of extensive ice sheets at the conclusion of the last glacial period roughly 12,000 years ago resulted in sea level rises far surpassing contemporary rates. By analyzing the response of coastal ecosystems during these historical instances, scientists gained valuable insight into potential future scenarios.

Secondly, researchers examined the behavior of contemporary coastal ecosystems — including 190 mangroves, 477 tidal marshes and 872 coral reef islands — over the past few decades along different coastlines from around the world. This approach allowed them to measure how ecosystems like tidal marshes and mangrove forests have changed as a response to dynamics such as accretion and sinking.

This comparative analysis showed that the Paleo record and the recent observational record align very closely, suggesting an upper bound of adaptation exists. This tipping point at which ecosystems could succumb to sea-level rise might be lower than previously thought. It lies somewhere between 5 and 7 millimeters of sea level rise. Past this threshold, tidal marshes and mangroves face the risk of drowning, causing the loss of these coastal ecosystems.

“In both contexts, both the Paleo context and the modern coastlines, we’re actually seeing a situation where once you get above around five millimeters a year, usually seven millimeters a year, you’re moving into a rate of sea level rise where it’s highly likely their systems are not going to be able to keep up. They’ll fall behind and eventually drown,” said Saintilan.

Reflecting on the research findings, Prof. Saintilan expressed his astonishment at the remarkable consistency between two distinct lines of evidence. Despite the vastly different nature of these sources, they exhibited strikingly similar outcomes, underlining the credibility of the research.

Coastal ecosystems at the crossroads

At 3 °C of warming, the study projects that nearly all of the world’s mangrove forests and coral reef islands and 40% of mapped tidal marshes will be exposed to more than 7 mm of sea level rise by the end of this century. As a result, coral reef islands could start falling apart due to shoreline erosion and wave overtopping, while tidal marshes and mangroves will face the risk of drowning. This doesn’t mean that these ecosystems will be obliterated but rather that they may not be able to adapt with rather uncertain consequences playing out into the next century.

We’re seeing some of these effects already in some places that exhibit abnormally high sea level rise, such as in the Solomon Islands where a combination of tectonic shifts and intensified trade winds has led to the disappearance of smaller coral islands. This has resulted not only in the loss of land but also in the salinization of groundwater wells, compelling residents to seek refuge in countries like New Zealand and Australia.

Coastal ecosystems are extraordinarily important for a number of reasons. Take mangrove forests, for instance. They protect tropical coastlines from storm surges by absorbing wave energy, reducing storm impacts on soft sediment shores. These forests also serve as essential fish nurseries, supporting economic activity vital for many global coastal communities that depend on these stocks for their livelihoods. As such, the health of these ecosystems directly affects food security and economic stability in these areas, particularly in Southeast Asia and other tropical regions.

Furthermore, mangroves offer important ecosystem services. They act as carbon sinks, capturing and storing carbon dioxide, thereby aiding in climate change mitigation. Notably, they exhibit enhanced carbon capture and storage with slower sea level rise. However, rapid sea level rise could potentially shift them from carbon sinks to carbon sources, impacting global greenhouse gas emissions and contributing to a negative feedback loop that drives more warming and potential ecosystem collapse.

These findings underscore the vital significance of the Paris Agreement to curb global temperature rise. The two-degree threshold is not only crucial in the context of broader global warming impacts but also serves as a stark tipping point for coastal ecosystems.

“We have a very clear threshold, it’s like a tipping point that is crossed once you move beyond two degrees. It’s not often the case that you see that as clearly as we’re seeing in our data,” said Saintilan.

The findings appeared today in the journal Nature.