Coastal environment at risk with less than 10mm sea level rise

A sea level rise of about 5–7mm a year is enough to cause coastal landscapes like mangroves and wetlands to start to retreat, according to analysis from researchers published in Nature.

The report warns: “Widespread retreat of coastal habitat is likely at warming levels above 1.5°C.”

Coastal ecosystems have long been recognized as indispensable to the well-being and subsistence of millions of people by protecting shorelines; providing an important breeding ground for sea life; and storing CO₂.

The water level in the ocean rose by 3.6mm a year from 2006–2015 on average, but this varies greatly around the planet. In 2023, global mean sea level was 101.2mm above 1993 levels, making it the highest annual average in the satellite record (1993–present).

The US National Oceanic and Atmospheric Administration also says global mean sea level has risen about 21–24cm since 1880.

The rising water level is mostly due to a combination of melt water from glaciers and ice sheets, and thermal expansion of seawater as it warms. 

The importance of mangroves is the subject of the first Cosmos Science Detectives podcast, where our science journalists receive apparently straight-forward questions, and then gather evidence to build a case that delivers an answer.

Professor Neil Saintilan, a coastal wetlands specialist from Macquarie University and lead researcher of the Nature analysis, says there is now a greater awareness of the rates of sea level rise which will significantly affect coastal zones around the world.

“What we know from that record is that once you get above 5mm a year, or in places like the Indo-Pacific, 7mm a year, it’s highly unlikely that the mangroves will stay in place,” Saintilan told the new Cosmos Science Detectives podcast.

“The rate of sea level rise eventually will exhaust their capacity to keep pace [with added levels of silt] and they’ll start retreating.”

Saintilan says that mangroves expanded massively across the globe around 8,000 years ago, when sea levels were starting to fall.

“By 8,000 years ago the rate of sea level rise was around seven millimetres a year and falling. The rate seemed to trigger the expansion of mangrove ecosystems right through the tropics as if that was the point at which they were finally able to actually hold their ground against sea level rise.”

Conversely, rising sea levels at the end of the last glacial period were correlated with very few mangroves surviving.

“There are a number of really important ecosystem services that mangroves provide,” says Saintilan.

“What people would be most familiar with is the role they play in fisheries. Mangrove plants provide a habitat, a nursery ground particularly for small fish.”

Fish are also feeding on products from the mangrove environment.

“At certain times of the month, particularly with the big high tides, crabs will synchronise their spawning so that you’ll get the highest concentration of larvae anywhere in an estuary and that represents a really important feeding opportunity for young fish,” says Saintilan.

“They also provide coastal protection: waves that will come into a mangrove forest from a storm or a small tsunami might then be dissipated as they move through sometimes several kilometres of mangrove forest.

“That protects soft shorelines around the tropics from erosion.”

Mangroves are also significant carbon sinks.

“Mangroves take carbon dioxide out of the atmosphere which is locked away for thousands of years below ground. It’s what we call blue carbon,” says Saintilan.

To understand more about the value of mangroves, listen to Science Detectives.