Falling sea levels due to El Niño will hit mangroves and corals

Higher sea levels in the eastern Pacific due to the monster El Niño are expected to cause record coastal flooding and erosion in the coming months. But on the other side of the Pacific, falling sea levels around Australia and the Coral Triangle – an area to the north of the continent – pose another threat that is only just starting to be recognised: mass die-offs of mangrove forests, with corals and seagrass meadows also impacted. During the super El Niños in 1982/83 and 2015/16, there were mass diebacks of mangrove forests in northern Australia, says Norman Duke at James Cook University in Australia. In the 2015 event, around 76 square kilometres of mangroves on a 2000-kilometre-long stretch of shoreline in the Gulf of Carpentaria died. This was due to a prolonged 40 to 50 centimetre drop in sea level for at least six months. Whether there is a similar or worse event during this El Niño will depend in part on how much rainfall there is. Advertisement “Both [previous] events coincided with periods of low to no rainfall,” says Duke. “Mangroves, lacking inundation or other sources of moisture, died of thirst.” During neutral and La Niña years, trade winds blowing west across the equatorial Pacific pile up warm water around Australia’s northeastern coast. When these winds weaken or reverse during El Niños, those waters spill across the Pacific. The result is a kind of see-saw effect, with sea levels falling in the west and rising in the east. Because the current El Niño is so strong, it is producing a greater swing in sea levels than usual, with sea levels already up to 40 cm higher than typical in parts of the eastern Pacific near the equator but lower in a large swathe of the western Pacific, including around northern Australia and the Coral Triangle. Long-term tidal cycles can amplify or counteract the effects of falling sea levels, says Neil Saintilan at Macquarie University in Australia. Mangroves grow in the upper part of the tidal range, so if the tidal range decreases at the same time sea level falls, they are left high and dry, he says. “This happened in the Gulf of Carpentaria in 2015/16.” During the ongoing El Niño, the tidal range in the Gulf of Carpentaria should be higher than normal, reducing the risk of dieback – though sea levels may fall further than in 2015. However, along the coastline west of the gulf, the tidal range will be smaller than usual, says Saintilan. “We are anticipating extensive mangrove dieback there.” The full scale of these sea-level effects on tidal ecosystems around the Pacific is only just becoming apparent. The full scale of these sea-level effects on tidal ecosystems around the Pacific is only just becoming apparent. A 2025 satellite study concluded that in 2015/16, 34 per cent of the anomalies in mangrove leaf area index – a measure of canopy health – globally were due to sea-level changes. Changes in sea level also affect corals and seagrass, Duke suspects. Shallow reefs and seagrass meadows are exposed during low tide and can dry out if falling sea levels leave them exposed too long. Even if they remain submerged, temperatures will be higher – not just because the water is shallower, but because global warming and the El Niño event have pushed them up. “Samoan colleagues refer to El Niño events as ‘Taimasa’ – periods marked by the smell of dead fish and coral, as they die on exposed coral reefs,” says Duke. “It is quite possible that the sea-level drop contributes to the depth of reef dieback, though this would be dominated by the marine heatwaves themselves,” says Saintilan. However, it appears that no studies have attempted to quantify the relative contributions of sea-level fall and marine heatwaves to the coral bleaching events and die-offs in the western Pacific during recent El Niños. “Despite several attempts, I have been unable to convince research colleagues working on corals and seagrasses that these events also contribute to those habitats as well,” says Duke.