Rising Seas Could Drown Mangroves and Release Stored Carbon

A new study warns that rising sea levels could drown mangroves and release the large amounts of carbon these coastal forests store. The finding comes from research on coastal ecosystems that asks how reliable mangroves really are as natural carbon sinks under conditions that are changing fast. The message is cautious but clear: an ecosystem long assumed to work in our favour could, under certain conditions, cancel out part of that benefit.

Mangroves are salt-tolerant tidal forests that grow at the boundary between land and sea, in the shallow coastal zones of tropical and subtropical regions. They are often cited as a prime example of blue carbon, the term for carbon locked away in coastal and marine ecosystems. Unlike forests on land, mangroves trap much of their carbon in the soil and mud beneath them, where organic matter can stay out of contact with oxygen for centuries. That is precisely why mangroves are considered among the most efficient natural systems for long-term carbon storage.

The study behind this article asks what happens when the sea begins to rise faster than mangroves can keep up. Mangroves normally have the ability to adapt to gradual sea-level rise by trapping sediment and lifting the ground they grow on. The problem arises when sea-level rise crosses the threshold at which that natural mechanism can no longer keep pace. In that case the forest drowns, the soil breaks down, and carbon that was locked in the sediment for decades can escape back into the atmosphere as carbon dioxide.

Why would drowning mangroves be a problem for the climate?

Most of the concern is not about losing the forest itself, but about what the forest holds inside it. When coastal soil rich in organic matter is flooded and disturbed, microorganisms break down the accumulated carbon, and some of it returns to the atmosphere. That means mangroves in this scenario do not merely stop being useful, they can temporarily become a source of emissions rather than a sink. The study suggests this effect would reduce the net carbon-capture benefit at the scale of entire forests, not only on the most exposed individual patches.

This matters because many climate models and national emissions plans count on coastal ecosystems as stable, long-term carbon sinks. If that sink turns out to be more sensitive to sea-level rise than assumed, part of the calculation about how much nature can offset human emissions would have to be revised. The finding fits into a wider set of warnings that nature-based climate solutions are not infinitely elastic and depend on how fast the climate itself changes.

What this finding does not prove

It is worth stressing the limits of this kind of research. The study describes a risk and a mechanism, but it does not mean every mangrove on Earth is doomed to vanish, or that all stored carbon will be released. Much depends on local conditions, the pace of sea-level rise, the availability of sediment, and whether there is room for the forest to retreat further inland. This is about a risk linked to sea-level rise, not a guaranteed outcome for every coastline.

What such work does make clear is that the role of mangroves in fighting climate change is not guaranteed on its own. It depends on how well we manage to limit warming and sea-level rise, and also on how the coasts themselves are managed. Protecting existing mangroves, restoring damaged ones, and leaving room for them to migrate landward are cited as measures that can help keep this ecosystem an ally rather than a new source of problems. How much, as a society, should we keep counting on nature as a carbon sink if it turns out that nature, too, has its limit?