Coral reefs are among the most productive ecosystems on Earth, and their importance to people is fundamental. They through small-scale fisheries, underpin across the Caribbean, and serve as that protect the coast from storms and reduce flooding events.
Caribbean reefs are eroding fast
, we found that across the Caribbean, the 2023 marine heatwave — combined with a deadly disease known as stony coral tissue loss disease — has pushed reefs over a threshold scientists thought was a decade or more away. They are now eroding faster than corals can rebuild them.
We studied reefs in the Mexican Caribbean and the Gulf of Mexico, comparing data collected before the heatwave (2018-2022) with surveys after it (2023-24). At each reef, we counted live corals and organisms that break down the reef, like parrotfish and sea urchins. From those counts, we estimated how much reef-building (carbonate production) and reef-breaking (bioerosion) was happening, then calculated the net result — whether the reef was gaining or losing material.
The results were stark: Between 70 per cent and 75 per cent of our Caribbean sites had tipped from net growth into net erosion. They are now losing calcium carbonate faster than corals can add it. The threshold that earlier models had suggested might be crossed over during the next decade or so has .
This shift was driven by the loss of fast‑growing, branching and plate‑forming corals, especially the Acropora species, which have very high growth rates and disproportionately contribute to reef building.
One of our most unsettling findings is that the Caribbean reef sites that still had high coral cover and high carbonate production before the disease and heatwave were the ones that lost the most. Some lost up to 8 kilograms of calcium carbonate per square metre per year.
Tale of two seas
Our survey also revealed a striking contrast. While Caribbean reefs collapsed, reefs in the Gulf of Mexico largely held their ground. The great majority of Gulf sites remained net positive after the heatwave.
The difference comes down to which corals are pre-eminent in each region. In the Gulf of Mexico, reefs are dominated by slow-growing, mound-shaped corals. They grow more slowly, but they are tougher when the heat kicks in. They bleached during the heatwave but mostly survived, keeping the reef’s positive.
This is the balance between the constructing and eroding processes. When more is added than removed, the coral reef can grow. When that balance flips, the reef stops growing and may even erode.
Moreover, sites in the Gulf of Mexico have not yet been affected by stony coral tissue loss disease, which preferentially kills the same massive, long-lived species that are keeping Gulf reefs alive. By the time the heat arrived, large parts of the Caribbean had already lost their most resilient corals because of . What it started, the heatwave finished.
Why reef erosion matters
All the benefits reefs provide rely on a delicate balance between reef construction and erosion.
Tropical reefs are essentially vast limestone structures, built slowly over centuries as corals deposit calcium carbonate skeletons. At the same time, waves and various reef organisms like parrotfish, sea urchins and boring sponges chip away at them.
An eroding, flattening reef begins to lose its capacity to provide benefits to other species, and people.
We did not expect to be documenting the moment at which a major region of the ocean crossed from growing to eroding. The fact that it happened this quickly, and at some of the most iconic and well-studied reefs in the Caribbean, suggests the timelines scientists have been using may be too optimistic.