Corals already face numerous problems, but now researchers have identified another one: the annual growth of the reefs probably cannot keep up with the rise in sea levels. This means that over time they will lie deeper and deeper beneath the surface of the water, where they will receive less sunlight and may die. Entire island states that are built on coral reefs or are protected by them are also sinking deeper and are increasingly in distress.
Tropical coral reefs are hotspots of marine diversity, but climate change poses numerous challenges. Among other things, rising temperatures, acidified and over-fertilized water and diseases affect them. Coral reefs all over the world are already increasingly bleaching, are being overgrown by algae and weed-like coral species, or are being decimated by voracious starfish.
Drilling into the past
As if these future prospects weren't already bleak enough, researchers led by Eberhard Gischler from the Goethe University in Frankfurt am Main have now identified another threat to coral reefs. To find out whether the current coral decline is indeed unprecedented in history, they analyzed 22 core samples from Belize. The largest reef system in the Atlantic lies there.
The eight to twenty meter long drill cores provide information about how the atolls and barrier reefs there have fared over the past 9,000 years - also known as the Holocene. By identifying the age and types of fossil corals in the cores, Gischler and his team were able to determine which coral communities lived off Belize at what time. Measuring the spatial distances between these corals also provided information about when and how much they grew.
Corals sink into the depths
The result: “Our data show an overall decline in coral growth rates in Belize during the Holocene,” reports Gischler. On average, the reefs there have grown 3.36 millimeters per year over the past millennia. However, given the current climate change, this is not enough, as the researchers explain. The Intergovernmental Panel on Climate Change (IPCC) predicts that sea levels will rise by up to 9.8 millimeters per year in the coming decades. Levels are already rising by a global average of around 3.7 millimeters per year, but in 2019 record levels of 6.1 millimeters per year were already reached on some coasts. Even if global warming is limited to two degrees, the IPCC predicts a total sea level rise of 30 to 60 centimeters by 2100.
So it could well happen that sea levels are rising faster than corals can grow, leaving them deeper and deeper beneath the water's surface. Since tropical corals require sunlight to live, this would be the final death knell for them. But not only the coral reefs themselves, but also the people who depend on them would suffer from this development. At least one percent of humanity currently lives on or is protected by low-lying reef islands. Their home would therefore disappear along with the corals. Only if sea level rise falls to less than around three millimeters per year in the near future through effective climate protection would humans and corals still have a chance.
Interestingly, the reefs off Belize have also faced major challenges in the past, as Gischler and his colleagues have discovered. Accordingly, there were longer time windows 2,000, 4,000 and 5,500 years ago in which the elkhorn coral (Acropora palmata) - the otherwise most common coral species of the Holocene - completely disappeared off Belize. The researchers assume that it may have once fallen victim to higher temperatures, violent storms or a lack of nutrients. At that time, the stocks were always able to recover, but whether they will be able to do so in the future remains to be seen.
Source: Goethe University Frankfurt am Main; Specialist articles: Scientific Reports, doi: 10.1038/s41598-023-38118-5