Climate change has already caused significant declines in coral reef coverage globally and is expected to do much further damage in the coming decades. Some scientists have held out hope that tropical corals could improve their viability by expanding their range — by finding refuge in more temperate seas. But a new study throws cold water on the idea.

Coral reef decline will far outpace expansion into temperate waters, the study finds. Most of the damage to corals will be done in the next 40 to 80 years, but expansion will take centuries, according to the study, which was published in the journal Science Advances on June 6.

The “mismatch between the timescales,” as the authors call it, could spell bad news for corals and the ecosystems that depend on them, though the severity of the loss will depend on levels of greenhouse gas emissions in the coming decades.

“The main finding is that we’re probably talking about centuries plus, in terms of how long it takes them to move to high latitudes,” Noam Vogt-Vincent, a postdoctoral fellow at the University of Hawai’i at Mānoa and lead author of the study, told Mongabay.

This finding shows “the urgent need to reduce greenhouse gas emissions,” the study says.

It’s well understood that coral reefs are at risk from rising temperatures. They’re currently facing the fourth mass bleaching event since 1998. Bleaching involves corals expelling symbiotic algae in response to stressors such as temperature swings or pollution, losing their color in the process. While corals can recover from individual bleaching events, prolonged or repeated events can lead to their death.

Dire predictions about corals’ future are commonplace among scientists. A 2018 Intergovernmental Panel on Climate Change (IPCC) report estimated that 99% of tropical coral reefs would be gone by the year 2100 under 2° Celsius (3.6° Fahrenheit) of global warming — a relatively low-emissions scenario. A 2022 study in the journal Earth’s Future projected that, in the next two decades, 99% of the world’s corals would regularly face bleaching events.

Yet, some other animals and plants, including marine species, have already started adjusting their ranges in response to climate change, and experts have posited that corals might, as well. The evidence so far has been mixed.

Research on “poleward” coral reef movement has fallen into two main camps: fieldwork looking at how much is already taking place and theoretical work assessing future possibilities. Fieldwork has been done in places such as Japan, where researchers found apparent evidence of expansion in 2011, but subsequent research indicated that it wasn’t happening at a large scale. Most of the “new” corals that researchers observed were just pre-existing temperate corals growing in number, it turned out.

Prior to Vogt-Vincent’s study, most of the theoretical research on corals taking refuge in temperate waters was based on statistical models that showed which species could survive under which conditions — at which temperatures, for example — and figuring out how much oceanic area would thus be habitable for corals. However, that work on its own isn’t enough to predict future expansion, Vogt-Vincent said.

“Just because an environment is habitable for corals doesn’t mean you’re suddenly going to get big coral communities forming there,” he said.

Vogt-Vincent and his team built a mechanistic model that predicts where coral cover will occur based on factors such as temperature, light intensity and pH. To test the model’s validity, they looked at how it predicts current global coral cover. They found that the model is unsuitable for fine-grained regional analysis but does a “remarkable” job of determining what the range limits for corals are, Vogt-Vincent said. So they set about using the model to predict the future.

The model found that, under a “middle of the road” emissions scenario broadly in keeping with current climate policies, coral reefs would decline by 71% by 2100, while under a low-emissions scenario broadly in keeping with Paris Agreement commitments, they would decline by 34%. (In both cases, the degree of decline for individual coral colonies, including those in non-reef assemblages, is a bit lower.)

Unlike some of the statistical models used in other studies, the model used in this study predicted very little coral reef range expansion over the next 75 years: less than 25 square kilometers (about 10 square miles). That would make up only a tiny fraction of the coral reef decline over that period.

Vincent-Vogt’s team expected the main barrier to expansion to be larval dispersal — corals can’t simply move to a new location the way fish can, and their larvae are at the whims of ocean currents. But the model found the biggest problems, in fact, were lack of light intensity at higher latitudes and slow growth in marginal environments where, for example, temperature fluctuates more than it does in the tropics, even when the average temperature is suitable.

Stephen Palumbi, a professor of marine biology at Stanford University who wasn’t involved with the study, said the work drew attention to a broader point about climate change.

“[T]his is a good illustration of why, from an ecological perspective, the biggest problem with global warming isn’t the amount of warming, it’s the speed of warming,” Palumbi told Mongabay in an email. “We know that a lot of species like certain trees won’t be able to keep up, and this paints a pretty dire picture for corals.”

Palumbi cited the saying that “‘all models are wrong, but some models are useful,’” saying it probably applied here.

“The true outcome will probably be fairly different from the modeled outcome, perhaps not so bad or perhaps even worse,” he said. “Despite the uncertainty, this paper serves as an important call to action.”

Vogt-Vincent suggested that the findings, in fact, provide a rationale for a more urgent call to action than the conclusions drawn by the authors of the coral reefs section of the 2018 IPCC report, which he said didn’t account for coral evolution in response to changing conditions. Vogt-Vincent’s finding provides more hope and more of an obvious rationale for mitigation than their conclusion that corals would be nearly eliminated at 2°C (3.6°F) of warming, he said.

“There’s a really big difference [in our model] between the predictions under current policies and the predictions under 2 degrees of warming,” he added. “And all that means is that any amount of warming that we manage to avoid is going to have a really positive effect on the future of coral reefs.”

Vogt-Vincent, who has a background in geology and likes to reference corals’ paleontological record when giving talks, said he was also skeptical of the “‘all the coral will be wiped out’ narrative” because corals have survived major changes to their conditions in the deep past.

Vogt-Vincent’s model predicts that corals will expand their range over centuries if emissions stop by 2100.

“Without a doubt, they’re going to be severely damaged by climate change, and the impacts on humans will be really, really bad as that goes along,” Vogt-Vincent said. “But we’re not going to see the complete loss of coral reefs.”

If the model is correct, roughly 30% of the planet’s current coral reef cover would still be in place in 2100, which he said was “sad” but “enough to maintain quite a lot of coral diversity.”

Palumbi said that it was “good news” that range expansions will happen over time.

“Many species may indeed survive, although they will pass through a bottleneck of low population sizes and scattered corals that don’t form proper reefs,” Palumbi said. “That will be a tough time for all of the fish and invertebrates that rely on coral reefs for habitat, and for the people [who] eat those fish and shellfish.”

Palumbi said managing emissions to reduce biodiversity loss is essential.

“Nature is inherently resilient, but humans have the ability to overwhelm that resilience if we are not careful.”

Banner image: Paddle boarders over a coral reef in Indonesia. Image by Grant Thomas / Ocean Image Bank. 

Scientists warn coral restoration can’t keep pace with global reef collapse

Citations:

Vogt-Vincent, N. S., Pringle, J. M., Cornwall, C. E., & McManus, L. C. (2024). Anthropogenic climate change will likely outpace coral range expansion. Science Advances. doi:10.1101/2024.07.23.604846

Yamano, H., Sugihara, K., & Nomura, K. (2011). Rapid poleward range expansion of tropical reef corals in response to rising sea surface temperatures. Geophysical Research Letters, 38(4). doi:10.1029/2010gl046474

Kalmus, P., Ekanayaka, A., Kang, E. L., Baird, M. E., & Gierach, M. M. (2022). Past the precipice? Projected coral habitability under global heating. Earth’s Future. doi:10.1002/essoar.10510201.2