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In a published today in Science Advances, researchers at UH Hawaiʻi Institute of Marine Biology (HIMB) Marine Ecological Theory Lab reveal that coral reefs are creeping toward the poles in response to warming oceans, but the pace is too slow to beat the heat and escape impacts of climate change. The study also offers a hopeful alternative: immediate actions to reduce greenhouse gas emissions can significantly improve the future outlook for coral reefs globally.

"As the ocean warms, species tend to move poleward," explains Dr. Noam Vogt-Vincent, a NOAA Climate & Global Change Postdoctoral Fellow at HIMBʻs Marine Ecological Theory Lab and lead author of the study. "We know from the fossil record that coral reefs have previously expanded their ranges in response to past climate change, but we didn't know whether this was a matter of decades or millennia."

To predict changes in coral reef distributions, the team turned to sophisticated simulation models run on the University of Hawaiʻi's Koa supercomputer.

"It is impossible to investigate future coral range shifts entirely experimentally, because these shifts take place over very large distances and long timescales. A computer simulation approach is the only way to predict this," notes Vogt-Vincent.

The team designed a global simulation model that included roughly 50,000 coral reef sites, incorporating processes essential to coral reef health, such as how corals grow, disperse, evolve, and adapt to heat stress. They included experimental data and then tested three future emissions scenarios: low warming (~2℃ by 2100), moderate warming (~3℃), and high warming (>4℃).

"By modeling coral reefs globally and incorporating evolution and connectivity, this study provides an unprecedented long-term view of how these complex ecosystems will respond to climate change," says Dr. Lisa McManus, Assistant Research Professor at HIMB and Vogt-Vincent's postdoctoral advisor.

"We discovered that it takes centuries for tropical-type coral reefs to expand significantly beyond their current distribution," Vogt-Vincent shares of their findings. The implications of this discovery are profound.

"It was previously suggested that tropical coral species might be able to find 'refuge' in subtropical and temperate seas by expanding poleward and establishing new, higher latitude coral reefs. Unfortunately, while we've confirmed that coral reef range expansion will indeed eventually occur, the biggest coral losses are expected in the next 50 years, meaning these new, higher-latitude reefs won't form fast enough to save most tropical corals."

Places like northern Florida, southern Australia, and southern Japan might eventually see new reefs, but not soon enough to help many tropical coral species survive the 21st century.

But here's the good news: coral reefs are not doomed. The study shows that significant cuts in greenhouse gas emissions, like those outlined in the Paris Climate Agreement, could dramatically reduce coral loss. Instead of losing a further 70% of coral reefs, we could limit losses to around 30%. This finding underscores the critical importance of urgent human action.

"Our findings show that coral reefs are not a lost cause, but immediate and significant reductions in greenhouse gas emissions are essential for mitigating the damaging effects on our reefs," Vogt-Vincent says. "In fact, our study suggests that reductions in greenhouse gas emissions will not just improve coral reef futures this century, but for hundreds to thousands of years into the future. Our actions over the next few decades will therefore have incredibly long-lasting consequences for coral reefs globally."

HIMB's Marine Ecological Theory Lab will continue to harness supercomputer power to better understand the threats and possible solutions for coral reefs around the world. Future research directions include developing more sophisticated genetic simulations to understand coral larval dispersal. They are also exploring how naturally cooler parts of coral reefs might create local conditions for coral survival.

"I find it really exciting to be able to forecast what the world will look like decades, or even centuries, into the future," Vogt-Vincent emphasizes. "This study provides a source of hope, because it shows that there is still meaningful action that we can take to protect our beautiful coral reefs, but it will require ambitious, global efforts to reduce our greenhouse gas emissions."