Tropical coral reefs are under pressure due to global warming. The colorful habitats are becoming increasingly bleached, the plants and animals who live there are going elsewhere or dying off, and recovery is becoming increasingly difficult due to the frequent marine heat waves. A global bleaching event has been in progress since 2023 – the fourth one since 1998. According to the International Coral Reef Initiative, 84 percent of tropical and subtropical reefs are currently affected. These numbers show how vulnerable coral reefs are to rising sea temperatures, and highlight the need for research on this topic.

Since 2022, the “Tropical Climate Variability & Coral Reefs” (SPP 2299) DFG Priority Programme has been investigating how coral reefs respond to stress caused by high water temperatures and why some populations seem to be more resilient. Thomas Felis, geoscientist at the University of Bremen’s MARUM – Center for Marine Environmental Sciences, had the idea for the program with geoscientist Miriam Pfeiffer from the University of Kiel, and has been coordinating the network since 2022. Throughout Germany, 42 researchers from 15 different institutions work together in the network. One of the leading hypotheses is that the corals that have so far been less likely to bleach may have previously experienced heat stress and have adapted over time.

Calcium Carbonate Layers Document Ocean Temperatures and Environmental Conditions

To test this hypothesis, researchers have developed timelines of temperature developments in different reefs and have analyzed the chemical composition of coral skeletons. As part of this process, they extracted cores from coral colonies – a procedure that makes the environmental history of a reef comprehensible.

The fine annual layers of corals become visible with the help of X-rays. Similar to the rings of a tree trunk, the layers appear in lighter and darker shades over the course of the year. “These yearly bands are on average one centimeter thick and provide a calendar that documents the environmental conditions,” says Felis. “We can track a given temperature on a monthly, and sometimes even weekly, basis.”

In a second step, small holes are drilled into the coral core and lime powder samples are taken and then analyzed in a lab. This creates temperature timelines that can go back several centuries. In addition to providing information about temperatures, chemical timelines also document information on salinity and pH values of the water, showing the effects of additional environmental influences such as local ocean acidification, pollutants, or an overabundance of nutrients – all further stress factors for corals.

How Corals Handle Heat Stress

Preliminary results after three years of research show fluctuating curves whose average has been increasing over the last decades and is repeatedly characterized by distinctive peaks. “The upward fluctuations are from years with an El Niño event,” says Felis. El Niño is a climate phenomenon that occurs every few years in the tropical Pacific Ocean, where winds and ocean currents temporarily shift and temperatures in the equatorial Pacific increase. “Given the higher water temperatures during El Niño, we are able to use the drill cores to reconstruct the corals’ response to heat stress,” says Felis.

The interactions between corals and the microalgae they host are vital for coping with heat stress. Corals have a symbiotic relationship with the microalgae that give them their characteristic blue color. The algae receive protection and in return provide their host with nutrients and energy. However, when the water becomes too warm, the algae produce toxins, and the coral then expels them. This results in the coral losing its color and turning a pale white. If water temperatures return to normal quickly enough, the algae are able to return and the corals recover.

“We have observed in aquarium experiments that after a bleaching event, different, more heat-tolerant algae may move in,” says Thomas Felis. This suggests that some corals may be able to adapt to changing conditions. However, chemical analyses have repeatedly shown that these changes are temporally or spatially limited, and the underlying causes of this are currently being investigated. Differences between sea regions play a major role. “We have conducted research on reefs in the Andaman Sea, west of Thailand, as part of our research at the University of Bremen,” Felis reports. “In this case, our research showed that a deep, natural current can cause cold water to rise up and temporarily cool reefs during marine heat waves. In this region, bleaching is more limited, not due to increased resistance, but because of regional conditions.”

Close Collaboration within the Team

Felis emphasizes the importance of close collaboration in the interdisciplinary DFG Priority Programme. Researchers from 16 countries and various fields of climate, environmental, and ecosystem research work closely together to gain a better understanding of tropical climate fluctuations and their effects on coral reef ecosystems.

“The reconstructed periods from the past to the present provide a crucial foundation for assessing the developments in the tropical marine climate. We have only been able to achieve this thanks to the interdisciplinary collaboration,” Felis summarizes at the end of the first funding period. “In the coming years, we want to deepen this understanding with the goal of helping to preserve at least a portion of the threatened coral reef ecosystems.”

In the first half of 2026, Miriam Pfeiffer from Kiel will take over the role of coordinator for the second funding period, which lasts an additional three years until 2028.

Further Information

The project website „Tropical Climate Variability & Coral Reefs“

Or on Instagram, Bluesky, and Mastodon – each with the handle @climatereefs