idw – Informationsdienst Wissenschaft
Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
New study by researchers from Kiel University reveals underestimated threat to Indonesia’s coral reefs from cold and heat stress
The Indonesian seas are a biodiversity hotspot, harboring the highest coral diversity in the tropics and home to an extraordinary variety of marine life. Yet these unique ecosystems have been under growing pressure for years, particularly from increasing heatwaves in the ocean. However, heat stress is not the only stressor affecting these sensitive marine creatures. A new study led by researchers of Kiel University (CAU) together with, the KIKAI Institute for Coral Reef Sciences (Japan), and the Indonesian Research Institute (BRIN) now shows that extreme cold events can also cause coral bleaching. These events are often more intense than heatwaves and rival the negative records set by heatwaves. The researchers have systematically investigated the consequences of both extremes for Indonesia’s coral reefs over several decades for the first time. Their findings were published in the journal Geophysical Research Letters.
“Our findings show that we need to reassess stress factors affecting corals. Rising temperatures are not the only threat; unusually cold water can also cause massive damage to coral reefs,” explains lead author Dr. Takaaki K. Watanabe from the Institute of Geosciences at Kiel University.
Cold as a previously underestimated stress factor
In their new study, the international research team analyzed both heat and cold stress in Indonesian waters over the past forty years. Heat stress events, triggered by El Niño, have increased significantly during the study period and affect larger ocean areas. Cold stress, on the other hand, occurs primarily along the coasts of Sumatra and Java when a so-called positive Indian Ocean Dipole brings cold deep water to the surface. Although these cold events are more spatially limited, they often reach higher intensities and last on average about 20 days longer than heat events.
Corals are sensitive to temperature fluctuations
Tropical coral reefs are the ecosystem most severely affected by rising temperatures. Corals live in symbiosis with single-celled algae, which provide them with nutrients through photosynthesis and give them their characteristic coloration. If the water temperature deviates by at least 1 degree Celsius from the average summer temperature over several weeks, the corals expel these algae. The result is bleaching: without their algae, the corals starve and die.
However, unusually cold water also has consequences for the sensitive ecosystems in Indonesia’s seas. The cold stress levels identified in the study reached intensities comparable to the most extreme heat events documented to date — including the one off the coast of Florida in 2023, which led to the functional extinction of certain coral species there. “What is surprising is that these extreme cold events are nearly equivalent in intensity to extreme heat waves. They occur in regions that were previously considered refuges, as the bleaching caused by cold water has largely been overlooked until now,” says Professor Miriam Pfeiffer, head of the Paleontology and Historical Geology research group and spokesperson of the DFG Priority Program Tropical Climate Variability and Coral Reefs (SPP 2299).
Climate Change Amplifies Impacts on Coral Reefs
Tropical coral reefs suffer particularly in years when multiple climate phenomena occur in succession. Strong El Niño events, followed by a negative Indian Ocean Dipole, can significantly prolong and intensify periods of heat stress. Such combinations have already been linked to widespread coral bleaching, most recently in 2016, the worst heat stress event in the region since records began. Global climate change further exacerbates this dynamic. Previous studies have shown that the tipping point for coral reefs is already exceeded at 1.5 degrees Celsius of global warming. “We don’t yet know how cold stress events will develop in the future. To assess whether they will decrease or intensify, we need even more reliable long-term data,” says Takaaki K. Watanabe, who is also conducting research in the priority program.
Protected Regions for Coral Reef Conservation
In addition to the risks, the study shows that regions such as the Karimata and Makassar Straits are comparatively well protected from extreme temperature fluctuations due to their complex ocean currents. These straits in the Indonesian archipelago could play a key role in the recovery of damaged reefs as so-called thermal refuges. They serve not only as refuges but also as sources of coral larvae, which could contribute to the repopulation of endangered reefs throughout the region via ocean currents.
“These refuges are crucial for the preservation of these valuable ecosystems. Their targeted protection could help ensure that more severely affected reefs are repopulated and recover in the long term,” says Takaaki K. Watanabe, looking to the future.
Pictures/Materials to download:
https://www.uni-kiel.de/de/pressemitteilungen/2026/075-korallenbleiche1.png
Healthy coral reefs, such as those found here in the Indonesian seas, are biodiversity hotspots; however, they are increasingly exposed to stressors such as heat and cold events, which could be further exacerbated by climate change.
© Takaaki K. Watanabe, Kiel University
https://www.uni-kiel.de/de/pressemitteilungen/2026/075-korallenbleiche2.jpg
Corals bleach and die when exposed to unusual stresses affecting ecosystems over extended periods of time. Researchers at Kiel University have now systematically investigated this phenomenon in the Indian Ocean.
© Saori Ito, Kiel University
Further information:
About the Paleontology and Historical Geology Research Group at Kiel University
https://www.palaeontologie.ifg.uni-kiel.de/en?set_language=en
About the Priority Program Tropical Climate Variability & Coral Reefs (SPP 2299)
https://www.spp2299.tropicalclimatecorals.de/
Press contact:
Friederike Balzereit
Science Communication Kiel Marine Science (KMS), Kiel University
fbalzereit@uv.uni-kiel.de
+49 (0) 431/880-3032
About Kiel Marine Science
Kiel Marine Science (KMS), the Center for interdisciplinary marine science at Kiel University, is the centre for interdisciplinary marine sciences at Kiel University (CAU). KMS is the organisational unit for all researchers in the natural sciences, the humanities and the social sciences who are concerned with oceans, coasts and their impact on humanity. The expertise of the groups comes from fields, such as climate research, coastal research, physical chemistry, botany, microbiology, mathematics, computer science, economics or from law, business and social sciences. In total, KMS is comprised of over 70 working groups in seven faculties and over 26 institutes. Together with actors outside of science, they work worldwide with a transdisciplinary approach on solutions for a sustainable use and protection of the ocean.
https://www.uni-kiel.de/en/research/priority-research-areas/kiel-marine-science
Dr. Takaaki K. Watanabe
Institute of Geosciences
Research Group Paleontology and Historical Geology
takaaki.watanabe@ifg.uni-kiel.de
Prof. Dr. Miriam Pfeiffer
Institute of Geosciences
Research Group Paleontology and Historical Geology
miriam.pfeiffer@ifg.uni-kiel.de
Watanabe, T. K., Ito, S., Nurhidayati, A. U., Cahyarini, S. Y., & Pfeiffer, M. (2026). Coral reefs in the Indonesian seas threatened by heat and cold stress. Geophysical Research Letters, 53, e2025GL121003
https://doi.org/10.1029/2025GL121003
https://www.uni-kiel.de/en/details/news/075-korallenbleiche
Criteria of this press release:
Journalists, Scientists and scholars
Biology, Environment / ecology, Geosciences, Oceanology / climate
transregional, national
Research results
English
You can combine search terms with and, or and/or not, e.g. Philo not logy.
You can use brackets to separate combinations from each other, e.g. (Philo not logy) or (Psycho and logy).
Coherent groups of words will be located as complete phrases if you put them into quotation marks, e.g. “Federal Republic of Germany”.
You can also use the advanced search without entering search terms. It will then follow the criteria you have selected (e.g. country or subject area).
If you have not selected any criteria in a given category, the entire category will be searched (e.g. all subject areas or all countries).
