What is tropicalization and how does it change coral reefs?

Written by Larissa Marques Pires Teixeira

In November 2025, representatives from approximately 195 countries met in Brazil during the 30^th Conference of the Parties (COP30) to the United Nations Framework Convention on Climate Change to discuss strategies and establish joint commitments for addressing environmental and climate challenges. For the first time, oceans were placed at the center of climate discussions [1], acknowledging the fundamental role of coastal and marine ecosystems in regulating the global climate.

Oceans are crucial for climate regulation as they capture approximately 30% of the carbon dioxide emitted globally [2] and absorb nearly 90% of the excess heat generated by global warming [3]. However, marine species are negatively affected by ocean warming. One of the most significant consequences of global warming is the increasing frequency, intensity, and duration of marine heatwaves, which can greatly reduce species productivity and alter their geographic distributions [4, 5]. Beyond these short-term extreme events, the long-term warming of ambient sea surface temperatures is driving persistent shifts in species distributions toward higher latitudes. Such poleward expansion of warm-affinity species leads to novel ecological interactions that alter ecosystem functioning, a process known as tropicalization [6].

Climate change and coral reefs

The Sixth Assessment Report from the Intergovernmental Panel on Climate Change identifies coral reefs as among the ecosystems most vulnerable to climate change, facing a high risk of species extinction [7] and marine bioinvasions [8]. On temperate reefs, the arrival of species typically dominant in tropical waters, such as algal mats, surgeonfish, and parrotfish, alters trophic interactions and transforms energy flow within these ecosystems [9].

The functional and structural consequences of tropicalization range from modifications in reef architecture to the loss of ecosystem functions and shifts in trophic dynamics [8, 9]. For instance, a reduction in trophic interactions on tropical reefs is predicted to occur as reef-associated fish migrate toward colder regions [10]. Because these ectothermic (cold-blooded) organisms cannot regulate their body temperature, they must therefore seek environments with suitable thermal conditions to survive, such as subtropical reefs.

Alteration of ecosystem services: the socioeconomic consequences of tropicalization

Coral reefs provide a wide array of ecosystem services, including tourism and recreation, coastal protection, fisheries, and cultural and aesthetic value, which generate approximately US$30 billion annually in direct economic benefits [11]. The tropicalization of reef environments can drive profound changes in these services, with significant socioeconomic implications. Indeed, ecological and evolutionary analyses have shown that increased poleward migration of tropical fish has led to higher overall fishery landings [8]. Similarly, the expansion of mangroves into temperate areas may enhance carbon sequestration and storage, as mangrove ecosystems capture more carbon than temperate salt marshes [8].

However, tropicalization can also drive species turnover—a shift in community composition caused by the extinction of native species and the arrival of new species from different regions—along with biodiversity loss and habitat degradation that can initiate a cycle of economic decline [8]. For example, even where total fish catch increases, newly tropicalized reefs may lose commercially important temperate species [8]. These changes can reduce long-term fisheries productivity and disproportionately affect vulnerable coastal communities [12], exacerbating social inequality and destabilizing traditional livelihoods.

Future perspectives

The future of coral reefs points toward an intensification of tropicalization in the coming decades, driven by continued long-term ocean warming. Although the most recent COP30 meeting formally recognized the critical role of the oceans in climate regulation, international agreements and proactive strategies aimed at safeguarding reef ecosystems under climate change are still insufficient. Addressing these governance and policy gaps is essential to ensure an equitable and resilient future for both marine ecosystems and the human communities that depend on them.

References

[1] United Nations Framework Convention on Climate Change. (n.d.). Ocean action under the UNFCCC. UNFCCC. January 12, 2026, from https://unfccc.int/topics/ocean/ocean-action-under-the-unfccc

[2] Gruber, N., Clement, D., Carter, B. R., Feely, R. A., Van Heuven, S., Hoppema, M., Ishii, M., Key, R. M., Kozyr, A., Lauvset, S. K., Monaco, C. L., Mathis J. T., Murata, A., Olsen, A., Perez, F. F., Sabine, C. L., Tanhua, T., Wanninkhof, R. (2019). The oceanic sink for anthropogenic CO₂ from 1994 to 2007. Science, 363(6432), 1193–1199. https://doi.org/10.1126/science.aau5153.

[3] IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. IPCC, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3–32. https://doi.org/10.1017/9781009157896.001.

[4] Oliver, E. C., Donat, M. G., Burrows, M. T., Moore, P. J., Smale, D. A., Alexander, L. V., Benthuysen, J. A., Feng, M., Sen Gupta, A., Hobday, A. J., Holbrook, N. J., Perkins-Kirkpatrick, S. E., Scannell, H. A. Straub S. C., Wernberg, T. (2018). Longer and more frequent marine heatwaves over the past century. Nature Communications, 9, 1324. https://doi.org/10.1038/s41467-018-03732-9.

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[6] Poloczanska, E. S., Burrows, M. T., Brown, C. J., García Molinos, J., Halpern, B. S., Hoegh-Guldberg, O., Kappel, C. V., Moore, P. J., Richardson, A. J., Schoeman, D. S., Sydeman W. J. (2016). Responses of marine organisms to climate change across oceans. Frontiers in Marine Science, 3, 62. https://doi.org/10.3389/fmars.2016.00062.

[7] IPCC, 2023: Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 1-34, doi: 10.59327/IPCC/AR6-9789291691647.001

[8] Zarzyczny, K. M., Rius, M., Williams, S. T., & Fenberg, P. B. (2024). The ecological and evolutionary consequences of tropicalisation. Trends in Ecology & Evolution, 39(3), 267–279. https://doi.org/10.1016/j.tree.2023.10.006. 

[9] Pessarrodona, A., Vergés, A., Bosch, N. E., Bell, S., Smith, S., Sgarlatta, M. P., & Wernberg, T. (2022). Tropicalization unlocks novel trophic pathways and enhances secondary productivity in temperate reefs. Functional Ecology, 36(3), 659–673. https://doi.org/10.1111/1365-2435.13990. 

[10] Inagaki, K. Y., Pennino, M. G., Floeter, S. R., Hay, M. E., & Longo, G. O. (2020). Trophic interactions will expand geographically but be less intense as oceans warm. Global Change Biology, 26(12), 6805–6812. https://doi.org/10.1111/gcb.15346.

[11] Cesar, H., Burke, L., & Pet-Soede, L. (2003). The economics of worldwide coral reef degradation. Retrieved January 12, 2026, from https://www.icran.org/wp-content/uploads/2022/11/cesardegradationreport.pdf

[12] Cinner, J. E., Caldwell, I. R., Thiault, L., Ben, J., Blanchard, J. L., Coll, M., Diedrich, A., Eddy, T. D., Everett J. D., Folberth, C., Gascuel, D., Guiet, J., Gurney, G. G., Heneghan, R. F., Jägermeyr, J., Jiddawi, N., Lahari, R., Kuange, J., Liu, W., Maury, O., Müller, C., Novaglio, C., Palacios-Abrantes, J., Petrik, C. M., Rabearisoa, A., Tittensor, D. P., Wamukota, A., Pollnac, R. (2022). Potential impacts of climate change on agriculture and fisheries production in 72 tropical coastal communities. Nature Communications, 13, 3530. https://doi.org/10.1038/s41467-022-30991-4.