Climate change is threatening coral reefs even more than previously thought. A study shows that even areas that have been able to largely compensate for previous heat waves will disappear if temperatures continue to rise. Without these thermal refuges, more than 90 percent of existing coral reefs will die from heat stress. In order to protect the corals, it would therefore be necessary to limit the rise in global average temperatures to well below the 1.5 degrees agreed in the Paris Agreement.
Coral reefs represent a unique ecosystem. Built by tiny cnidarians that grow only a few millimeters per year, they form the largest man-made structures on earth. They provide a habitat for numerous marine animals, protect the coasts and ensure the livelihood of around half a billion people who benefit from income from fishing and tourism. The corals are threatened on the one hand by local factors such as marine pollution, on the other hand by the global rise in water temperatures and the acidification of the seas as a result of increased CO2 levels.
Slow recovery from heat damage
A team led by Adele Dixon from the University of Leeds in the UK has now investigated the extent to which the world’s current coral reefs are at risk as climate change progresses. To do this, they used the latest climate models from the Intergovernmental Panel on Climate Change (IPCC) and combined them with high-resolution satellite measurements that record the surface temperatures of the oceans worldwide with an accuracy of one kilometer.
The researchers placed a special focus on so-called thermal refugia. These are sea areas that offer good temperature conditions for corals despite globally rising sea temperatures, for example because local currents bring colder water up from the depths. In this way, they can cushion heat waves that would otherwise cause the corals to die as a result of thermal stress. “After a heat wave, it takes at least ten years on average for coral communities to reestablish themselves,” the researchers explain. They therefore counted areas in which such deadly heat waves are likely to occur less frequently than once in ten years in the future as thermal refuges.
The refuges are dwindling
Currently, 84 percent of all coral reefs are in thermal refugia, where they have enough time to recover after a heat wave. “However, if global temperatures rise to 1.5 degrees above pre-industrial levels, only 0.2 percent of coral reefs will have enough time to recover before the next heat wave hits,” the researchers warn. “With a temperature rise of two degrees above pre-industrial levels, the thermal refugia will disappear altogether.”
“Our finding underscores the fact that there is no safe limit to global warming for coral reefs,” says Dixon. Even 1.5 degrees Celsius still means significant warming for the ecosystems that are at the forefront of climate change.” Coral reef refugia would disappear in Australia, Brazil, the Caribbean, East Asia, the East Pacific, Fiji, Hawaii, the Indian Ocean, the Persian Gulf and the Red Sea. Only in the Coral Triangle in the western Pacific and in Polynesia would small refuge areas be preserved if the 1.5 degree target was met.
New conservation efforts required
“This analysis confirms that action to reduce greenhouse gas emissions is urgent and needs to be taken within this decade, but we also need to step up local management actions to help reefs survive the predicted impacts,” says co-author Scott Heron from James Cook University in Australia. Previous protective measures often focus on preserving existing refuges and protecting them by restricting fishing and tourism. This still makes sense, but could only be a short-term solution given rising sea temperatures, as the refugia will disappear in the long term despite local conservation efforts, the researchers said.
“Instead, to ensure the survival of coral reefs, adaptation to higher temperatures must be encouraged and migration facilitated,” they write. In their analysis, they also identified regions where the corals are already exposed to large temperature fluctuations and have adapted to them, such as in the eastern Pacific. “These are the most promising candidates for survival by adaptation,” say the researchers. “The assisted evolution and translocation of heat tolerant corals needs further research, particularly for the coral reef regions that are projected to lose all thermal refuges with warming of 1.5 degrees Celsius.”
Source: Adele Dixon (University of Leed, UK) et al., PLOS Climate, doi: 10.1371/journal.pclm.0000004