Recurring extreme climate events are devastating to coral reefs

Climate change induces both gradual shifts, such as changes in mean temperature, and extreme climate events, such as heatwaves. Extreme climate events offer little or no opportunity for species to adapt or acclimatise to change. These events can be especially devastating for sessile species, such as corals, which do not have the option to move to cooler places and avoid unfavourable conditions.

In 2019, a group of Australian scientists published an article¹ that synthesised results from earlier studies with regards to the effects of repeated extreme climate events on marine habitat forming communities. The article included information on back-to-back marine heatwaves that struck the coral reef communities of the Great Barrier Reef in 2016–2017.

Marine heatwaves induce coral bleaching, since corals when under stress expel algae that normally live in their tissues. The authors state with regards to the 2016 event that “Bleaching of corals in 2016 was the most severe and extensive so far recorded”. The event coincided with the warmest sea water temperatures ever recorded in parts of the Central Western Pacific. In parts of the Great Barrier Reef, more than 60 percent of the corals experienced bleaching.

Another tragic record was set in 2017, when for the first time there was a consecutive second year of bleaching. This event overlapped spatially with large areas of the previous event, and caused bleaching in approximately 50 percent of the remaining corals.

Since recovery times are years to decades, frequent bleaching events suggest a grim future for these corals. The authors suggest that such events are “changing ecosystems in profound ways that in some cases are unlikely to be reversible”.

As extreme climate events already affect marine habitat forming communities (including not only corals, but also kelps, mangroves and seagrasses) along 45 percent of the Australian coast, the report on these events is truly alarming. The article consequently suggests that in addition to studying gradual shifts due to climate change, more attention should be given to extreme and abrupt events – also globally. The authors in their conclusions state that the impacts of extreme climate events on ecosystems “are likely to become more severe and extensive in the near future. Indeed, they are happening now, and based on this Australian analysis, may be more common globally than currently appreciated […].”

Unfortunately, the bad news for these corals has – in line with the scientists’ conclusions above – continued during this Australian summer. In February 2020, the highest monthly sea surface temperatures on record were observed for the Great Barrier Reef.

Scientific studies that fully describe the impact of this latest extreme marine heat event are yet to come. Already, however, the reefs have been surveyed from the air and underwater by the ARC Centre of Excellence for Coral Reef Studies and the Great Barrier Reef Marine Park Authority. Results of the survey have since been reported and commented on in several newspapers, such as the New York Times². According to the survey, the proportion of severely bleached corals was exceeded only during the event in 2016. Additionally, for the first time, severely bleached corals could be observed along the whole length of the reef.

This most recent bleaching event could be viewed as a confirmation of conclusions in the scientific article that synthesised information from earlier events. Indeed, the Director of the ARC Centre of Excellence for Coral Reef Studies, Professor Terry Hughes, told the New York Times that “the heat waves of recent years were creating a cumulative effect that was drastically altering the makeup of the reef”. Professor Hughes also said “We’re surprised by the pace of this acceleration in bleaching. We had a 14-year gap between 2002 and 2016, and now in five years we’ve had three severe events.”

If the future is not only grim for these corals, but is already here, then corals will remain at risk even if strong measures are taken to mitigate greenhouse gases, such as those in line with the Paris Agreement (see also https://airclim.org/acidnews/not-even-15%C2%B0c-good-enough). The need for protection is urgent. For an environmentalist, it is very obvious that corals deserve protection in their own right. No species should be threatened by extinction. In the case of corals, however, the implications for biodiversity reach far beyond this group of species alone. In fact, coral reefs host more than 83,000 other species! Although not all species of corals are equally sensitive to extreme temperatures, changes in the species composition of corals will have effects on biodiversity.

According to the interview with Professor Hughes, the corals most likely to die are species of root and branch corals, which are particularly important habitats for fish. The sturdiest types are dome-like corals, which have other important functions (e.g. protection against erosion), but are less important for fish. On that note, Professor Hughes stated that “This is a transition from high diversity and lots of species, to lower diversity, with fewer tougher species.”

The fate of coral reefs is an ecological and a biodiversity issue. In addition, it is to a large extent an issue with vast economic, social and political consequences, as entire ecosystems are dependent on them.

Ultimately, the key message for protecting species from extreme climate events (and ocean acidification, which also threatens coral reefs) is mitigation of greenhouse gases. Additionally, because the resilience of coral reefs is also compromised by factors such as construction, impaired water quality and overfishing, local protection and management measures remain vital.

Marko Reinikainen

1. Babcock et al. 2019: Severe continental-scale impacts of climate change are happening now: Extreme climate events impact marine habitat forming communities along 45% of Australia’s coast. Frontiers in Marine Science, vol. 6

2. https://www.nytimes.com/2020/04/06/world/australia/great-barrier-reefs-b...

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