Over the past century, the sea surface temperature (SST) has increased by almost 1 °C and is predicted to continue increasing approximately 1-2 °C every century. Increasing SST has become a growing concern for coral reefs, especially for the Great Barrier Reef. Extended warm periods not only induce coral bleaching events but also hurt the reef’s ability to protect itself from such events, ultimately threatening its sustainability.
What is Coral Bleaching?
Coral bleaching is a stress response that occurs when the SST exceeds the thermal tolerance of corals and zooxanthellae, the symbiotic algae that live in coral tissues. This causes corals to expel the zooxanthellae and results in the corals losing pigment and turning completely white. Reefs can survive bleaching events, but cell death is often the response to the stress they experience.
How does the Great Barrier Reef protect itself from bleaching?
Over the past few decades, the Great Barrier Reef has employed certain pre-bleaching mechanisms to increase physiological preparation for imminent rising temperatures. These mechanisms expose corals to an SST above the average monthly SST before they are exposed to the temperatures that cause bleaching. This allows corals to acquire a higher degree of thermal tolerance, meaning that they are able to tolerate slightly higher temperatures with less of an impact from bleaching.
Three different thermal trajectories are involved in pre-bleaching events on the Great Barrier Reef, each of which produces a distinct warming period to prepare coral for future temperatures. The first trajectory—the “protective trajectory”—has predominated throughout the reef for the past 27 years. Corals exposed to this trajectory acquire the thermal tolerance necessary to reduce the effects of bleaching. However, the two other thermal trajectories—the “single bleaching trajectory” and the “repetitive bleaching trajectory”—do not allow corals to acquire a high amount thermal tolerance. Their gene expression profiles are associated with increased levels of cell death and symbiotant loss. They therefore do not offer as great preparation for increasing temperatures or protection from bleaching events.
How is climate change threatening the reef?
As SST continues to increase, the pre-stress events that best protect the coral are likely to begin disappearing. With an increase in only 0.5 °C (which is estimated to occur within the next four decades), reefs will begin to experience more single and repetitive bleaching trajectories than protective trajectories. By 2100, the number of events that experience a protective trajectory is expected to fall from 75% to 22%, while the number that experience a single bleaching trajectory will increase from 21% to 71%. This suggests that climate change will not only increase the number of bleaching events (because higher temperatures will lead to more thermal stress), but also that coral bleaching events will become far more lethal as reefs lose their primary protective mechanism—the protective trajectory—and adopt the single bleaching trajectory, a mechanism that is significantly less effective in protecting against bleaching events.
Ainsworth, Tracy D. “Climate Change Disables Coral Bleaching Protection on the Great Barrier Reef.” Science 352.6283 (2016): 338-42. Web. <http://science.sciencemag.org/content/352/6283/338>.
High-Guldberg, Ove. “Climate Change, Coral Bleaching and the Future of the World’s Coral Reefs (Abstract).” CSIRO Publishing. N.p., n.d. Web. 24 Apr. 2016.
“What Is Coral Bleaching?” NOAA. N.p., n.d. Web. 24 Apr. 2016.
Caroline Marone ’17