WASHINGTON, Dec. 22 (UPI) --
Global temperatures are the highest they've been in modern history. Predictably, the ocean isn't far behind. And with global warming continuing to push both air and ocean temperatures higher and higher, scientists are predicting one of the worst coral bleaching episodes in 20 years.
When coral expels the algae-like protozoa that live in and on their tissue -- known as zooxanthellae -- it is called bleaching. Healthy coral is reliant on the unicellular organism for both the pigmentation it provides as well as its photosynthetic abilities. When coral expels its zooxanthellae it doesn't die, but it becomes much more vulnerable to other environmental stressors and susceptible to disease. It also becomes unattractive to fish.
Because coral reefs act as a sort of ecological backbone for the oceans' many, many food chains -- serving as both food and shelter for a wide variety of smaller fish and other marine creatures -- their health is vital to sustainable biological diversity.
But as waters warm and become more acidic, bleaching events are becoming larger and more frequent.
As the ocean becomes more acidified, the bleaching threshold for corals drops, more carbon dioxide makes corals more sensitive to thermal stress,
Mark Eakin, head of National Oceanic and Atmospheric Administration's Coral Reef Watch program, told Mashable in a recent interview. Not only are we seeing more thermal stress ... but we're making them more sensitive at the same time.
Historically, large bleaching events only happen during El Niño events, a periodic warming phase in the Pacific that occurs every two to seven years -- lasting several months to a year and setting off a chain reaction of atmospheric and climatological events. But with ocean temperatures nearing all-time highs (since record-keeping began in 1880), it's becoming easier and easier for warming events to set off a domino effect of coral bleaching events.
We're seeing a rising background temperature, we're seeing this increase in the thermal content of the oceans, and as that happens it doesn't take as nearly as big of an event to set off this chain of bleaching,
Eakin said.
Scientists have already witnessed Kiribati, Tuvalu, the northern Marianas Islands, Guam, Hawaii and Florida; and researchers are warning that large bleaching events are expected throughout the Pacific Islands and along the coasts of Australia.
The last massive bleaching episode, the worst in modern history, occurred in 1998 when global warming and a particularly large El Niño event triggered coral colonies across the globe to send their zooxanthellae packing. That event killed some 15 percent of the world's coral. This year's ongoing episode is expected to keep getting worse for the next several months.
Many coral reef scientists are expecting something similar to 1997-98 to unfold in the next six to 12 months,
Ove Hoegh-Guldberg, a marine biology professor at the University of Queensland and coral reef expert, told The Guardian.
Corals typically recover from bleaching, asking zooxanthellae to move back in with them -- so to speak. But they need time to heal in the wake of these stressful episodes, and with waters warming and bleaching events more frequent that's becoming increasingly difficult.