Record Ocean Temperatures Threaten Hawaii's Coral Reefs

Record warm sea surface temperatures in Hawaii's waters threaten to bring a second consecutive year of record coral bleaching to their precious coral reefs this summer. According to NOAA, ocean temperatures in the waters near and to the south of the Hawaiian Islands were 1 - 2°C (1.8 - 3.6°F) above average in June, which was the warmest these waters have been since record keeping began over a century ago. With the waters surrounding Hawaii expected to warm to their highest values of the summer by September, and likely remain 1 - 2°C above average, NOAA's Coral Reef Watch has placed the islands under a Coral Bleach Watch, and their experimental coral bleaching forecast gives a 50 - 90% chance that Hawaii will experience "Level 2" thermal stress this summer--the highest category of danger, likely to result in widespread coral bleaching and mortality. The record warm ocean temperatures are due to a strong El Niño event that is pushing large amounts of record-warm water into the Central Pacific, in combination with the steady rise in ocean temperatures due to global warming. Mass coral die-offs commonly occur during strong El Niño events; the United Nations Environmental Program found that 16% of the worlds coral reefs were effectively lost during a nine-month coral bleaching episode associated with the 1997 - 1998 record-strength El Niño event. With this year's El Niño event likely to be almost as strong as the 1997 - 1998 one, coral reefs are going to take a beating again.


Figure 1. NOAA's experimental coral bleaching forecast gives a 50 - 90% chance that Hawaii will experience Satellite Bleaching Alert Level 2 thermal stress this summer--the highest category of danger, likely to result in widespread coral bleaching and mortality. El Niño conditions have produced an extremely warm band of water from the central equatorial Pacific to the South American coast, and Level 2 thermal stress has already been reached in the Gilbert, Phoenix, and Northern Line Islands in Kiribati, as well as in Micronesia, the Howland and Baker Islands, and to the east in the Galápagos.


Figure 2. Unusually warm waters are also in place along the northern coast of Cuba and in the Bahamas, and NOAA's experimental coral bleaching forecast gives about a 70% chance coral reefs in these waters will experience Satellite Bleaching Alert Level 2 thermal stress this summer--the highest category of danger, likely to result in widespread coral bleaching and mortality.

Second consecutive year of severe coral bleaching in Hawaii
Hawaii's reefs are already reeling from their worst coral bleaching event in recorded history in 2014, when record warm ocean temperatures caused 50 - 70% of the corals sampled in Northeast Oahu's Kāneʻohe Bay to bleach. When the sea surface temperature is 1°C warmer than the highest monthly mean temperature corals usually experience, coral polyps will expel the symbiotic algae that live in their tissues, exposing the white skeleton underneath, resulting in a white "bleached" appearance. Death can result if the stress is high and long-lived. In Hawaii's waters, corals cannot tolerate water temperatures above 83°F (28.3°C) for multi-week periods without suffering bleaching. Corals typically recover from mild bleaching, gradually recovering their color by repopulating their algae. However, if the bleaching is severe or prolonged, individual polyps or whole colonies will die. With Hawaii likely to undergo a second consecutive year of record warm waters and coral bleaching in 2015, widespread mortality in many of Hawaii's coral reefs is possible, particularly around the Big Island.


Figure 3. During June 2015, the ocean areas near and to the south of Hawaii were at their warmest levels since record keeping began in 1880. Image credit: National Centers for Environmental Information (NCEI) .

Could a hurricane help?
When hurricanes and tropical storms churn the waters, they upwell large amounts of cooler waters from the depths that can cool the surface waters, potentially reducing the thermal stress on coral reefs. The heavy rains from the storm can also potentially cause cooling. This occurred in the Virgin Islands in 2010, when Hurricane Earl and Hurricane Otto helped relieve a potentially dangerous coral bleaching episode (Figure 4.) So, should Hawaii hope for a hurricane this September to help save its coral reefs? Well, be careful what you wish for. Hurricanes cause damage to reefs. Following Tropical Storm Iselle, which hit the Big Island of Hawaii on August 7, 2014, with 60 mph winds, researchers at the University of Hawaii, Hilo documented that one coral reef on the Big Island (the Wai`ōpae tide pools) suffered physical damage from pounding waves that broke up to 18% of the coral colonies of one species of coral with long slender branches--cauliflower coral. Other corals suffered lesser damage, 0 - 10% breakage. In addition, these corals were subjected to sewage contamination due to damage of cesspools and septic tanks.


Figure 4. Mean daily temperatures (on left Y-axis) with standard error collected at reef-depth (26'-58') monitoring sites on St. John US Virgin Islands, and total daily rainfall (on right Y-axis) from August through October 2010 from Newfound, Haulover, Tektite, Yawzi and Mennebeck Reefs in the Virgin Islands. Image credit: Rafe Boulon, Resource Management Chief, Virgin Islands National Park.

Long term outlook for world's coral reefs: bleak
The large amount of carbon dioxide humans have put into the air in recent decades has done more than just raise Earth's global temperature--it has also increased the acidity of the oceans, since carbon dioxide dissolves in sea water to form carbonic acid. The oceans are acidifying faster than at any time in the past 300 million years. Corals have trouble growing in acidic sea water, and the combined effects of increasing ocean temperatures, increasing acidity, pollution, and overfishing have reduced coral reefs globally by 19 percent between 1950 - 2008. Another 35 percent could disappear in the next 40 years, even without the impact of climate change, according to a report released in October 2010 by the World Meteorological Organization and the Convention on Biological Diversity.

Coral expert J.E.N. Veron, former chief scientist of the Australian Institute of Marine Science, had this to say in an excellent interview he did with Yale Environment 360 in 2010: "the science is clear: Unless we change the way we live, the Earth's coral reefs will be utterly destroyed within our children's lifetimes...Reefs are the ocean's canaries and we must hear their call. This call is not just for themselves, for the other great ecosystems of the ocean stand behind reefs like a row of dominoes. If coral reefs fail, the rest will follow in rapid succession, and the Sixth Mass Extinction will be upon us--and will be of our making."


Figure 5. An example of coral bleaching that occurred during the record-strength 1997-1998 El Niño event. Image credit: Craig Quirolo, Reef Relief/Marine Photobank, in Climate, Carbon and Coral Reefs

Jeff Masters