Posted by: JeffMasters, 4:50 PM GMT on January 07, 2011
Record warm ocean temperatures across much of Earth's tropical oceans during the summer of 2010 created the second worst year globally for coral-killing bleaching episodes. The warm waters, fueled in part by the El Niño phenomena, caused the most coral bleaching since 1998, when 16 percent of the world's reefs were killed off. "Clearly, we are on track for this to be the second worst (bleaching) on record," NOAA coral expert Mark Eakin in an interview last month. "All we're waiting on now is the body count." The summer 2010 bleaching episodes were worst in Southeast Asia, where El Niño warming of the tropical ocean waters during the first half of the year was significant. In Indonesia's Aceh province, 80% of the bleached corals died, and Malaysia closed several popular dive sites after nearly all the coral were damaged by bleaching. However, in the Caribbean's Virgin Islands, coral bleaching was not as severe as experienced in 2005, according to National Park Service fisheries biologist Jeff Miller. I'll discuss the reasons for this in a future blog post. In other portions of the Caribbean, such as Venezuela and Panama, coral bleaching was worse than that experienced in 2005.
Figure 1. 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
What is coral bleaching?
Coral bleaching is a whitening of the corals that occurs when stresses such as high water temperatures, increased water acidity, or pollution disturbs the symbiotic relationship between the corals and the algae that live inside them. Bleaching episodes occur when ocean temperatures rise above 85 - 87°F (29.5 - 30.5°C.) Peak warming events took place in the western Indian Ocean and north-western Pacific in 1997/98, in the north of Australia and central Pacific during 2003/04, and in the Caribbean in 2005. About half of the reefs affected by bleaching in these episodes have recovered, and one recent study cautions that non-lethal bleaching episodes and subsequent recovery of corals is often under-reported.
Australia's Great Barrier Reef at risk
With summer now in full swing in the Southern Hemisphere, coral bleaching concern now shifts to Australia's Great Barrier Reef. Ocean temperatures along the reef are currently up to 1°C above average, due, in part, to the current moderate to strong La Niña event. NOAA's Coral Reef Watch has issued its highest level of coral bleaching alert for the northern 2/3 of the Great Barrier Reef, since the La Niña event is predicted to persist into at least April. Also of concern is the tremendous run-off occurring in the wake of the record flooding that has affected the neighboring Australian province of Queensland. While the floods have now peaked and the rivers of Queensland are now falling, the $5 billion disaster dumped a large amount of sediments, pollutants, fertilizers, and pesticides into the southern portion of the Great Barrier Reef, and this will act to increase the stress on the corals. However, the floods may end up indirectly benefiting some portions of the Great Barrier Reef. The cloud cover and strong winds that accompanied the flooding rain storms also acted to cool the waters along the reef. According to an analysis I did of the UK Met Office Hadley Centre global ocean temperature data, sea surface temperatures along the southern portion of the reef, between 15°S and 20°S latitude, were the warmest ever for September, 1.27°C above average. These waters cooled significantly, relative to average, during October and November, and were just 0.12°C warmer than average during November. Cooler waters will mean less potential for coral bleaching, though the pollution in the flood run-off water may end up killing some corals.
Figure 2. Forecast stress on coral due to warm ocean temperatures for Australia, Jan - Apr 2011. The northern 2/3 of the Great Barrier Reef are under the highest alert level for coral bleaching. Waters are cooler along the southern portion of the reef, due, in part, to the storms that have brought record flooding to portions of Queensland, Australia. Image credit: NOAA Coral Reef Watch.
Long term outlook for world's coral reefs: grim
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. 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 since 1950. 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 loss has been the most severe in Earth's hottest ocean, the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles. Global warming has heated up most of the tropical ocean surface waters by about 0.5°C (0.9°F) over the past 50 years, and the remarkable bleaching episodes of 1998 and 2010 both occurred when strong (natural) El Niño episodes heated up Pacific tropical waters to record levels. If the Earth continues to heat up this century as expected, coral bleaching episodes will grow more frequent and intense, particularly during strong El Niño episodes. The twin stresses of ocean acidification and increasing ocean temperatures will probably mean that by 2050, it will be difficult for any coral reefs to recover when subject to additional stresses posed by pollution or major storms, according to a talk presented by Stanford climate scientist Ken Caldeira at last month's American Geophysical Union (AGU) meeting.
Figure 3. Departure of sea surface temperature in the Australian region over the past one hundred years, year-by-year (red line), and decade-by-decade (grey bars.) The 2010 value is preliminary and does not include data for December 2010. If ocean temperatures and ocean acidity continue to rise in Australian waters at the same pace as has occurred over the past 100 years, the Great Barrier Reef will be in significant danger by 2050. Image credit: Australian Bureau of Meteorology.
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 last year: "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.
"You may well feel that dire predictions about anything almost always turn out to be exaggerations. You may think there may be something in it to worry about, but it won't be as bad as doomsayers like me are predicting. This view is understandable given that only a few decades ago I, myself, would have thought it ridiculous to imagine that reefs might have a limited lifespan on Earth as a consequence of human actions. It would have seemed preposterous that, for example, the Great Barrier Reef--the biggest structure ever made by life on Earth--could be mortally threatened by any present or foreseeable environmental change. Yet here I am today, humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for our children's children to enjoy unless we drastically change our priorities and the way we live."
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.
I might add that not only are reefs the ocean's canaries, they are incredibly valuable in their own right. According to the World Meteorological Organization, coral reefs provide economic services--jobs, food and tourism--estimated to be worth $30 billion per year. NOAA put this figure at twelve times higher, $375 billion each year. Corals cover just 0.2% of the world's oceans, but contain about 25% of all marine species.
Check out wunderground weather historian Christopher C. Burt's post on the notable weather extremes of December 2010. It was truly an extreme month!