Human Hothouse Spurs Longest Coral Die-Off on Record

23 February, 2016

The big coral die-off began in the Western Pacific as a massive ocean temperature spike built up during 2014. Back then, ocean heat accumulation had hit a very high ramp. A vicious, century-and-a-half long increase in atmospheric greenhouse gasses re-radiated greater and greater portions of the sun’s energy hitting the Earth — transferring the bulk (about 90 percent) to the world ocean system.

(A report out today from AGU finds that the world is now experiencing its longest coral die-off event on record. Image source: AGU.)

By 2015, as one of the strongest El Ninos on record began to extend its influence across the globe, a broad region stretching from the Western Pacific, through the Central Pacific and on into the Eastern Pacific and Caribbean were all experiencing mass coral die-offs. Into early 2016, die-off events again expanded taking in Australian waters and sections of the Indian Ocean off East Africa and Western India.

After 20 months of ongoing coral mortality, we are now in the midst of the longest coral die-off event on record — one of only four such events that the world has ever experienced.

The Fourth Major Coral Die-Off

Researchers have long known that corals are sensitive to changes in ocean temperature. Arise in ocean water readings by as little as 1 degree Celsius above average peaks over the period of a month can be enough to set off a life-threatening condition called a coral bleaching event. According to a recent report in AGU:

The bleaching, or whitening, occurs when the corals expel the symbiotic algae that live in their tissues. Without the algae, corals lose a significant source of food and are more vulnerable to disease. In a severe bleaching event, large swaths of reef-building corals die. This causes reefs to erode, destroying fish habitat and exposing previously protected shorelines to the destructive force of ocean waves.

The typical bleaching threshold for most corals tends to be in the range of 29-30 degrees Celsius or about 84-86 degrees Fahrenheit over an extended period. And with the world ocean surface approaching a range near 1 C above 1880s averages, this threshold is hit more and more frequently — putting corals at greater and greater risk.

(World Resources Institute Published the above video in 2012 as a survey of, then current, threats to global coral reef systems. By 2030, heating of the world ocean system, ocean acidification and global warming related dead zones will provide an extreme existential challenge to the world’s beautiful and diverse coral reef systems.)

Prior to the 1980s, widespread coral bleaching events were unheard of. Though isolated events occurred, the world ocean system had not yet warmed enough to put corals at major risk. However, by the 1980s global ocean temperatures had begun to rise into ranges at which peak ocean warming periods could put corals in the firing line for major, globe-spanning die offs.

The first such major, global coral die-off occurred during the, then record, 1982-1983 El Nino. At the time this event was unprecedented. And it held the dubious standing as the only such event until the 1997-1998 Super El Nino set off a similar, though longer-lasting mass die off. By the late 2000s, global ocean temperatures had again risen — hitting marks high enough to enable a weak 2010 El Nino to set off the third mass coral de-off.

The fourth mass die off began in 2014 prior to the most recent super El Nino — which has only exaggerated and lengthened its impact. It is now the longest lasting coral die-off ever recorded. And researchers expect it to continue on through at least much of 2016 and possibly into 2017.

Corals Entering a Period of Killing Heat

As the oceans are predicted to continue warming over the next few decades, corals are expected to come under ever-worsening stress. A recent report by the World Resources Institute (WRI) found that regions experiencing the current mass die-off were 70-90 percent likely to experience similar events at a frequency of once every two years by 2030. And a much larger region was expected to have a 50 to 70 percent risk of experiencing a bleaching event over a two year time-frame.

(World Resources Institute in 2012 found that mass coral bleaching and related die-off would occur with extraordinary frequency post 2030. Image source: The World Resources Institute.)

By the 2050s, under business as usual fossil fuel burning, WRI expects that much of the world’s temperate and tropical oceans would experience coral bleaching events bi-annually.

Taking this stark prediction into account we find that the threat to corals over the coming decades will eventually exceed El Nino periodicity and become common during most ocean climate states. The current, likely two year to 30 month, coral die off should serve as a warning for the worse and more frequent hits to corals that will, sadly, be stacking up over the coming decades. Eventually, mass coral die-offs in the continually warming world ocean will become continuous and ubiquitous unless the current trend somehow draws swiftly to a halt.

In addition, given an expanding ocean acidification proceeding southward from the poles and more and more widespread zones of ocean anoxia (areas of water containing very little oxygen), what we are seeing is that threats to coral health are rapidly multiplying due to influences directly related to human-forced climate change.

Links:

El Nino Prolongs Longest Coral Bleaching Event

NOAA: Coral Bleaching Background

World Resources Institute Shows Widespread Coral Bleaching by 2030

The World Resources Institute

Hat Tip to TodaysGuestIs

Hat Tip to DT Lange