From October of 2014 through June of 2016, the world was in the grips of a powerful El Nino. And throughout this event, the oceans spewed back some of the massive volume of heat they’ve been accumulating in their depths due to global warming. As a result, atmospheric and ocean surface temperatures hit new record highs. And during 2016, global surface temperatures will likely average 1.2 C hotter than 1880s levels. This amount of warming is as considerable as it is harmful.

(A global coral bleaching event that began in 2014 continues. It is the longest coral bleaching event on record. But unless oceans somehow cool off, it won’t really end. With only a weak La Nina emerging following a strong El Nino and a record spike in global temperatures, there is some risk that this ongoing event will ebb and flare on a nearly indefinite basis. Continued fossil fuel burning, meanwhile, will continue to add heat to the global climate system — presenting worsening medium and long term bleaching pressure for corals. Image source: Coral Reef Watch.)

The Worst Global Coral Bleaching Event Ever Recorded…

This new record spike in global surface temperatures set off the worst coral bleaching event ever recorded. Around the world, reef systems came under severe stress as sea surface and near surface temperatures exceeded 28-30 degrees Celsius.

Among the hardest hit regions were the reefs of Kiribati. There, sea surface temperatures hit up to 31.4 C on an extended basis. Such hot waters are now expected to have wiped out all but 1 to 5 percent of Kiribati’s living corals. So, for all practical purposes, the reefs of that island republic have been wiped out.

Overall, the event was very wide ranging — impacting corals throughout the Pacific, Atlantic, and Indian Oceans as well as in the Persian Gulf, Mediterranean, and Red Sea. As an example, 95 percent of corals in US territories from Florida to the Gulf of Mexico, Caribbean, and Pacific experienced some level of bleaching.

(A weak La Nina has probably already cooled ocean surfaces as much as they will be cooled during 2016 and 2017. But despite this cooling, ocean near-surface waters are still too hot for corals in many places. Relative, if mild, ocean surface warming should occur as ENSO is predicted to shift into neutral status. If coral bleaching is ongoing through La Nina, then it is unlikely to cease as the global ocean starts to warm again. Global sea surface temperature anomaly image source: Earth Nullschool.)

In Australia, the Great Barrier Reef (GBR) also saw its worst bleaching event on record. There, 93 percent of corals are reported to have experienced bleaching. Meanwhile, about 50 percent of corals have died in the northern section of the GBR. In the media, a controversy has raged over whether or not this event is the start of the great reef’s swansong. To be clear, the GBR was not killed off by the most recent large bleaching event. But it was dealt a very severe blow. With the world continuing to warm as fossil fuel burning remains ongoing, a similar blow could occur as soon as the next El Nino or the one after. And the story for many of the world’s remaining reefs could well be the same.

…Is Still Ongoing…

For after about two years now, and as the world has settled into the periodic natural cooling of ocean surfaces called La Nina, the global coral bleaching event that has so damaged the vital species that build the world’s reefs is still ongoing. Though diminished, and as ocean surface heat backed off during late 2016, NOAA has still identified numerous regions that are high risk for coral bleaching through at least February.

The austral summer is expected to bring bleaching over far-flung regions encircling the southern part of the globe. Thankfully, most of the GBR is only under a bleaching watch for now. But bleaching warnings and alerts abound and, unfortunately, many reefs are likely to see continued die-offs even after El Nino has long since faded.

… And May Just, For all Practical Purposes, Continue

As the current La Nina is rather weak, and as it is predicted to shallow into an ENSO neutral state by spring, it appears that sea surface temperatures may be in the process of bottoming out. Global fossil fuel emissions, meanwhile, continue to add heat to the ocean system. As a result, the coral bleaching pressure that we are seeing during the period of November 2016 through February of 2017, unless we see a resurgence to a stronger La Nina event over the next year or two, could be the minimum we will see over the coming years. And if that is the case, then the coral bleaching event that hasn’t ended for the past two years may not really end at all.

Links:

NOAA Coral Reef Watch

NOAA El Nino

Earth Nullschool

Dr Gavin Schmidt of NASA GISS