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Sobering News on Coral Reefs – But We Knew It Would Come to This


The journal, Science, began 2018 with an issue including an update on the status of coral reefs around the world.  Led by Terry Hughes of James Cook University, Australia, a consortium of 25 authors, from around the world though mostly from Australia, compiled data on the history since 1980 of 100 coral reef locations from 54 countries around the world.  For each year from 1980 to 2016, each location (10 to 10K km2 in area) was reported as bleached, severely bleached (more than 30% of coral bleached), or not bleached.  This data set was compiled from published records and from direct observation by individuals known to the authors.  By using only 100 well-documented sites, the authors were minimizing the errors that creep in when well-studied or easily accessible sites are over-sampled relative to others, or when new sites are continually added (almost always in the year when they are seen to bleach).  Temperature histories were also compiled for each location, and years were classified as el Niño. La Niña, or neutral years.  This article (as usual locked behind a paywall) was well covered in the mainstream media.

Hughes and colleagues report a trend to diminishing ‘return-time’ for bleaching events that will be devastating for coral reef systems.  The global mean return-time between one bleaching event at a location and the next is now ~6 years, a decrease from 25 to 30 years in the 1980s.  Since faster growing corals take a decade or more to recover from the mortality caused by a bleaching event (and slower growing corals much longer) it’s obvious that the damage now being caused by bleaching is beyond the capacity of coral reefs to repair.  This information on shortening of return-times confirms a hypothesis put forward in a 2005 paper in Global Change Biology by Simon Donner (then at Princeton, now at UBC) and colleagues that return-time will continue to diminish as the planet warms and could be less than 2 years for the great majority of reefs by 2035 or so.  One intriguing twist – Donner and colleagues based their estimate on projected tropical sea surface temperature (SST); Hughes and colleagues have found no link between annual mean SST and frequency of bleaching, and argue that it is local patches of unusually warmed water, sort of marine weather, that leads to bleaching in a location.  Looking into the future, using projected mean SST, is not likely to be an accurate way to predict bleaching frequency, although it remains true that with SST projected to continue rising, the frequency of bleaching events will surely go up until bleaching is an annual, or near annual event.  That will surely happen within the next couple of decades.

The primary results from the study of 100 reef locations, showing A) the pattern of bleaching across the 100 locations from 1980 to 2016, B) the cumulative pattern of bleaching events, and the progressive loss of unbleached locations during the same time period, C) the pattern of severe and of all bleaching events among locations showing that most locations have been bleached multiple times, and D) a comparison between return-times for successive bleachings at specific locations during the first 19 and the final 17 years of the record – return-time is clearly becoming a lot shorter.  Figure © Science.

Hughes and colleagues also report that of the 100 locations they examined, only 6 have yet to experience a severe bleaching event, and all 100 have been bleached at least once.  The severe 2016 bleaching event impacted 75% of the 100 locations monitored (remember these are places from 10 to 10k km2 in area, and ‘severe’ means more than 30% of all coral there bleached).  This is comparable to the severe 1997-98 bleaching event, which impacted 74% of these sites.  As of the end of 2016, the median number of severe bleaching events per site is now 3 since 1980, and 31% of locations have experienced 4 or more (up to 9) such severe events as well as many less severe events.

Globally, the annual risk of bleaching, for any reef location, has increased from 8% in the early 1980s, to 31% by 2016, while the risk of severe bleaching has increased from 4% to 17% over the same period.  The pattern of bleaching likelihood, severity, and frequency differs among the four major geographic regions considered.  The Caribbean/West Atlantic began to experience regular bleaching events earlier, and the average location there had been bleached over 4 times by 1998.  Widespread bleaching, affecting more than 50% of locations in this region has now occurred 7 times, more than twice as often as in the other three regions.  In the 1980s, bleaching risk was highest in the Caribbean/West Atlantic, and the Pacific, but bleaching risk has increased most strongly in the Indian Ocean and in Australasia, so that in 2016, risk is comparable across all four regions.

Plot showing the trend in frequency of bleaching per year for the 100 locations monitored.  The risk of bleaching (proportion of locations bleached) has increased in all regions since 1980, but the pattern of increase in risk is quite different for the Caribbean/West Atlantic than for the other three regions.  Figure © Science.

While Hughes and colleagues are appropriately objective and dispassionate in reporting their results, this paper shows that coral reefs are on a journey toward oblivion.  Quoting their own words,

“The time between recurrent events is increasingly too short to allow a full recovery of mature coral assemblages, which generally takes from 10 to 15 years for the fastest growing species and far longer for the full complement of life histories and morphologies of older assemblages. Areas that have so far escaped severe bleaching are likely to decline further in number, and the size of spatial refuges will diminish. These impacts are already underway, with an increase in average global temperature of close to 1°C. Hence, 1.5° or 2°C of warming above preindustrial conditions will inevitably contribute to further degradation of the world’s coral reefs. The future condition of reefs, and the ecosystem services they provide to people, will depend critically on the trajectory of global emissions and on our diminishing capacity to build resilience to recurrent high frequency bleaching through management of local stressors before the next bleaching event occurs.”

Using the reef story to motivate action on climate change

The imminent demise of coral reefs as we know them provides the reef science community with a powerful weapon for the battle to convince the global community that the need to deal with climate change is critical.  The empirical evidence is abundant that coral reefs have already sustained substantial damage from human impacts, and that the damage caused by climate change has reached a level that exceeds the regenerative capacity of reefs.  What we are witnessing is the progressive elimination of an important, complex, and productive ecosystem from this planet.  It is happening because the pace and extent of our warming of the planet has exceeded the capacity of the chief structural architects of this ecosystem to survive and rebuild, and it is happening over the span of one human lifetime or less.  What is happening to reefs is akin to the disappearance of ALL rainforests on the planet in the space of one human lifetime.  That our modest warming of the planet can result in so extreme a consequence for one sensitive ecosystem should be an urgent warning to us.  What other serious consequences of climate change are happening, or about to happen?  And isn’t it time to stop our foolish dumping of greenhouse gases into the atmosphere?

Yes, it’s a powerful weapon.  But are we going to be able to use this weapon effectively to sway the minds of others?  To do that, the reef science community has to be able to project a clear and unambiguous message to the world community.  I think we have been making strides towards doing that, but our message is frequently mixed, because we are scientists first, rather than communicators.

So, in the interest of effective communication, I suggest framing a message around the following 6 points:

  • ONE Coral reefs are important ecologically, economically, culturally, wherever they occur; and they directly sustain the lives of millions of people on this planet, many of whom have played only a tiny role in releasing greenhouse gases (GHGs) to the atmosphere. There is ample information on the value of coral reefs, but many people still do not appreciate how valuable they are;

To casually permit the eradication of coral reefs on this planet through our careless dumping of GHGs into the atmosphere removes vital coastal ecosystem services, while also degrading the lives of millions of people with only tiny roles in that pollution, who are directly dependent on reefs economically, culturally and spiritually.  Photo of reef at New Britain, Papua New Guinea.  Image © David Doubilet, National Geographic Creative

  • TWO Coral reefs are deteriorating globally at a rapid, and increasing rate due to several human impacts. Coral bleaching, a stress response caused by ocean warming can result in substantial mortality of corals, and is now a major factor in reef decline.  Our releases of GHGs to the atmosphere cause the warming, and the frequency of bleaching events causes damage that already exceeds the regenerative capacity of reef systems;

Bleached coral off Lizard Island, Australia, in 2016.  Photo © XL Caitlin Seaview Survey

  • THREE The pace of deterioration is such that the future condition of reefs, and the ecosystem services they provide to people, is now critically dependent on the effectiveness of global GHG emissions reduction;

Even ‘large-scale’ propagation of corals for transplant to degraded reefs is ineffective when the return-time of severe bleaching events is 6 years or less.  It’s too little, too slowly, and corals are limited in how fast they can grow.  Photo of Indonesian coral farm © SA Simbolon

  • FOUR The future that is likely even under the most optimistic IPCC scenarios (i.e. +2o or +1.5oC above preindustrial) looks marginal for coral reef systems. Current trajectories of GHG emissions, which are insufficient to restrict warming to +2oC, are also woefully inadequate if we want to have any coral reefs on this planet beyond mid-century;

Although the Paris climate agreement commits the world to reduce emissions of GHGs and keep warming below +2oC, global emissions continue to rise.  Warming well above +2oC looks very likely by the end of the century.  Image © Global Carbon Project and Inside Climate News.

  • FIVE On the other hand, with aggressive GHG emissions abatement, coupled with vigorous management of reef systems to address our other negative impacts, we could achieve a post-2050 world with viable reef ecosystems continuing to provide the important ecosystem services they now provide;

It will be difficult, perhaps impossible, to retain reefs as rich as this one on our planet later this century.  But achieving this is a goal worth striving for.  It will require aggressive containment of GHGs as well as excellent management of local stressors of coral reefs.  Photo of Red Sea reef, winner in 2015 Underwater Photographer of the Year competition, © Lynn Wu.

  • SIX Coral reefs are not alone among ecosystems in being affected by climate change. While the impacts of climate change on reefs are already both extreme and starkly clear, reefs provide an early glimpse of the danger we are causing to many other parts of this planet’s biosphere through our careless dumping of GHGs into the atmosphere.  We need to take much better care of this planet.

Poem by Joe Miller.  Text Image © The Friend