Most coral reefs are at risk unless climate change is drastically limited

Only under a scenario with strong action on mitigating greenhouse-gas emissions and the assumption that corals can adapt at extremely rapid rates, could two thirds of them be safe, shows a study now published in Nature Climate Change. Otherwise all coral reefs are expected to be subject to severe degradation.

Coral reefs house almost a quarter of the species in the oceans and provide critical services – including coastal protection, tourism and fishing – to millions of people worldwide. Global warming and ocean acidification, both driven by human-caused CO2 emissions, pose a major threat to these ecosystems.

“Our findings show that under current assumptions regarding thermal sensitivity, coral reefs might no longer be prominent coastal ecosystems if global mean temperatures actually exceed 2 degrees Celsius above the pre-industrial level,” says lead author Katja Frieler from the Potsdam Institute for Climate Impact Research. “Without a yet uncertain process of adaptation or acclimation, however, already about 70% of corals are projected to suffer from long-term degradation by 2030 even under an ambitious mitigation scenario.” Thus, the threshold to protect at least half of the coral reefs worldwide is estimated to be below 1.5 degrees Celsius mean temperature increase.

A more comprehensive and robust representation than in previous studies

This study is the first comprehensive global survey of coral bleaching to express results in terms of global mean temperature change. It has been conducted by scientists from Potsdam, the University of British Columbia in Canada and the Universities of Melbourne and Queensland in Australia. To project the cumulative heat stress at 2160 reef locations worldwide, they used an extensive set of 19 global climate models. By applying different emission scenarios covering the 21st century and multiple climate model simulations, a total of more than 32,000 simulation years was diagnosed. This allows for a more robust representation of uncertainty than any previous study.

Corals derive most of their energy, as well as most of their famous color, from a close symbiotic relationship with a special type of microalgae. The vital symbiosis between coral and algae can break down when stressed by warm water temperatures, making the coral “bleach” or turn pale. Though corals can survive this, if the heat stress persists long enough the corals can die in great numbers. “This happened in 1998, when an estimated 16% of corals were lost in a single, prolonged period of warmth worldwide,” says Frieler.

Adaptation is uncertain and ocean acidification means even more stress

To account for a possible acclimation or adaptation of corals to thermal stress, like shifts to symbiont algae with a higher thermal tolerance, rather optimistic assumptions have been included in the study. “However, corals themselves have all the wrong characteristics to be able to rapidly evolve new thermal tolerances,” says co-author Ove Hoegh-Guldberg, a marine biologist at the University of Queensland in Australia. “They have long lifecycles of 5-100 years and they show low levels of diversity due to the fact that corals can reproduce by cloning themselves. They are not like fruit flies which can evolve much faster.”

Previous analyses estimated the effect of thermal adaptation on bleaching thresholds, but not the possible opposing effect of ocean acidification. Seawater gets more acidic when taking up CO2 from the atmosphere. This is likely to act to the detriment of the calcification processes crucial for the corals’ growth and might also reduce their thermal resilience. The new study investigates the potential implications of this ocean acidification effect, finding that, as Hoegh-Guldberg says: “The current assumptions on thermal sensitivity might underestimate, not overestimate, the future impact of climate change on corals.”

This comprehensive analysis highlights how close we are to a world without coral reefs as we know them. “The window of opportunity to preserve the majority of coral reefs, part of the world’s natural heritage, is small,” summarizes Malte Meinshausen, co-author at the Potsdam Institute for Climate Impact Research and the University of Melbourne. “We close this window, if we follow another decade of ballooning global greenhouse-gas emissions.”


  1. Quite logically, coral research based on hard data tends to come from (or near) the more readily accessible tourist areas equipped with small diving craft. These areas are a tiny, tiny fraction of 1% of the world’s coral. Not an impressive sample. The fact is that we have very little data on global coral reefs.

    You surely have to ask yourself, “Where is the hard data behind this study?” It seems to be BASED ENTIRELY ON COMPUTER MODELS. Presumably these were fundamentally the same IPCC models that have been so widely reputed to be crude and unreliable, even by IPCC modelers. Dr. James Lovelock made that very clear in his April 23rd remarks.

    So if I am understanding this correctly, what we have here is an e-data study based entirely on collections of speculations about future climate and hydrosphere conditions, which are then plugged into 19 models climate models known to be unreliable.

    Yet its authors have the gall to pontificate with profound ‘might happen” predictions and “could be’ conclusions. It is embarrassingly obvious that the guys are fishing for a continued flow of IPCC related grant money. Something seems amiss about all this, don’t you think?

    It doesn’t have to be that way, although the mass media and university research complex obviously shun and generally badmouths the good science This recent study (link below) used new data, real data, on coral reefs and came to falsifiable conclusions, one of which was that “distributions of deep-sea corals are not constrained by carbonate levels below saturation as broadly supported by the literature.” See for yourself at:

    Or try this one. Using original new data, it found that a previously unrecognized factor disasterously affecting at least one tourist accessible reef is human herpes virus! Further study might be needed to determine the source of the virus, but one obvious suspect looms very, very large — human divers. And one question that needs to be asked is “Do you suppose that needs to might be what is ‘killing’ coral instead of CO2? See the article on the coral herpes study for yourself:

  2. Exaggerated science disqualifies any and all scientific consensus:
    Climate science only agrees a crisis “COULD” happen, “MIGHT” happen and “PROBABLY” will happen, not WILL happen.
    The scientists do not all agree that a CO2 crisis will happen, they only that it could happen as they have NEVER said a crisis “will” happen and every single IPCC warning is laughably sprinkled with “maybes” and “probablys” and “potentiallys” . How close to the edge will the world of science take us before they say climate crisis will happen? Help, my planet is on fire maybe?


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