The current global hope regarding climate change is that we can minimize the warming to 2 degrees Celsius above pre-industrial levels, however this will not be enough if we want to save the coral reefs that are so necessary in the overall health of the oceans.
Only if we look to drastically and speedily halt climate change can we expect the coral reefs to survive, and even then the outlook does not look good.
These are the findings of a new report published in the journal Nature Climate Change, which ran over 32,000 years worth of simulations based on an extensive set of 19 climate models in an effort to project the heat stress likely to take place at 2160 coral reef locations worldwide.
With almost a quarter of all species living in the oceans relying on coral reefs to survive, not to mention the critical services of coastal protection and the somewhat less important job of providing fish and tourist locations, it is no wonder that coral reefs are the focus of such an intensive study.
“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.”
The research was done in conjunction with the University of British Columbia in Canada and the Universities of Melbourne and Queensland in Australia. It is the first study to provide a comprehensive global survey of coral bleaching to express results in terms of global mean temperature change.
The researchers based their simulations on 19 global climate models, and then applied different emission scenarios covering the 21st century and multiple climate model simulations, totalling more than 32,000 simulation years and providing a more robust representation of uncertainty thanany previous study.
Coral bleaching is the process by which a coral is deprived of it’s famous colours by the depletion of a special type of microalgae that forms a close symbiotic relationship with the coral. The symbiosis between coral and algae breaks down when stressed by temperatures too warm, which in turn makes the coral “bleach” or turn pale.
“This happened in 1998, when an estimated 16% of corals were lost in a single, prolonged period of warmth worldwide,” says Frieler.
In an attempt to find a possible acclimation or adaptation of corals to thermal stress, the researchers included some rather optimistic assumptions in their 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 studies have estimated the effect of thermal adaptation on coral bleaching thresholds, but have failed to include the possible opposing effect of ocean acidification, a process that is likely to act detrimentally to the calcification process crucial for the growth of coral reefs, and may in fact prove a hindrance to their ability to resist temperature increases.
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.”
All in all, this study paints a dreary picture of 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.”