Most corals will bleach at 1.5°C

Corals derive most of their energy, as well as most of their colour, from a close symbiotic relationship with a special type of microalgae that breaks down at warm temperature. Photo: © Lee Chin Yong - Fotolia.com

A quarter of marine biological diversity depends on coral reefs. Now researchers estimate that only 10 per cent of the reefs might survive at a temperature increase of only 1.5°C.

Coral reef ecosystems are one of the first global ecosystems whose existence is threatened if temperatures rise over 1°C due to global warming.

A study published in Nature gives the first comprehensive global survey of coral bleaching to express results in terms of global mean temperature change. The study shows that preserving more than 10 per cent of coral reefs worldwide would require warming to be limited to less than 1.5°C relative to pre-industrial levels.

The study was conducted by scientists from Potsdam in Germany, 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 twenty-first century and multiple climate model simulations, a total of more than 32,000 simulation years were examined. This allowed for a more robust representation of uncertainty than any previous study according to the authors.

The introduction to the study says that corals derive most of their energy, as well as most of their spectacular colour, 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 per cent of corals were lost in a single, prolonged period of warmth worldwide. At present, reef-building corals persist only within relatively narrow environmental conditions associated with shallow, sunlit and alkaline waters of tropical coastal areas. The carbonate reef structures that result from their calcium carbonate skeletons commonly build up in regions where temperatures exceed 18°C in winter.”

Although corals can re-establish themselves after mass bleaching events, in some cases it takes one to two decades for the ecosystem to return to the pre-bleaching state. An increase in the frequency and severity of mass coral bleaching could overwhelm the ability of coral reefs to recover between events. If this happens, coral reef ecosystems would shift towards systems that are dominated by other organisms such as cyanobacteria and algae, the study argues.

Coral reef ecosystems provide habitat for over a million species, almost a quarter of the species in the oceans. They are important for the socio-economic well-being, including coastal protection, tourism and fishing, of approximately 500 million people.

The results of the study indicate “that there would be long-term degradation of coral reef ecosystems in all present coral reef cells without a change in thermal tolerance at 2°C global mean temperature rise, an upper limit agreed to in international climate policy negotiations. Even at 1.5°C global mean warming, an alternative international temperature goal to be reviewed for international policy in 2015, the results suggest that around 89 per cent (63–100%) of coral reef ecosystems would face long-term degradation assuming no change in thermal tolerance. At the present rate of warming (0.2°C per decade), a 1°C warming above pre-industrial levels is going to be surpassed in the coming one or two decades, which might already put 16 per cent (3–29%) of reef locations at risk.”

The scientists write that: “despite the inclusion of optimistic scenarios concerning rates of evolutionary adaptation, our results confirm that coral reef ecosystems face considerable challenges under even an ambitious mitigation scenario that constrains global warming to 1.5°C above pre-industrial temperatures. The projections suggest that most coral reefs will experience extensive degradation over the next few decades given the present behaviour of corals to thermal stress to protect at least 50 per cent of the coral reef cells, global mean temperature change would have to be limited to 1.2°C (1.1–1.4 °C), especially given the lack of evidence that corals can evolve significantly on decadal timescales and under continually escalating thermal stress”.

The scientists argue that there is little doubt from the analysis that coral reefs will no longer be prominent within coastal ecosystems if global average temperatures exceed 2 °C above the pre-industrial period.

“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 °C 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 per cent 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°C mean temperature increase.”

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, concludes the study. Otherwise all coral reefs are expected to be subject to severe degradation. Coral reefs house and provide critical services to millions of people worldwide.

Reinhold Pape

Article is based on text from following source:
K. Frieler et al. Limiting global warming to 2°C is unlikely to save most coral reefs, Nature Climate Change 3, 165–170 (2013)