The Headline: 99% Coral Reefs Are Endangered Due To Climate Change, Warns Study Sophia Williams/Science Examiner  January 6, 2017

What the actual study says: “Under RCP8.5, ASB [annual severe bleaching] is projected to occur within the 21st century for 99% of the world’s coral reefs” (van Hooidonk et al, 2016). Without going into details, “RCP8.5” is a worst-case scenario associated with over 4° C warming by 2100.

Some context: The goal is to limit global warming to 1.5° C by 2100 but a lot of scientists think warming will be over 2° C by the end of the century. Still, 2° C is a lot less than 4° C. The above headline was thus based on a worse-case scenario, not a likely scenario. However, a different paper (Schleussner et al, 2016) does project a 99% loss of tropical coral reefs with a 2° C rise in global temperature by 2100. This paper also projects a 90% loss of coral reefs by 2050 even if warming were limited to 1.5° C by 2100. Reading the Schleussner et al paper, it does not appear* the authors incorporated interventions known to increase the resiliency of coral reefs in any of their computer models.

Additional context: from 2014 to 2017 we had a global coral bleaching event that is estimated to have weakened or killed 75% of the world’s corals. Since then, several coral reefs have been showing signs of recovery, including reefs around Hawaii,  Australia, and the Caribbean. So coral die-offs don’t necessary herald coral extinction. After all, corals date back to the late Cambrian period, about 500 million years ago, and have suffered die-offs repeatedly since then. None of this is to be lackadaisical about the fate of corals this time around.

Coral Reefs as a Political Tool: This year’s Green New Deal (GND) repeats the claim that “global warming at or above 2 degrees Celsius beyond preindustrialized levels will cause…a loss of more than 99 percent of all coral reefs on Earth”. The GND references an October 2018 report by the Intergovernmental Panel on Climate Change (IPCC). I reviewed the IPCC report’s references and the only one making that specific 99% claim was by Schleussner et al. So we’ve come full-circle. An iffy scientific claim based on iffy computer models stays alive to support a particular political agenda.

What Can Be Done? Lots! To be enumerated in the next post.

* Schleussner et al say their approach only includes the effects of increased CO2-concentrations and does not account for other stressors for coral reef systems, such as disease, invasive species, and rising sea levels. They seem to assume these stressors will only get worse over the course of the century, making their estimate of a 99% coral die-off “conservative”. However, the authors did not address the possibility of improved management of coral systems to reduce stressors such as pollution, sediment run-off, over-fishing, and coral harvesting. These improvements could increase the resiliency of coral to increased ocean temperatures and acidification.

Next: What can be done to increase the resiliency of coral reefs?

References and Links:

Schleussner, C.-F. et al., 2016: Differential climate impacts for policy-relevant limits to global warming: The case of 1.5°C and 2°C. Earth System Dynamics, 7(2), 327–351, doi:10.5194/esd-7-327-2016.

van Hooidonk, R., J. Maynard, et al. (2016). "Local-scale projections of coral reef futures and implications of the Paris Agreement." Scientific Reports 6: 39666.