[Coral-List] Coral Reefs in 2100

[Steve Mussman]

Dear Coral-Listers,

Two recent papers have me questioning my understanding of what the future holds for coral reefs. I’m hoping that some of you with far greater expertise can help me sort this out.

1. Present and future bright and dark spots for coral reefs through climate change

https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.16083

This paper uses Representative Concentration Pathways (RCPs) to determine that by 2100 under RCP8.5 (which represents a business as usual/worst case scenario with SSts more than 3 degrees C over pre-industrial levels) they project a relative decrease in coral cover of >40%. I understand that greater than 40% could be 90%, but they don’t emphasize high end possibilities although I was under the impression that even under a less extreme regime RCP4.5 (2.4C) most corals world-wide would not be able to survive. The authors further assert that “overall, 26% of the 2949 sites were projected to lose at least 50% of their relative coral cover by 2100 under RCP 8.5 . . . Considered together, no country is projected to maintain more than 75% of its total coral cover in the next 80 years”. Based on everything I’ve read, this prognosis seems to understate the threat. I don’t see how any contemporary coral reef could be expected to maintain anywhere near 75% of its total coral cover by 2100 under RCP 8.5 - I wouldn’t think we could be that hopeful under RCP 4.5 for that matter.

2. Future loss of local-scale thermal refugia in coral reef ecosystems:

https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000004

This paper uses different metrics and is focused on thermal refugia, but still it makes the projection that climate change will overwhelm global thermal refugia to the extent that declines would be from 84% of global coral reef pixals today to 0.2% at 1.5C and 0% at 2.0C. Refugia are defined by their ability to maintain favorable conditions under thermal stress therefore I’m assuming that if refugia are predicted to be at 0% at 2.0C under this model then coral reef pixals outside of refugia would be at 0% well before reaching this threshold.

Although both papers reinforce the need to reduce carbon emissions and adjust coral reef management strategies, (to me) these two papers paint distinctly different pictures of the future. I’d like to know to know if others view it similarly or if not, perhaps someone could help me to more accurately evaluate these findings.

Thanks and warm regards,

Steve Mussman