Through a Reef Darkly

Simon Donner provides a gliss on the fate to come of coral reefs in a warming world, and, to be frank, it ain't pretty.  The reefs are toast in a 2 C warmer world.  Commenting on Freiler, Meinshausen, Golly, Mengel, Lebek, Donner and Hoegh-Guldbergs' paper "Limiting global warming to 2 C is unlikely to save most coral reefs", Simon points to the obvious, if 2 C is an average, then the extremes are worse, and it is the extreme hot spells that doom the reefs.

Frieler et al. relates the projected frequency of heat stress events – what I often call ocean “heat waves” – in coral reefs worldwide – that can cause coral bleaching to global mean temperature change, the metric discussed so often in policy circles, the public and the press.  In past studies, including several of my own (Donner et al., 2005, 2007), we estimated the frequency of bleaching events under different future greenhouse gas scenarios. In those studies, we are able to show the difference in the outcome for coral reefs between futures with different levels of atmospheric carbon dioxide (and other greenhouse gases). For example, my “Coping with Commitment" (open access) paper concludes  with a discussion of the level of atmospheric CO2 necessary to avoid one definition of "dangerous" impacts to coral reefs.

In those past analyses, though, if you wanted to know what the coral reefs picture looked like under some specific global mean temperature increase – like, say, the proposed 2 deg C threshold – you’d need to take apart the results and find the point in the emission scenarios were global average warming reaches (or, depending on your question, stabilizes) at that level. In response to a request in advance of the UN Copenhagen climate summit in 2009, a group of colleagues and I did that analysis (pdf) for the Caribbean and found a clear difference between the coral reef outcomes in +1.5 C and a +2 C world. Frieler et al is the first paper to do a comprehensive analysis of coral reef outcomes under different temperature thresholds for the whole planet.

Now some, not Eli to be sure, might point out how useful proper framing could be in this matter.  Eli is simply depressed.