If Earth warms by 2 degrees The Great Barrier Reef is a goner, or maybe not. Tropical reefs are generally about 28C but even a one degree rise above normal temperatures can bleach corals.
This latest paper by Hume et al, showed that some corals survive in the hottest reefs on Earth which are in the Arabian/Persian Gulf and are a whopping 36 degrees C. In order to survive, corals do deals with symbiotic algae, but these are very sensitive to changes in temperature (or so we thought):
Reefs are made up of many species of coral, each of which have a mutually beneficial, or “symbiotic”, relationship with algae living in their tissue. These algae supply vital nutrition to the host but are sensitive to environmental changes including increases in seawater temperature.
Even a temperature rise of just one degree Celsius can harm the symbiotic algae, which in turn can increase mortality in corals. The associated loss of symbiotic algae is known as “coral bleaching” because the white skeletons of the corals become visible through the tissue depleted from the algal pigments.
Obviously those Gulf coral survive those wildly high temperatures with freak heat-loving-symbiotic-algae that can’t survive in normal oceans right? No. No. No. It was a plumb ordinary type, not even well known for living in warm areas. There goes that theory…
…the scientists were surprised to discover that the algae in Gulf corals belong to a group not known for its thermal tolerance.
“We see that the algae are indeed special but in a way that we did not expect,” said Dr Wiedenmann. “The algae that we found in most of the corals in Abu Dhabi reefs were previously described as a ‘generalist strain’ that is usually not found in corals exposed to high levels of heat stress.”
“The system seems to be more complex than it is commonly thought …”
A few things to learn still about coral reefs then?
I did wonder how they evolved during times of high CO2 and hotter conditions, but became endangered by a mere 0.9C rise. Somehow, incredibly, the Barrier Reef survived the Holocene. Of course, some would say we’re changing things faster than an ice age melt, but then, daily pH shifts on some reefs can be larger than the expected changes this century. (Hofmann et al).
There is the obligatory climate-caveat, but cause and effect is the wrong way around here (my bolding):
“Gulf corals are living at the limit of their tolerance,” said co-author Professor John Burt from the New York University Abu Dhabi. “We have observed an increased frequency of coral bleaching events in this area, and we need to act now to protect and understand these ecosystems that hold the answers to many important climate change related questions.
How do we protect something we don’t understand? First we study it. Then we pick a policy.
PS: Of course, this one paper about extremely hot conditions doesn’t suggest all corals would cope, or most reef-life would survive 36C temperatures, but it does tell us that what we don’t know is larger than what we do. We could spend billions trying to protect the reef only to find we did more harm than good. Think of the Nature-Climate-Change brainwave that we cover part of the Great Barrier Reef with shade cloth. It’s only 348,000 square kilometers, which part do we cover? Or we could spend 0.0001% of that on real scientists who use real observations instead of computer simulations.
Hume, B., D’Angelo, C., Burt, J., Baker, A., Riegl, B., and Wiedenmann, J. (2013): Corals from the Persian/Arabian Gulf as models for thermotolerant reef-builders: Prevalence of clade C3 Symbiodinium, host fluorescence and ex situ temperature tolerance. Marine Pollution Bulletin, 2013; DOI: 10.1016/j.marpolbul.2012.11.032
Hofmann GE, Smith JE, Johnson KS, Send U, Levin LA, et al. (2011) High-Frequency Dynamics of Ocean pH: A Multi-Ecosystem Comparison. PLoS ONE 6(12): e28983. doi:10.1371/journal.pone.0028983 [PLOS paper and graphs sourced here]
H/t To Brice.