UBC climate scientist Simon Donner published a gloomy paper a few months ago in PLoS One titled “Coping with Commitment: Projected Thermal Stress on Coral Reefs under Different Future Scenarios” link to the paper

The paper is the most sophisticated analysis to date of how climate change will increase the frequency of mass coral bleaching.  It is a step forward from Ove’s pioneering work on the same topic in his classic paper “Climate Change, coral bleaching and the future of the world’s coral reefs” (see two of the key figures from Ove’s review below).  In it, Ove reviewed the history and mechanisms of coral bleaching and made predictions about how the frequency of bleaching will increase as the oceans warm and high temperature anomalies become more and more frequent.

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Simon took the next step by modeling the future frequency of bleaching under different IPCC scenarios (Fig 4 below). The depressing thing is how little difference there is among them, including between the very optimistic B1 scenario and the pessimistic (if more realistic in my view) A1b scenario.  [ I find the naming and assumptions of these scenarios somewhat confusing and I plan to post a guide to them in the near future.  For now consult the wikipedia article about them here or look at the draft notes I made based on that article below].

Methodology/Principal Findings/Conclusions: This study uses observed sea surface temperatures and the results of global climate model forced with five different future emissions scenarios to evaluate the “committed warming” for coral reefs worldwide. The results show that the physical warming commitment from current accumulation of greenhouse gases in the atmosphere could cause over half of the world’s coral reefs to experience harmfully frequent (p≥0.2 year−1) thermal stress by 2080. An additional “societal” warming commitment, caused by the time required to shift from a business-as-usual emissions trajectory to a 550 ppm CO2 stabilization trajectory, may cause over 80% of the world’s coral reefs to experience harmfully frequent events by 2030. Thermal adaptation of 1.5°C would delay the thermal stress forecast by 50–80 years…Without any thermal adaptation, atmospheric CO2 concentrations may need to be stabilized below current levels to avoid the degradation of coral reef ecosystems from frequent thermal stress events.

Donner used existing SST anomaly forecasts (coupled atmosphere-ocean CGMs) to model the future frequency of bleaching under different IPCC emissions scenarios (Fig 4 below).  SST anomalies and the probability of coral bleaching were estimated in each of 1687 0.5° x 0.5° grid cells.  Coral reef scientists use “Degree Heating Months” (DHMs), among other metrics, as an indication of accumulated thermal stress experienced by a coral colony or population. The metric is based on both magnitude and duration of a temperature anomaly. See the NOAA tutorial on the related Degree Heating Weeks metric here and on coral bleaching thresholds here. DHWs are measured and reported in real time by the NOAA Coral Reef Watch Program.  These bleaching thresholds are based on the well-documented fact that tropical corals are highly sensitive to anomalously high temperatures and live within ~ 1° C of their upper thermal tolerance.

DHW figure from the SW Pacific from the NOAA coral reef watch program.

Satellites only measure the temperature of the “skin” of the ocean, yet provide a surprisingly good estimate of the temperature of the bottom, where the corals are, at least in shallow depths, i.e., < 8 m. For example, we compared satellite-based SST measurements with data from temperature loggers on the ocean floor on nine reefs on the GBR and found a good match, with R2 values ranging from 0.90 to 0.96 (Selig et al. 2006 – if you are interested, this paper can be download here).

The results are presented in terms of probabilities or frequencies of DHM values in excess of the upper bleaching threshold. GCMs are best suited to describe the evolution of the statistical properties of the climate system over time (e.g. bleaching events per decade), rather than the specific state of the climate at a particular moment in the future (e.g., bleaching event occurs on Jan 31, 2036). Therefore, studies of the effect of climate change on discrete events like mass coral bleaching or heat waves typically translate the time series of occurrences of the events into a time series of frequencies or probabilities.

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Figure 4. Frequency distribution of the year in which the probability of severe mass bleaching events (DHM≥2°C-month) exceeds 20% for each the 1687 coral reef cells.

Figure 4 basically suggests that reefs are doomed, at least if any of the currently realistic scenarios come to pass.  But in a sense, that isn’t really news – people have been arguing for years that regulatory frameworks like Kyoto are not nearly enough and just a starting point.  The reality is that we need to quickly reverse CO2 accumulation and stay well below > 500 ppm.  (Or as Ove suggests in a recent post, get back to 350 ppm).

But one neat thing Simon did in his paper is compare the projected effects on bleaching frequency of different scenarios with and without a modest degree (1.5 C) of adaptation.  The differences between A1b and B1 are striking:

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Figure 6. Frequency distribution of the year in which the probability of severe mass bleaching events (DHM≥2°C-month) exceeds 20% for each the 1687 coral reef cells.

With adaptation (which in this case includes acclimation, natural selection, shifts in species composition and local management actions) there is very little bleaching, at least in this century, under B1.  Is this a realistic degree of adaptation?  Maybe.  But this is being debated pretty intensively in the coral reef world.

Another question is how realistic the model assumption of coral recovery five years after a mass bleaching event is.  I think this is pretty optimistic.  But as Simon points out, that doesn’t affect the value of the scenario comparisons.

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