Coping with Commitment: New study on the challenge facing coral reefs

Maribo's back from a long hiatus, during which the host was overseas collecting data, samples and unfortunately some sort of parasite.

Good timing, for the return home, not the parasite, since a new study of mine on climate change and coral reefs appeared in PLOS-One today (link - no subscription required).

As many of the regular readers have heard before, one of the biggest challenges posed by climate change is the timing.

First, you have the time lag between greenhouse gas emissions and the climate effect of those emissions. Basically, it takes time for the big complicated mix of atmosphere, ocean, land and ice to come to equilibrium. That’s why you sometimes here the climate compared to a big ship. You can hit the brakes but it will take a while for ship to actually come to a stop. Similarly, even if we froze emissions today, the climate would continue to warm a bit, because that warming is physically built into the system. This is often referred to as "committed warming".

Society may impart a second warming commitment. There is a lag between the decision to take action and the action itself. Using the ship analogy, it takes time for the officers on deck who first see the iceberg to relay the message to the captain. In other words, even if we decided to drastically emissions tomorrow, those reductions would not occur for some time. Hence all the hysteria about the construction of new coal-burning power plants. Since coals plants may last for decades they come with a considerable emissions “commitment”.

The new study looks at the implications of “committed warming” for conserving the world's coral reefs.
 
The dangerous impacts of climate change on coral reef are expected to occur sooner than most other prominently discussed climate change impacts (e.g. ice sheet melt, rainfall shifts in the topics). Why? Water temperatures, only 1-2 degrees Celsius over the usual summer maximum temperatures can cause bleaching of corals and some other reef organisms. Bleaching, described here many times, is the paling of the corals caused by a breakdown of the symbiosis between the reef-building animals themselves and the colourful algae that live in the coral tissue. A bleached coral is still alive, but is deprived of its primary energy source. If the conditions persist, the bleached coral can die.

Now, corals can grow back after a bleaching event, just as trees grow back after a fire, but it takes time. If bleaching events happen too often – say, because of ocean warming - most corals and the ecosystem as a whole will be unable to recover. Now, combine that with the rise in CO2 levels reducing the ability of corals to actually build the reef, and you’ve got one of the most threatened ecosystems on the planet.

This new study evaluates the “committed" frequency of bleaching events, and what it all means for coral reef conservation and for climate policy. The results show that the physical commitment alone is enough to make bleaching events harmfully frequent at over half of the world’s reefs by the end of the century. A possible additional commitment, caused by the time required to shift from a “business-as-usual” future to a GHG “mitigation” future, may cause over 80% of the world’s coral reefs to experience harmfully frequent events by 2030.

There is a possible silver lining. Thermal adaptation of 1.5 degrees C, whether via biological mechanisms, coral community changes or extreme management interventions, could postpone the forecast for 50-80 years in the “business-as-usual" case. That could provide time to change the trajectory of greenhouse gas emissions and then prevent the majority of the world's reefs from experiencing harmfully frequent bleaching events this century.

Let's be clear. This is no panacea - the ecological costs of proposed adaptive mechanisms and the implications for climate policy are outlined in the discussions. Here's the final paragraph:

In summary, the results of this study indicate that a combination of greenhouse gas mitigation and improved coral reef management will be required to avoid the degradation of the world’s coral reef ecosystems from frequent mass coral bleaching events. Actions that enhance reef resistance and reef resilience - including protection of bleaching-resistant reefs, reduction of other stressors, and possibly even more radical suggestions like “seeding” reefs with more temperature-tolerant species of Symbiodinium – may be necessary to help coral reef ecosystems endure through the committed warming over the next several decades. These management actions, while important, will alone prove to be insufficient to protect coral reefs through the latter half of the century. The difference between the future scenarios presented in this study demonstrates that protecting the world’s coral reefs from increasing thermal stress will require a dramatic reduction in greenhouse gas emissions over the next several decades.