Coral responses to 110 years of climate change, and lessons on how to

[Harlequinmania]

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Mounding corals of the genus Porites tend to be among the longest-lived corals on reefs throughout the Pacific and Indian oceans. It is not unusual to find individuals of these corals which have grown more than 6 ft high and are over a century old. In fact, the oldest known tropical corals are mounding Porites more than 30 ft tall and estimated to be on the order of 1000 yrs old. Because these are often long-lived animals they can be used as recorders of coral health and growth as well as certain environmental conditions over long periods of time.

Corals produce bands in their skeleton as the seasons change, just as trees make tree rings. After taking a core sample of a coral skeleton the annual growth rate (i.e., linear extension) and quantity of skeleton it produced (i.e., calcification) can be determined by measuring the distance between bands and the density of the skeleton. Several scientists at the Australian Institute of Marine Science (AIMS), among others, have used this technique extensively over the last several years to better understand how climate change and ocean acidification which have already occurred due to human activities has affected coral growth rates over time.

Let me start with a bit of background. In 2009 Glenn De’ath, Janice Lough and Katherina Fabriscius of AIMS published a study in the journal Science which examined the growth of mounding Porites along the Great Barrier Reef (GBR) over the last 400 yrs. They sampled from the hotter reefs in the north to the cooler reefs in the south and found that coral growth didn’t change much or do anything particularly interesting over most of the last 400 yrs, until about the year 1990. Beginning around 1990 coral growth rates started to decrease across the GBR and overall they decreased about 14% from 1990-2005. Since this decrease occurred over thousands of miles one can be confident that local stressors like sedimentation, overfishing, etc. which might affect one part of the reef but not another are unlikely to be the cause of the decreased growth rates. Instead, a regional or global stressor is almost certainly to blame with the most likely culprits being increased temperature (associated with climate change) and reduced pH (associated with ocean acidification).

Christmas tree worms growing on Porites attenuata.

On the GBR the annual average temperature in the north today is ~82 °F whereas in the south the annual average is ~77.5 °F (note, these are the annual averages—temperatures are higher in summer and lower in winter). Along the GBR temperatures increased about 1-2 °F over the last 100 yrs (less in the north, more in the south) with about half of that warming occurring after 1990. Coral bleaching along the GBR due to high summertime temperatures was practically nonexistent prior to 1990 but has become widespread since then. My gut feeling and based on various lines of evidence is that elevated temperatures and not reduced pH are likely the bigger player in this case of reduced coral growth since mounding Porites seem to be less sensitive to moderate reductions in pH as compared to many other corals, but they are sensitive to elevated temperature.

Fast-forward to 2012 and AIMS scientists (Timothy Cooper, Rebecca O’Leary, and Janice Lough) have just published a similar study in Science, but examining coral growth along Western Australia (as opposed to the GBR, which is off eastern Australia). Like the 2009 study, they used cores from mounding Porites to examine coral growth, concentrating on the years 1900-2010. Similar to the GBR, the reefs in the north  along Western Australia have an average annual temperature today of ~82 °F whereas the reefs in the far south along Western Australia have a chilly annual average today of ~72 °F. These far southern reefs grow in some of the coolest water of any coral reefs worldwide. Over the last 110 yrs the reefs in the north off Western Australia have experienced relatively little warming—less than 0.4 °F. Cooper et al. found that the growth rate of these corals has not changed significantly during this period. The reefs  near the middle of the geographic range have an annual average temperature today of ~77.5 °F which has increased about 1 °F over the last 110 yrs and coral growth rates have decreased 3-11%. In contrast, the reefs in the south and far south have a mean annual temperature today of ~75 and 72 °F, respectively, and have each warmed by about 1 and 2 °F over the last 110 yrs. The corals on these reefs have increased their growth rates by 6 and 24%. Hence, these southern reefs had an annual average temperature of ~74 and 70 °F 110 yrs ago, now have an annual average temp of ~75 and 72 °F, and the corals are growing faster as a result.

Context here is very important. Drawing from these two studies as well as others that have been conducted in a similar way in Thailand, the Red Sea, and Belize we see that corals that are found at typical temperatures for coral reefs have responded negatively to increased temperatures. As the temperature goes higher, most corals grow slower. Corals that are found in some of the few places that have not warmed much (i.e., northern reefs along Western Australia) have not experienced much of a change in their growth rates. Corals found at the extreme cold fringes of where they can grow have responded positively to a little bit of warming so far (i.e., southern reefs along Western Australia).

This is exactly how we would expect most any group of organisms to respond to a change in temperature. Any physiological process (in this case coral growth) occurs fastest at an optimal temperature and decreases if we either raise or lower the temperature from that optimum. Corals along the GBR and on the majority of reefs worldwide are already close to their temperature optima. Hence, when the temperature increases they grow more slowly. Corals at the extreme cool fringe of coral reefs are below their temperature optima and a small increase in temperature increases their growth rates. As a result, we would expect to see the growth rates of most corals on most reefs decrease as the climate warms and the growth rates of corals on cold reefs to initially increase, reach a maximum and then decrease as the climate warms further.

Coral on the Great Barrier Reef. Photo by Steve Evans.

One of the beautiful things about science is that data, not opinions, ultimately settle scientific disagreements. Questions of fact are settled with the facts and an individual’s political, social, or other positions don’t count in science, just the evidence. An honest assessment of all the data shows that the climate change we have had so far has harmed most corals but has benefitted at least some corals which grow on unusually cold reefs, though they represent a very small proportion of reefs worldwide. Hence, climate change has been mostly, but not entirely, bad for coral reefs whereas additional warming will be much worse.

It is therefore very disheartening to see articles like, “Century of ocean warming good for corals, research shows”, a summary of Cooper et al. (2012). The Herald Sun managed to put dramatic political spin on scientific data. Does research show that a century of warming is generally good for corals, as the article title suggests? No, research including just the Cooper et al. (2012) study, shows that a century of warming has been bad for most corals and has only benefitted a minority which grow on unusually cool reefs. Do “the findings undermine predictions that global warming will devastate coral reefs”? No, not remotely. The findings reinforce our understanding of how a warming climate will affect coral reefs. Indeed, that understanding tells us that a large degree of warming like we are currently on track for will prove devastating to almost all coral reefs. Do these data “add to a growing body of evidence showing corals are more resilient than previously thought - up to a certain point”? Again, not really. These data show that the predictions made about how climate change should affect the physiology of organisms is being born out in the real world. Decades worth of laboratory and field study of corals along with centuries worth of research on physiology have allowed us to make correct predictions of how corals and other organisms will react to climate change.

In the article by the Herald Sun we see a classic case of cherry picking—presenting just that piece of information that supports ones argument and ignoring the rest, especially when the rest of the data invalidates the argument. The argument is similar to suggesting that the recession of 2008 was a good thing for the economy because some bank CEOs were payed huge bonuses. That doesn’t make much sense! It is disturbing to see this happen, but I stress that no fault lies with the Cooper et al., the authors of the scientific article, nor anyone else at AIMS. Indeed, I’ve seen my own words and data used to make idiotic, non-scientific arguments by individuals or groups sceptical of the seriousness of ocean acidification. Such dishonesty doesn’t fly in science simply because it is difficult to pull a fast one on experts in any field, but these tactics are used rampantly by some publications and groups to misinform the public for their own political, social, or other gains.

Make no mistake, most of the climate change of at least the last 50 yrs has been the direct result of the release of greenhouse gases from human sources, primarily CO₂. This warming has had a negative effect on most corals, though it has benefitted a tiny minority. Business-as-usual greenhouse gas emissions would prove devastating to almost all reefs worldwide, but there is still a lot of relatively healthy coral left. We can save reefs for the future, but to do that we need to get to work reducing CO₂ emissions now.

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