By Jim Steele
Good news continues to accumulate regards corals’ ability to rapidly adjust to changing climates. The view of coral resilience has been dominated by the narrative of a few scientists. In the 1990s they advocated devastating consequences for coral reefs due to global warming, arguing coral cannot adapt quickly enough. Since the Little Ice Age ended, they believed rising ocean temperatures had brought coral closer to a “bleaching threshold”, a more or less fixed upper temperature limit above which corals cannot survive. Their model predicted the speed of recent global warming “spells catastrophe for tropical marine ecosystems everywhere”. Their assertions that “as much as 95% of the world’s coral may be in danger of being lost by mid-century” was guaranteed to capture headlines and instill public fear. However, a growing body of scientific research increasingly casts doubts on such alarming predictions. Unfortunately, that good news gets much less attention.
A recent peer-reviewed paper titled A Global Analysis of Coral Bleaching Over the Past Two Decades (Sully 2019) compared 20 years of ocean temperatures at which coral bleaching was initiated. From 1998 to 2006, the average sea surface temperature that initiated bleaching was 82.6 °F. But that temperature limit proves not to be “fixed” as earlier researchers incorrectly believed. From 2007 to 2017 the average temperature limit that initiated bleaching was higher, 83.7 °F. This indicates coral have been rapidly adapting to warmer regional climates much faster than once believed.
Based on these new observations the scientists concluded, “past bleaching events may have culled the thermally susceptible individuals, resulting in a recent adjustment of the remaining coral populations to higher thresholds of bleaching temperatures.” Furthermore, they suggested, “Localities that commonly experience large daily, weekly, or seasonal SST ranges [Sea Surface Temperature] may harbor corals, and strains of coral symbionts, that are more resistant to SST extremes.”
Other studies also observed similar rapid adaptations. Studies in Indonesian waters determined that two coral species, both highly susceptible to bleaching, had experienced 94% and 87% colony deaths during the 1998 El Nino. Yet those same species were among the least susceptible to bleaching in the 2010 El Nino despite a similar increase in water temperatures with only 5% and 12% colony deaths.
In the context of coral evolution over thousands and millions of years, such rapid adaptation was suspected by many scientists. After all, none of the coral reefs we observe today, that depend on symbiotic algae, existed 18,000 years ago. The last Ice Age Maximum lowered sea level by 400 feet, killing all coral above those depths. As ice sheets melted, oceans warmed, sea levels rose, and coral rapidly adapted to those ever-changing conditions. More recently, estimates of ocean temperatures just 3000 to 5000 years ago range from 1.8°F to 9°F warmer than today. And clearly those warmer temperatures did not result in massive coral extirpations, thus casting further doubt on predictions of massive coral deaths by 2050. Evidence of bleaching thousands of years ago also reveals it is not just a recent phenomenon.
Studies of coral reefs that existed thousands and millions of years ago, find the lowest extinction rates occurred in the warmest tropics. Sully 2019 similarly found “coral bleaching was less common in the equatorial regions.” In contrast to earlier “models that predict minimal coral survival in the tropical oceans within the next 100 years, recent field work shows considerable geographic variability in both temperature stress and coral survival”. Thus, they argue there is an “urgent need to develop better models” to more accurately predict coral bleaching.
Sully 2019 hypothesized “localities that commonly experience large daily, weekly, or seasonal SST ranges may harbor corals, and strains of coral symbionts [symbiotic partners], that are more resistant to SST extremes.” Increased resilience to a variety of bleaching events, whether induced by anomalous warmth or cold, prompted the Adaptive Bleaching Hypothesis first proposed in 1993. That hypothesis suggests that although bleaching events are a response to stress, by ejecting susceptible symbionts, coral create the potential to acquire totally new and different symbiotic partners that are better suited to new stressful conditions. A broader analysis of the Adaptive Bleaching Hypothesis is discussed in the article “The Coral Bleaching Debate: Is Bleaching the Legacy of a Marvelous Adaptation Mechanism or A Prelude to Extirpation?”
Because coral live in nutrient depleted environments, many species require single-celled photosynthesizing symbionts that typically provide ~90% of the coral’s energy needs. Just 40 years ago it was believed all corals were host to just one photosynthesizing symbiont. But thanks to technological advances in genetic sequencing, we now know a coral species can harbor several potential symbionts, each capable of responding optimally to a different set of environmental conditions. As predicted by the adaptive bleaching hypothesis, genetic techniques have now revealed a wondrously diverse community of symbionts with which coral can partner.
The more alarmist researchers had argued coral can only adapt very slowly over thousands of years via genetic mutation and natural selection. They incorrectly believed coral’s upper temperature limit is “fixed” for decades and centuries. But corals are now seen as an “eco-species” that can rapidly evolve and adapt to changing climates by expelling and acquiring new symbionts. Various symbionts enable various temperature tolerances.
To summarize Sully 2019, they found:
1. Coral now require higher ocean temperatures to bleach than the temperatures that caused bleaching a decade ago. This suggests rapid coral adaptation.
2. Coral bleaching was significantly lower in localities with a high variance in temperature anomalies. Localities with high variability likely maintain a wide variety of symbionts and coral genotypes.
3. There has been no universal response to global warming. Despite similar changes in temperature, bleaching was much less likely in equatorial region where coral diversity was highest.
4. Rapid changes in temperature can result in more bleaching, but the causes of rapid temperature change, such as an El Nino, were not analyzed.
Unfortunately, the last sentence in Sully 2019, reveals how some editors and journals are politicizing the science, and downplaying any optimism. Sully 2019’s last sentence read “immediate action globally to reduce carbon emissions is necessary to avoid further declines of coral reefs.” But Sully 2019’s research never tested or analyzed the effects of CO2 on temperature and bleaching. Their research only revealed resilience and rapid adaptation to warming, whether that warming was natural or CO2 induced. Furthermore, their research reported susceptibility to bleaching varied over time and location and did not detect a CO2 fingerprint. Their research did not determine whether rapid changes in regional ocean temperature were caused by changes in El Nino, shifting ocean currents, changes in upwelling, cloud cover or CO2 concentrations. In the past, honest and objective scientific journals restricted comments to conclusions based on the author’s actual research.
Over the years I have had several researchers thank me for posting information in my blogs that their editors had not allowed. They tell me editors have insisted on more catastrophic CO2-biased conclusions in order for them to publish. We also know from published emails that alarmist scientists like Michael Mann and Kevin Trenberth have actively “persuaded” journal editors, via bullying or other means, to obstruct publication of any skeptical scientific research that undermines Mann’s and Trenberth’s dire predictions. Sully’s CO2-alarmist, non-sequitur closing sentence is most certainly the fingerprint of another such enforced distortion that is now being superimposed on otherwise objective science.
Jim Steele is retired director of the Sierra Nevada Field Campus, San Francisco State University