Jim Steele

The coral that now form our modern reefs evolved 240 million years ago during the Age of Dinosaurs when coral formed a marvelously fluid symbiotic relationship with symbiodinium algae. The new algae symbionts absorbed CO2 and produced sugars via photosynthesis for the coral. In turn coral respiration produced CO2 that supported the algae’s photosynthetic production. This symbiotic relationship evolved when CO2 was 4 times above today’s concentrations and global temperatures were several degrees higher. Takashima (2006) reports the mid-Cretaceous earth “surface temperatures were more than 14°C higher than today,” a time when dinosaurs roamed Antarctica. Atlantic warm pool sea surface temperatures reached a maximum of 42°C (107°F).

That evolutionary history explains why shallow-water coral are now restricted to the warmest ocean waters. Graphic A shows the location of today’s coral. Graphic B shows the oceans regional temperatures. Corals are most abundant in the warmest waters of the Indian Ocean and Western Pacific Warm Pool, also known as the Coral Triangle. There, sea surface temperatures (SST) reach 28-30°C (82-86°F), sometime a little higher.

In contrast, the eastern tropical Pacific, experiences cooler SSTs due to upwelling typically in the range of 20°C (68°F). With the eastern tropical Pacific 8-10°C (14-18°F) cooler than the tropical western Pacific, there coral reefs are scarce and restricted to much smaller Atlantic and eastern Pacific warm pool regions. So why are today’s warm temperatures blamed for coral bleaching? Why does CNN report bleaching as catastrophic?

To survive 240 million years of extreme environmental change that required a fluid symbiosis. More recently during glacial maximums sea levels fell by 120 meters, killing all the shallow water coral reefs. As the ice melted, sea levels then rose 10 times faster than sea level rise today. Such rapid sea level changes greatly altered the sunlight reaching the surviving coral and the amount of nourishment the algae could produce.

Increased sunlight produces excessive dangerous Reactive Oxygen Species (ROS) like hydrogen peroxide and superoxides during the light reactions of photosynthesis that produce highly charge electrons and singlet oxygen by splitting water. Normally algae have several methods of neutralizing those dangerous ROS molecules but a rapid increase in sunlight, and thus ROS, sometimes overwhelms their protective systems. Thus, many corals threatened by increasing ROS, have adopted the survival strategy of ejecting their mal-adapted algae (i.e. bleaching). Coral can re-absorb those algae later when the extreme light conditions subside or absorb new symbiodinium algae adapted to the higher light conditions.

Today the major factor affecting solar insolation is changing cloud cover associated El Nino-La Nina oscillations. As seen in graphic C, during La Nina-like conditions heavy cloud cover reduces solar insolation over the coral triangle. During an El Nino, that heavy cloud cover shifts eastward causing greater solar heating around the Great Barrier Reef. The El Niño events of 1982-83, 1998, 2010, and 2015-2016 are all associated with massive coral bleaching.

Great Barrier Reef bleaching can also happen during a La Nina, but only when a high-pressure weather system sits over the reef and locally reduces cloud cover. Either way, reduced cloud cover both increases surface temperatures and overwhelms the algae’s photosystems by increasing ROS production. That prompted some scientists to argue that surface warming alone, greater than 1°C degree, will cause bleaching and that fit their climate crisis narratives. Alarmists like Fred Pearce on Yale 360 fear mongered global warming threatens coral writing, “The coral reefs of the tropics have looked doomed” … “Some experts say they will be gone by mid-century, the first great ecosystem casualty of the climate emergency.”

However, the science of coral bleaching now reports it is changes in the sun’s insolation that primarily drives bleaching by increasing ROS production during photosynthesis along with solar heating. If global warming alone is causing coral bleaching and rising coral deaths, we would expect a steady decline in coral cover as CO2 rises. However, changes in coral cover have oscillated as we would expect from the El Nino oscillation. After recent El Nino-related massive bleaching events impacted nearly 90% of Australia’s corals, in 2022 the Australian Institute of Marine Science (AIMS) reported the highest levels of coral cover across two-thirds of the Great Barrier Reef (GBR) in over 36 years (graphic D).

Sleep well! Corals are not threatened by rising CO2.