Data for the Phoenix Islands over the past 18 years have shown a significant increase in their resistance to high temperatures. This could mean that coral reef ecosystems as a whole are markedly more resilient to climate change than previously thought. Just a decade ago, a number of authors predicted the extinction of corals by 2050. The new work suggests that they will continue to exist at the end of the 21st century.
Anthropogenic CO2 emissions have significantly increased the acidification of seawater - that is, reduced its pH below typical pre-industrial times. In addition, they cause warming. When overheated, many corals undergo bleaching, in the process "expelling" symbiotic photosynthetic organisms, from which they themselves normally receive the bulk of the nutrients.
All these processes have caused and are causing concern among biologists: they expect the extinction of corals by 2050. At the same time, a number of scientists question such expectations. It is not entirely clear why corals, which have flourished for hundreds of millions of years in a hotter Earth climate than would be observed as a result of global warming, should die out now - and why they did not die sooner. Because of all this, field observations are extremely important, which can indicate how the situation in the coral habitat is actually developing.
Researchers from the United States have summarized data on the development of corals in the Phoenix Islands - an archipelago in the equatorial Pacific Ocean, thousands of kilometers from the nearest continent. A related article was published in Geophysical Research Letters.
The authors used observation data for 2000-2018. The fact that the Phoenix Islands are located at a great distance from any other coral reefs gave particular value to the observations, therefore they are rarely invaded by starfish - predators that dramatically change the number of corals of the Great Barrier Reef and a number of other places. Because of this, the new work data reflects only the balance of the effects of warming on corals, without the "noise" from invasions of coral-eating predators - making it extremely difficult to unambiguously interpret studies in other regions of coral distribution.
Starfish cover the area of the coral to feed on and begin to dissolve it. Their periodic outbreaks of reproduction seem unreasonable, and have been happening for thousands of years in a row. During such periods, a significant proportion of corals die. In some areas, they can die completely / © Wikimedia Commons
In 2002-2003, the Phoenix Islands experienced a strong El Niño - during this period the water there was two to three degrees warmer than usual, which led to a 76.4% loss in the area of living coral coverage at a depth of 76.4%. The average area covered by live corals eventually fell from 44.9% to 10.4%. By 2009, surviving corals showed growth and covered 24.4% of the entire reef area. Six months later, El Niño of 2009-2010 came, which gave a new serious heating of surface waters. Because of it, a new reduction in coral cover was expected. However, in practice, the 2012 expedition showed otherwise: instead of 24.4%, corals covered 30.4% of the entire reef area.
In 2015-2016, Super El Niño happened and surface waters in this area warmed up three degrees above normal, remaining at the achieved level for a very long time. Total thermal stress was double that of 2002. Then the researchers again expected mass death of corals, but in 2018 - two years after this event - the new expedition found a reduction in the area of corals by only 40% (against 76.4% in 2002), to about 18% of the total surface of the reefs. …
The authors of the work tried to find some third-party factors that would explain the significant reduction in the harm of overheating of the upper layers of water for the corals of the Phoenix Islands. For example, they tried to track if Super El Niño had an unusual amount of clouds that would soften the pressure on marine organisms. However, all searches of this kind yielded nothing.
The researchers concluded that the drastic reduction in the severity of the effect of water heating on corals is an adaptation of the latter. It is known that such phenomena are possible. This follows, for example, from the existence of corals in the Red Sea and the Persian Gulf, where average surface water temperatures are several degrees higher than those of the Phoenix Islands or the Great Barrier Reef. But so far there have been no examples of extremely rapid adaptation of coral communities to rising water temperatures.
In the case of a new job, we can talk about quick adaptation in less than 18 years. If it is typical of corals in other parts of the world, their extinction by the middle of the 21st century seems unlikely - as well as by its end. Scientists nevertheless end the article with the phrase: "Reversing global warming remains integral to the survival of coral reefs." Unfortunately, they do not explain how, in this case, the reefs did not die in previous periods of rapid warming on the planet, or when the Earth's climate was noticeably warmer than the climate that the scientific community expects from modern global warming.