Global warming and coral reefs renaissance instead of death

(ORDO NEWS) — New work in Nature shows that since the appearance of modern corals a quarter of a billion years ago, their range has been much wider than it is today. Only in the last tens of millions of years has the cooling of the planet forced coral reefs into the tropics.

This contradicts earlier estimates, according to which rising temperatures threaten them with destruction. The question arises: can the current warming bring corals back to their heyday? And what will this mean for life on Earth in general?

Coral reefs will be gone by 2100, the news warns us. Computer models show this – what can you do? The reasons, we are told, are two: global warming and acidification of the oceans (from the same carbon dioxide emitted in large quantities due to human activity).

Excessive heat causes corals to expel symbiont microalgae, and ocean acidification reduces the concentration of carbonate ions in seawater – and it is from them that corals build their hard shell. Sounds logical.

However, this logical picture naturally raises questions from empirical experience. During our school years, almost any of us touched a strange white material that stained our hands many times – chalk.

We wrote to them on school boards and did not particularly think about the fact that chalk is the remains of calcium-containing shells of ancient living inhabitants of the sea.

Settling to the bottom in many places, they covered (and thus preserved to this day) the corals of the Cretaceous period, the era of dinosaurs

The corals there are very similar to modern deep-sea ones. The question arises: why are there no corals in Denmark today, because it is approximately at the same latitude as then? And why were they there during the time of the dinosaurs and beyond?

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Ancient coral reef in Denmark , there are many remains of ancient corals Dendrophyllia candelabrum

Yes, back then, in the Cretaceous and early Cenozoic, the world was five degrees warmer than it is today. But in the Mesozoic, corals lived not only where Denmark is now, but also in hot tropical waters.

Why, if warming and acidification of water – and in the Mesozoic they were stronger than now – should kill them? And are those who predict the death of corals from these factors in our days right?

How to predict the past?

There are many different explanations that could be offered for what made past corals flourish. After all, corals are very complex creatures, and our knowledge of their past is full of gaps. For example, they are known to have existed in the Cambrian, half a billion years ago.

However, then, in the late Ordovician, for some reason their traces are lost for millions of years – and appear only in the following geological periods.

The reasons for this “disappearance” are unknown. Most likely, in reality, the corals did not go anywhere: they simply stopped leaving hard skeletons for a while, which are easy to find in the fossil record.

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Marine life in shallow waters in the pre-coral era as imagined by an artist

The fact is that these “external skeletons” are far from a prerequisite for their survival. Even the soft tissues of modern hard corals contain substances that prevent most marine predators from eating them (terpenoids give them a too peculiar taste).

As scientists discovered a decade and a half ago, even modern corals, which normally grow hard outer shells, can shed them when sea water conditions change and still survive. Why survive there: in such experiments, they became even larger than usual.

Another problem: scleratinia corals, similar to modern ones, appear only 247 million years ago (at least, there are no older finds of them yet). It is very difficult to understand what factors helped or hindered the corals of older groups, because their descendants did not survive to our time.

To resolve all these questions, the authors of a new paper in Nature took data from 535 coral reefs of the past and put them into a generalized digital model that took into account the climate and movement of the continents over the past quarter of a billion years.

Then the model was forced to make “predictions”: in which areas of the Earth during this time coral reefs should have flourished, and in which they should not?

An important point: the conditions in which reef-building corals could survive were laid down by the conditions in which they live today. The authors put into the model the assumption that corals will not live at +33 and above – as well as temperatures below +18, when they are too cold.

The model was based on more than just temperature. Depending on the dynamics of the movement of the continents, the number of shallow waters in zones with a suitable climate can either decrease (for example, when the continents diverge far in different directions) or increase (when they approach each other).

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The high bioproductivity of coral reefs serves as a kind of magnet that attracts fish

Based on these two factors, the model gave “predictions” that coincided with the places where the real coral reefs of antiquity were found. True, the coincidence of “predictions of the past” and the real past was observed only in 60-87% of cases.

For some reason, 72 out of 535 ancient reefs existed in places where the sea surface temperature was below +18 – that is, where modern coral reefs could not survive.

In 61 cases, ancient reefs were located in areas where the sea surface had a temperature of 33.8–35.6 ° C – that is, it was so hot there that modern reefs would not show (according to the authors of the model) growth there either.

The conclusions of the authors of the work are as follows: during the main part of the last quarter of a billion years, the zone of formation of coral reefs was much wider than today – going above 45 degrees north latitude.

The main reason for this is the warmer climate that dominated during the era of dinosaurs and the first half of the Cenozoic. Prior to that, a serious shrinkage of the coral reef range was observed only in the early Jurassic period.

It was at this time, the authors note, that the sea level dropped sharply. It is clear that the sea does not have a “drain hole”: falls occurred due to cooling, the temporary formation of ice caps in the polar regions that connected part of the sea waters.

A similar picture is observed in the last 37 million years – and on the rise. Since the appearance of the ice cap in Antarctica, sea levels have been decreasing and the climate has become colder.

What does the new work mean for the future of corals?

At first glance, this work is good news. But on the second… As its authors themselves write, “Although this may cause some optimism regarding the fate of coral reefs in a warming world, caution is needed here.

The coral reef ecosystems of the past originated on a greenhouse world where ecological communities were adapted to warm climates over millions of years. While it is possible that coral reefs will cope better with projected global warming than current models estimate… conservativeness in this kind of assessment is required.”

The idea of ​​the authors is simple: those corals were perfectly adapted to heat, and the current ones may not be very well. How to know something?

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However, not only fish

At the end of their paper, the authors even reinforce this point: “Although coral reef ecosystems can change their distribution under suitable conditions over very long periods of time, they are unlikely to be able to withstand the pace of rapid, anthropogenic climate change.”

It turns out that where we started, we ended up with that. Corals are dying out anyway – only now not from warming, as we were told before, but from its too fast pace – because it is artificial, not natural.

Something went wrong

The authors of the Nature article did a lot of work that gave clearly new results – they found, for example, that corals of the past could live in climates that today are considered too hot or too cold. But they did not rush to optimistic conclusions. Perhaps in vain.

The biggest problem with them (with the conclusions) is the very phrase “they [corals] are unlikely to be able to withstand the rapid pace of anthropogenic climate change.”

There is absolutely nothing in the work itself, no arguments that would support this conclusion – only a reference to two scientific papers by other scientists, published back in 2013.

The whole trouble is that the very idea of ​​“anthropogenic climate change is fast, and natural is smoother” does not find confirmation in the reality we observe. Recall: anthropogenic climate change for 1970-2020 brought a change in the average temperature on Earth by exactly one degree.

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And what were the natural changes in world temperatures during previous major climate changes? 14,700 years ago, the Boelling warming began on Earth . The ice sheets then melted so fast that the sea rose by 40-60 millimeters per year. Today, we recall, it rises by 3-4 millimeters per year – 10-20 times slower.

Maybe other laws of physics acted on the then Earth? And then the sea was rising 10-20 times faster than it is today, at a rate of warming slower than today? Let’s test this hypothesis: let’s turn to paleoclimatic data.

Data from South Alaska about 14,600 years ago show a warming of 3 degrees in no more than 90 years. And in the current global warming, no one has seen any increase in temperatures in Alaska by three degrees in 90 years.

Let’s turn to the last of the rapid natural warmings on our planet: the end of the Younger Dryas, 11.6 thousand years ago. According to the analysis of the Greenland ice, in that era it got warmer by at least 6 degrees.

The authors of the corresponding work write : “The climatic transition at the end of the Younger Dryas [in the area of ​​ice sampling]… happened in less than 50 years, most of it in 10-20 years.”

It turns out that the rate of climate change in Greenland at that time was at least six degrees in half a century – or at least 1.2 degrees in ten years.

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And it is precisely that “at least six degrees.” The most likely estimate is much higher – 10 degrees . It is possible that the real rate of warming then was even 14 degrees for all the same, geologically insignificant time.

However, with regard to the future fate of corals, it does not matter what the actual rate of warming at the end of the Dryas was – 6 degrees or 14. Another thing is important: during the modern warming in Greenland, average temperatures have not risen by six, five, or four degrees.

Take a look at the graph below: the coldest recorded year (1914) in Greenland of our era is only 3.46 degrees cooler than the hottest recorded year there (2016). Even if we take the period after 1980, it is impossible to get more than 1.0 degree in ten years.

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Strictly speaking, changes in mean temperature should be estimated from five-year averages. If this is done, then for the entire period of observations in Greenland it has not warmed even by two degrees. No matter how you rotate this graph, the same pace as 11.6 thousand years ago cannot be found on it

That is, in any case, now in Greenland it is getting warmer no faster than then. Meanwhile, absolutely all climatologists agree that the Arctic is the most sensitive region to modern warming.

In the rest of the world, temperatures rise much more weakly than there. 11.6 thousand years ago, the situation should have been similar: a ten-degree warming in Greenland over half a century corresponded to only a few degrees of warming on Earth as a whole.

However, even a few degrees in 50 years is much, much faster than the one degree we’ve seen in the last half century.

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In Europe, at the end of the Younger Dryas, in a geologically insignificant period of time, the paleotundra ecosystem turned into a modern one. This flower – the dryad, which gave the name to the youngest dryas – in the new, warm European climate, retreated high into the mountains, where it has survived to this day

Conclusion: from the data we have, it is impossible to conclude that the current anthropogenic warming is faster than the natural warmings of the past. On the contrary, according to the data we have, the last ice age ended with a much steeper warming than we see now, in the industrial age.

There is not a single scientific work that would analyze exactly the end of the last ice age on the basis of any empirical data, and would come to a different conclusion.

How do you actually know if corals are going extinct?

So, a good work on the fate of corals of the past did not correctly indicate their fate in the future, since its authors judged the rates of warming in the past from other people’s words. There is a natural desire to understand: what will happen to corals in the real world?

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The easiest way to know for sure is to find data on temperatures at which present-day reef-building corals begin to die en masse.

Not the way the media does, to take the temperature at which the Great Barrier Reef corals begin to bleach (expel symbiont algae), but otherwise: to find exactly those lines of corals that often encounter high temperatures.

It is clear that on the Great Barrier Reef, in the open ocean, sea water rarely reaches +30. That is, the corals there rarely encounter heat. To understand their future, the answer to the question must be sought where above +30 in the water happens regularly.

It is in such conditions that the corals of the Red Sea and the Gulf of Aden live. The latest work on this topic reports : if we take the coldest parts of the Red Sea, then corals in it normally exist from 34.72 ° C ( S. pistillata ) to 35.15 ° C ( P. verrucosa ).

But in the warmest parts of the coral “metropolis of heat” they do not die even at 37.70 ° C (species A. hemprichi ).

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An approximate assessment of the bioproductivity and species diversity of corals in different epochs is seriously hampered by the fact that most likely we have not yet discovered the vast majority of reefs of the past. And many of them will not be found: the collision of the edges of the lithospheric plates of the past with each other means that a significant part of them have already gone deep into the lithosphere and are inaccessible for scientific research

Furthermore. It is very possible that in reality, even in the warmest waters, warming will accelerate the growth of reefs – their flourishing, not death.

So, in 2017, a team of scientists took those same heat-resistant Red Sea corals ( Stylophora pistillata ) and placed them in water 1-2 degrees warmer than summer water (peak temperature) in their natural habitat.

At the same time – check so check – and the pH of the water was lowered to 7.8, approximately like in the Mesozoic, and much lower than in the modern seas of the Earth.

It turned out that Stylophora pistillata was not only unaffected by such conditions. Its symbiont microalgae also began to photosynthesize more actively – their pigmentation improved, they began to produce twice as much oxygen, and their primary photosynthetic productivity increased by 51% .

It seems that we have discovered the “great secret” behind the conclusion of the authors of the work on the reefs of the last 247 million years.

Their model failed to predict the existence of ancient coral reefs that lived at 33.8–35.6°C, simply because the authors of this model were not yet aware of another recent work on Red Sea corals.

If you know that even today 35.6 ° C is far from the limit for the survival of corals, then it is not surprising that in past eras they quite coped with such temperatures.

Corals (or rather, their millimeter-sized larvae) are carried by currents quite efficiently. Therefore, even one heat-resistant population sooner or later colonizes all places suitable for it.

And the fact that corals of the same species living in hot zones for a long time become more heat resistant than corals of the same species but in colder zones clearly shows that there is no reason to expect coral extinction due to warming.

Judging by the above figures, even today A. hemprichi tolerates heat no worse than Mesozoic corals or the Paleocene-Eocene climatic maximum.

Given that in the Mesozoic on Earth it was on average +20, and in the Paleocene-Eocene maximum – even +26, then high temperatures will obviously not destroy corals.

Fortunately, for the time being, the average +15 on the planet and not a single mainstream scientist promises an increase in temperatures by 5-11 degrees above current values ​​in the foreseeable future.

And this is good: if you take marine life, then it is highly concentrated around coral reefs. The biomass of the inhabitants of the ocean is 90 times less than that of the land living world: no matter how small the reef zones are, a very large part of the aquatic biodiversity falls on them.

But won’t sea acidification kill corals?

Carbon dioxide entering the water causes the pH to drop. And in less alkaline water, in theory, it should be harder for corals to create an outer hard shell based on calcium carbonate. Maybe warming alone will not kill reefs, but what if the negative impact of acidification, acting simultaneously with warming, will finish off the corals?

In 2021, a work was published in the Proceedings of the National Academy of Sciences , the authors of which conducted a full-scale experiment.

They built the mesocosm, a special experimental environment with an open entrance, in which they settled eight key coral species and the main species of fish and algae.

Then an open waterway was built into the mesocosm from the vicinity of the nearest coral reef, so that new species typical of such places could freely penetrate there.

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The special value of the work in PNAS is that it was carried out on islands that were reliably isolated from the periodic invasions of the so-called crowns of thorns. These starfish are predators, one of the few able to digest corals (for which they literally “turn inside out”, such is their way of digestion). There are especially many of them on the Great Barrier Reef, where they make a great contribution to the destruction of corals

There were four types of media in the mesocosms. In the first – control, where the water and pH are like in the ocean today.

In the second, there was no acidification of the water, but it was heated 2 °C higher than in the environment. In the third, there was acidification (pH 0.2 lower than modern, this is expected by the end of the century), but there was no heating. In the fourth mesocosm, both warming and heating were combined.

It should be noted that an increase in the average water temperature in warm seas by 2 °C can happen only when global warming is noticeably higher than two degrees – fortunately, global warming changes the temperature mainly in high latitudes, where there are no coral reefs.

The tropics and the equator are warming many times slower than the average increase in planetary temperatures. That is, the conditions of this experiment correspond to the radical scenario of global warming – 3-4 degrees higher than modern values. Almost Mesozoic.

However, in none of the four mesocosms during the two years of the experiment did the death of corals, as well as the disappearance of any other species of the reef ecosystem, happen.

In the mesocosm with elevated temperature, the diversity was even greater than under standard conditions. And where there was both elevated temperature and acidity – exactly the same as in the “climatic norm”.

It turns out that even the distant future – with a severe version of global warming – has not reduced the biodiversity of typical coral ecosystems in any way.

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A typical fragment of the Great Barrier Reef near Australia. Corals in this region have experienced many severe temperature fluctuations, including the extremely rapid warming of the end of the Younger Dryas. About 15 thousand years ago, the old Great Barrier Reef began to flood the sea rising 120 meters. Millimeter coral larvae with currents simply migrated to higher places, which until recently were dry land, but the reef itself did not even think of disappearing. The entire projected rise in sea level, even under the most drastic scenario of modern warming, is half what happened then, at the end of the last ice age

Perhaps this could have been predicted in advance. The same Mesozoic saw water much more acidic than it is today, with a pH in the region of 7.8. But the decline of corals was not observed.

Let’s summarize. Every time we hear “global warming will kill so-and-so”, we have to ask ourselves the question: why didn’t it do it sooner? 11.6 thousand years ago, when was it even faster than today?

120 thousand years ago, when the warming was much more serious than today and hippos were splashing in the Rhine?

What makes us expect half-billion-year-old corals to rise and die today, under conditions inconceivably more favorable than those experienced 20,000 years ago, during the era of maximum shrinkage of the coral reef range?

Perhaps the only reason anyone predicts coral extinction (and many others) is purely computer simulations. As in the latest IPCC report, promising the extinction of all the same corals by 2100. No empirical evidence, outside of computer simulations, has yet pointed to the death of corals from global warming.

Any model is limited by what kind of data was entered into it. If you do not enter into the model that corals live even at +37.7, then you should not be surprised when it shows that part of the real reefs of the past could not exist, because it was warmer than +33.

If you do not enter into the model the fact that corals in today’s experiments and in the past of the Earth tolerated temperatures and acidification without noticeable problems, then you should not rely on the forecast that it gives for 2100.


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