In the last two millennia, there have been two periods of global cooling, which have led to rapid sea level fluctuations - up to 4.23 millimeters a year. This is noticeably stronger than today. It turns out that the current climate change is not unprecedented in human history. And the past retreats of the sea could cause two whole civilization crises. Let's try to understand why this is important for us now.
Sea level: different everywhere, despite the law of communicating vessels
We usually think of sea level as something fixed and the same all over the planet. This is not the case: the sea level in Ireland is more than 150 meters higher than that of the Maldives, and even its current rise in some places is faster, but in some it is exactly the opposite: there the average ocean level "sags".
It is clearly seen that in the New Guinea region the sea is more than 80 meters higher than the global average, and in the Maldives - - more than 100 meters lower.
At first it seems that this contradicts the law of communicating vessels: after all, the World Ocean is one, and water from a higher place must flow into a lower one, leveling the surface. In fact, there is no contradiction. Our planet is not a sphere, but a geoid, and those parts of it where the sphere is slightly "chipped" have a different surface shape. Restrictions from gravity and the shape of the Earth create a huge, up to a couple of hundred meters, difference in sea level in different places.
The places of the greatest convexity of the geoid are highlighted in red, in blue - places of its greatest concavity.
In order not to get confused and understand what is happening on the planet as a whole, they use the concept of global sea level. It averages all the irregularities of the geoid, which makes it possible to obtain a single measure of the change in the “average” sea level.
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So, until the 1980s, it grew by 1.8 millimeters per year, and since the 1980s - by 3.2 millimeters per year (now it is 3.3 millimeters). The value itself is small, but there are places where, due to the local specifics, the sea comes especially quickly: for example, in the Pacific Ocean.
Specifically on the islands of the Pacific Ocean, this has not yet affected badly: on the contrary, the area of Tuvalu and Kiribati, recorded by satellite images, has grown. But, definitely, in the future, somewhere on Earth, a situation may arise when the rise of the sea will force the coast to be strengthened by pouring sand on it.
However, it should be remembered that the sea is capable of not only advancing, but also retreating. And such processes can be even more dangerous.
An ancient crisis: the first cold snap
One of the most important properties of sea level change is that it clearly reflects what is happening to the climate. If he changes, the sea changes with him. Because of global warming, the sea has risen to a level of up to 21 centimeters since 1900. Mainly due to the expansion of sea water from heating. An additional factor was the melting of part of the ice in Greenland and other regions. Rising sea levels are one of the key pieces of evidence for global warming.
The question arises: has the sea level changed before this? Previous measurements on the Atlantic Ocean gave different data: somewhere changes over the past two thousand years were recorded, somewhere - not, and it seemed that all these fluctuations in any direction did not exceed 0.25 meters. Not much, but still more than the current sea rise due to global warming.
The problem with these numbers is that they were counted by the level of salt water in coastal swamps. This is not a direct measurement of sea level and may contain significant errors. The data on the Pacific Ocean are scattered, not always clearly dated, and therefore some researchers consider them speculative.
An international team of scientists recently published a paper in Nature Geoscience exploring the coral atolls of the Maldives, islands in the Indian Ocean close to the equator. The advantages of this research point are that there is no noticeable tectonics, the subsidence and uplift of the land for geological reasons are extremely slow. Coral atolls are closely related to sea level: when the sea drops noticeably, the corals end up in the open air at low tide and die.
Maldives atoll where samples of work-dated corals were collected.
In addition, the authors used an accurate uranium-thorium dating of ancient corals. It is based on the fact of the decay of uranium with the formation of thorium atoms: and, since the rate of such decay is known, it makes it possible to accurately calculate the chronological frame of changes in sea level (more precisely than in most previous works on the topic).
One of the sampling points of corals during the measurement phase.
It turned out that in 2000-100 BC the sea level in the Maldives was 0.5 meters above the average in our era, which means a rather warm climate. In 234-605 AD, the level began to fall sharply, at a minimum it was 1.34 meters below the current average sea level. Two samples even indicated a level 1.45 meters below current.
The event lasted 371 years and cannot be explained by anything other than a noticeable drop in temperature in the equatorial regions of the Indian Ocean. Equatorial waters are often more susceptible to warming because the water in them raises the temperature faster, expanding and leading to the advance of the sea - but the same pattern works in the opposite direction.
It turns out that in 234-605 the planet could be noticeably colder than today. Interestingly, this period coincides with the decline and collapse of the Roman Empire and the Great Migration.
Historians associate the decline of the Roman Empire with systematic crop failures, which often lasted for several years in a row.
Earlier, on the basis of narrow tree rings (the norm during a cold snap), other scientific groups have already put forward the hypothesis of the "Late Antique Little Ice Age". However, then it was about the 500-600s, and there was no data for an earlier time. Now it becomes clear that the cooling could have begun as early as the 3rd century, coinciding in time with the crisis of ancient civilization.
The authors of the study note that these 371 years account for the 13 coldest decades of the last two thousand years. Since colder periods normally coincide with a decrease in precipitation, they should have had catastrophic consequences for agriculture in the form of droughts and crop failures.
After the 7th century, corals in this area did not grow until the 15th century: the authors believe that the sea level rose to at least modern or even higher, not giving the local coral reefs enough light to grow.
New time: the second wave of cold snap
In the years 1481-1807, the Maldives experienced a second period of low sea level: on average, at this time, it was 0.71 meters lower than today. By 1807, the level had reached a minimum - 0.89 meters below the modern average. It should be emphasized: the fall of the sea near the Maldives, both in modern times and in antiquity, is much stronger than its rise during modern global warming.
Illustration of February from the French Book of Hours 1412-1416. Light clothes, thin snow, and all this in the middle of winter. In the illustrations of the rest, less severe months, there is no snow at all. It seems that as early as the beginning of the 15th century, the climate in Europe was quite warm.
In 2017 alone, the sea level near the Maldives rose by 0.89 meters, that is, reached the current one. Over 210 years of anthropogenic global warming, the sea has grown by the same amount that it retreated over much shorter periods of time in the III-VII and XV-XIX centuries.
The peak sea retreat recorded in the work for the last two thousand years is 2.8 millimeters per year. The peak rate of sea level rise today is 3.4 millimeters per year (this is the limit, commonly referred to as 3.3 millimeters per year). This shows that the current advance of the sea is comparable in speed to its retreat in the last thousand years.
In addition, after the retreat of the sea in late Antiquity, its rapid advance followed - and by the beginning of the 700s, according to the work, the rate of the sea advancing was 4.23 millimeters per year, that is, significantly more than today.
It is no less interesting that in the case of the Maldives, the current sea level is still well below the level of the heyday of the ancient world - 2000-100 BC (a difference of half a meter).
Canals of Holland, 17th century. Clothing is noticeably warmer than in the Middle Ages. In Western Europe, the use of sleds has become the norm.
The events of modern times - this period is also called the "little ice age" - included a series of wars and famine years. The most brutal of them date back to the first half of the 17th century, when a series of several hunger years led to the Troubles in Russia, and the population of Germany fell by 50 to 70% from a combination of wars and famine.
Definitely, this is also very similar to a civilization crisis. Key countries of 1481, such as Spain, Portugal, leading Italian republics such as Venice, Florence and Genoa, by 1807 became only a pale shadow of themselves, and China was so weak that it experienced conquest by another wave of barbarians.
Nevertheless, a number of other states have noticeably strengthened their positions during this time. England, France and Russia by 1807 had become much larger, had more population and opportunities. True, in all three cases this coincided with the active territorial expansion of these states, which allowed them to obtain resources that were absent there in 1481.
Why is it important
The sea cannot recede either 89 centimeters (peak from work) or 0.7 meters (average for two cold periods) if there is no cold snap. Today the Maldivian atolls are the most accurate measured by sea level over the past thousand years.
And from measurements on local corals it follows that during this time there were two fairly strong cold snaps, in amplitude similar to the current warming. In terms of speed - judging by the record-breaking sharp retreat of the sea by 2.8 millimeters per year - these cold snaps could come even faster than the current warming.
Meanwhile, earlier in 2019, a work was published that generally denied the very reality of the Little Ice Age of the New Time, as well as the Late Antique Little Ice Age. Its authors argued that the most reliable signs of cold weather in the Little Ice Age for different regions happened at different times, therefore, on the whole, the temperature on Earth did not seem to change.
Fluctuations in sea level of up to 0.89 meters indicate the extreme doubtfulness of this conclusion, often drawn from historical records or indirect temperature indicators. Sea level is a more reliable measure of global temperatures than circumstantial evidence.
The authors of the new article note that their predecessors could not obtain accurate data on sea level changes in other places - mainly because coral reefs over the past 200 years have been actively growing in the wake of rising sea levels. In the process of re-growth, they often destroyed or strongly mixed materials from older reefs, which were preserved in only a limited number of places with extremely favorable conditions such as the Maldives. That is, the method used in their work allows a deeper and more realistic assessment of the changes in the climate of the past centuries.
An unprecedented climate change? Does not look like
The researchers conclude with the conclusion: "The magnitude and speed of sea level changes in the current Indian Ocean are not unprecedented in the past 2000 years." These words look dry and stingy - in fact, this is a scientific journal, what else can its text be in style?
But there is a lot behind them. If the sea in the last 20 centuries retreated at a speed of up to 2.8 millimeters per year, and came at a rate of 4.23 millimeters per year, then it turns out that its current growth of 3.3 millimeters per year is not only not unprecedented, but also quite moderate. The authors believe that the ancient period of sea level for the Maldives (half a meter above the current one) will be reached only in the 21st century.
More importantly, the temperature changes in the pre-anthropogenic era, when people could not influence the climate as they do today, are comparable in scope to the present. This is a very unpleasant sign. Random fluctuations in climate such as the Late Antique or Little Ice Age can lead to alarming consequences in the form of high cold mortality (and it is much higher than mortality from heat), crop failures and droughts.
It would be nice to understand and find out the exact culprit of the events of those years - in order to try to avoid their repetition. Or at least not be caught off guard.
Alexander Berezin