Ice, CO2 and time

Short introduction

I can consider today's climate changes only from the point of view of what I know about the climate of the past; I will reconstruct the natural atmospheres of the last geological system of the quarter period, whose age is slightly less than three million years. Such knowledge has been unusually useful recently. It suddenly turned out that not only “the present is the key to understanding the past” [ 1 ], but the past is the key to understanding the present and even the future.

So we agree that my colleagues, well versed in atmospheric physics, will talk about the mechanisms of the modern climate, and I - about centuries -old trends. In essence, they are inextricable.

Climate definition and its evolution

The most simplified, but correct definition of climate, is "the weather for a long period, about 30 years or more." Now it remains only to find out what the weather is and why it is not a climate. According to the definition of Noaa, the weather is the state of the atmosphere depending on temperature, humidity, cloudiness, pressure, wind and other meteorological indicators. The weather is determined by these conditions at a particular moment, and the climate is determined by the average weather conditions for a long period.

Let's add here a scientific definition given back in 1982 by A. Monin [2]:

“ The climate is the statistical ensemble of states passing through the atmosphere -ocean -drying system for periods of several decades.

The statistical ensemble here is understood as many $ \ MathScr {a} $ elements A with a probabilistic measure of P (A) specified for each measurable subset A ⊂ $ \ MathScr {a} $ - its probability p {a ∈ A} .

Since the state of the AOS system is multicomponent fields, the climate mathematically is defined as a multicomponent random field . ”

Remember the main thing: we are dealing with a huge number of variables of climate system, and they can change randomly. Under this condition, at first glance, the problem of reconstruction or, moreover, the predictions of the climate look frightening. But this is not so. Co -shaped processes that can ruin the forecast for the day after tomorrow, on a long centuries -old time scale, are leveled and averaged, and from the chaotic movement we get regular trends. To understand these patterns, you need to understand the theory of chaos, which cannot be made in these notes. But we know that there are climate of modern land, say marine, continental, tropical, etc. [3], they are determined by averages of temperature and humidity for certain territories, and this does not cause us surprise. So, we must agree that, knowing the initial conditions of the AOS system, we can try to restore both the past and future climate. Here the dog is buried. We must know the initial conditions in order to understand where our climatic system will move further.

The weather is chaotic, but the climate in general is not [4].

Scientists have made so many observations over the past few centuries, collected so many data, discussed so many theories, learned to determine the absolute age of ancient geological rocks, recognize the chemical composition of the deposits and understand on this basis, in what natural conditions they accumulated, that paleoclimatology has become more accurately than the usual story, which we still do not know how to measure it by isotopes of chemical elements and it Model to the future. We will talk about predictions the next time, but what knowledge paleoclimatology has given us.

1. Cycles of Milankovich

The climate of the Earth undergoes significant changes for 100,000–1,000,000 years. These changes occur because the climate is sensitive to planetary orbital changes. The Earth rotates with an inclined axis, making the so -called precession, where the top of the “top” moves around the circle with a period of about every 23,000 years. The slope of the Earth’s axis varies from 22.1 to 24.5 ° C with a period of ~ 40,000. The precession and the slope of the Earth axis occur as a result of gravitational attraction of the moon and planets of the solar system, mainly Jupiter. The earth's orbit around the Sun, slightly elliptically elongated, cyclically changes its eccentricity in the intervals of time at ~ 100,000 years.

Changes in the inclination of the earth's axis and its precession are reflected in the change in the incoming solar radiation, especially in high latitudes, while a change in the eccentricity affects the change in the resulting solar heat on all latitudes.

Currently, it is recognized that changes in the seasonal and latitudinal distribution of solar heat, reaching the Earth, generated by these orbital cycles, Milankovich cycles, lead to the occurrence, growth or reduction of the ice hats of the Earth. It is important to consider all three cycles together. None of these cycles individually would give any changes in the earth's climate.

Milankovich theory has gone a difficult path of adaptation by the scientific community, for several decades it was discussed and doubted, just as now the theory of global warming. But after the extensive evidence of these changes was obtained, based on the measurements of the isotopic composition of the oxygen over the past 130,000 years from the ice cores of Greenland and for 800,000 years in Antarctica, and later from the deep -sea bottom columns of the ocean, gossips, stalactites and stalagmites in deep -sea lake deposits, in corals, etc. d., there are no more doubts.

Milankovich cycles

Period, thousand years

Cause

Limits

The mechanism of action of the cycles: the most warm summer in the northern hemisphere (perihelia at the maximum ellipticality of the orbit and the large inclination of the axis) attracts global warming, and vice versa.

In the last million years of glaciation and interglacial on Earth, they are due to orbital causes and occur with a frequency of ~ 100 thousand years.

Two essential circumstances should be noted:

1) This cycle is unstable, it can vary, lengthening or shortening for several thousand or even a dozen thousand years. This is due to the mutual gravitational attraction of the planets of the solar system at different points of passing their orbits and needs constant adjustments to the current situation. But the cycle itself remains; 2) Starting a million years ago, the frequency of glaciation on Earth is close to ~ 100 thousand years, coinciding with the period of change in eccentricity. Prior to this, at least the last three million years, glaciations went with a frequency of ~ 40 thousand years, coinciding with the frequency of a change in the inclination of the earth's axis. The explanation of this leap has already dawn in numerous studies, but it is not yet ready for popular discussion. In our case, we take it for a given. To date, the glacial periods are replaced by interglacial periods, that is, the climate of the Earth changes radically once every 100 thousand years, for the reason that the Earth, as a result of the aggregate position of all three orbital changes, receives more solar radiation to its surface.

We do not live so long. Something should explain our minor climate problems.

2. The events of Dancegaard - Eshger and a new understanding of the role of CO2

While the orbital cycles forced glaciations have sinusoidal vibrations, the cycles of glacial and interglacial periods are not simple sinusoids. The glacial periods, on the contrary, abruptly end, and begin gradually, which indicates complex feedback between these processes.

After ice cores from Greenland columns with a very high temporary resolution were first obtained and investigated, it became obvious that there were sharp short -term changes in the climate within 100-10,000 years. Thus, against the background of orbital climate changes, there are also short -term ones, which are much more important to human life.

And now attention!

In 1972, the Danish scientist Danshaard first examined the oxygen content in the accumulated layers of ice cores and demonstrated that the ratio of heavy isotopes of 18o and the lungs of 16o in the ice systematically varies in accordance with a change in temperature, and this ratio of 18O/16O is reduced by 0.7 (PPM) by every 1 ° C of the derogation [5].

This ratio is simplified called "Delta 18o" and is determined by the formula

δ 18O = ((18O/16O) in the sample - (18o/16O) Standard/(18o/16Ostandart)) × 1000 ‰

Which since then has become the main tool for obtaining paleotem operators in almost all types of scraping.

A little later, already with the Eshger [6], they found that each sudden growth of 18O corresponded to 50 PPM increased concentration CO2 and vice versa. Thus, Dansgaard - Eshger (D -E) episodes of warming were opened, accompanied by subsequent cooling. They occur in quasi -aidic mode with an interval of ~ 1.2 thousand to 800 years. In total, there are 25 d -e episodes for 120 thousand years (Fig. 2).

If you begin to check the D-E-BECTION in Wikipedia, you will read that their cause is still not clear. This is not entirely true. Although there is still much to understand, one of the explanations is confirmed by the largest number of studies, and this phenomenon is called “overturning thermal circulation” (there is no corresponding Russian term), which is responsible for the formation of the meridional circulation of the Atlantic Ocean (“The Atlantic Meridional Overning Circulation” - AMOC is a standard English abbreviation, or Atlantic meridional heat transfer, AMPT in Russian, we will use the hybrid of these terms “Atlantic meridional thermal circulation” (AMTC).

The initial assumption about why the events of Danshaard - Eshger took place, was proposed by Walley Broker [7] and it remains a predominant opinion so far. This is a conveyor, which is part of the large -scale circulation of the ocean, adjustable by the global gradient of density created by the surface of the ocean and the delivery of fresh water. He is responsible for the large -scale transfer of water masses in the ocean, including the transfer of oxygen to the deep layers of the ocean. The entire cycle takes ~ 2000 years [ 8 ].

This schedule, without which you now you will not find a single textbook on any science of land, did not exist only 30 years ago! We all knew from the school bench about the existence of the Gulf Stream, but we did not know that this was just part of the global “conveyor belt” of currents and that symmetrically superficial currents transferred by the wind, there are tribed currents regulated by the difference in temperature, salinity and density of oceanic water in various latitudes of the planet. But the discovery of the broker was not that there were tributarous currents (this was gradually being clarified with the improvement of the methods of research of the ocean), but that this conveyor tends to drown out with sharp warmings on the surface of the Earth. And resume with cooling. How?

I will quote the author himself, V. Broker with small abbreviations.

“ This large -scale circulation is due to the descent into the deep layer of cold and salt water in two places on the planet: in the North Atlantic, in the vicinity of Iceland, and in the Southern Ocean on the perimeter of the Antarctic continent.

These flows are important for the climate of the Earth, because they redistribute heat. This redistribution is especially important for the land surrounding the North Atlantic. Replacing the water, which sank to the bottom of the North Atlantic, warm waters on the surface of the ocean are transferred north to Iceland in the surface sleeve of the conveyor. Since this upper sleeve of the current passes through low latitudes, it is heated by the sun. When it reaches the high northern latitudes, the accumulated heat is released into the atmosphere. In the winter months, this heat weakens cold Arctic air masses that move east through the Atlantic. This additional heat helps to maintain soft winters in Northern Europe. The scale of water transfer and energy in this conveyor is huge. It is equal to one hundred Amazons and corresponds to the amount of precipitation throughout the globe. The sleeve moving north carries water with an average temperature of 12 ° C to Iceland. Water plunging to a depth is on average only 2 ° C. Therefore, for each cubic centimeter of water transferred to the north the upper limb of the conveyor, 10 calories of heat are thrown into the atmosphere. This is generally a stunning value equal to about one quarter of the solar heat, sitting into the atmosphere above the part of the Atlantic Ocean located north of Gibraltar! "

And for some reason, this conveyor will drown out.

“What can cause the restructuring of circulation? Although we do not have a clear answer to this question, there is one probability that can be called a salt generator. According to models, the most effective means of intervention in the formation of deep water is to increase the flow of fresh water into the region, where “deep water” is formed. Such injections dilute the salt content in surface waters, thereby reducing their density. If this dilution continues to such a point that even in the coldest winters, water is dense enough to supplant the lower layers of water, no longer can be extinguished. That is why deep waters are not formed in the north of the Pacific Ocean. Its superficial waters contain so little salt that even when cooling to the freezing point (–1.8 ° C), water is not dense enough to penetrate into the deep sea. ”

Langmuir Charles H., Broeker WS How to Build a Habitable Planet (PP. 560–561). Princeton University Press. 2017.

So, warming causes an influx of fresh water into the ocean, probably mainly due to the melting of ice, which leads to a lack of injection of dense and salt water into the tribute cold, which inhibits the conveyor, in any case in its northern part.

30 years of study of D-E-BEDIES have shown that the process that initiates them is actually multi-stage, but the main conclusion has not changed: warming in the northern part of the Atlantic leads to weakening thermal circulation and subsequent sharp cooling in high latitudes.

The third mechanism of climatic changes: the “closure” of overturning thermocaline circulation caused by warming and accompanied by subsequent sharp cooling.

So, we realized that the circulation of the atmosphere and the ocean, complex, but still understandable transfer of matter and energy, can change the climate of the Earth, if not to the same extent as the orbital cycles of Milankovich, but very significant, and much faster! These changes, in contrast to the orbital cycles, occur sharply and not quite regularly, and they are always accompanied by parallel growth and a drop in the concentration of carbon dioxide in the atmosphere, which is proved by diverse paleoclimatic data.

But that's not all.

3. Hinrich event

In 1988, at that time, the young German geologist Hartmunt Heinrich, who studied sea deposits, made a striking observation [9]: in the precipitation of a deep-sea well drilled in the north-eastern Atlantic, he found six consistent layers similar to deposits of moraines, that is, to the deposits of melting glaciers, intermittent layers of normal thin marine precipitation. The layers were composed of a wide variety of rough precipitation, including large boulders, pebbles and gravel.

A lithological analysis showed that the age of stones is about two billion years, while the breeds containing them had an age of only a few hundred million years. Основываясь на этих наблюдениях, молодой Хайнрих сделал смелый вывод, что слои отложились в результате таяния айсберговых армад Лаврентийского канадского ледяного щита. Но как они туда попали? Хайнрих предположил, что они были принесены огромными айсбергами Лаврентийского ледникового щита, покрывавшего Канадский кристаллический щит, в процессе его внезапного таяния. Он назвал их Ice-rafted debris (IRD), дословно «сплавленные льдом обломки пород» (в русской терминологии я не нашла этого термина, связанного с Х-событиями; самый близкий термин «ледниковый рафтинг»).

Вслед за этим пошли годы исследований. Идея подтвердилась многими авторами, новыми анализами и разными палеоклиматическими методами. Оказалось, что Heinrich-event (Н-event), Х-событие, как его стали называть, совпадает с пиками Д—Э-событий, но оно более сильное и заметное, т. е. начинается очень внезапно и сопровождается бо́льшим похолоданием, чем при Д—Э-событиях. И конечно, то, что Х-события происходят гораздо реже, их периодичность 7–10 тыс. лет.

Самый важный вывод из этой работы, как в дальнейшем показал В. Брокер [10], Х-событие, заглушает Атлантическую меридиональную термохалинную циркуляцию (АМТЦ) в Северной Атлантике и приводит к резкому похолоданию на Европейском континенте и даже сказывается глобально на всей планете! Это заключение заставило по-новому посмотреть на Д—Э-события, и после многолетних исследований было установлено, что каждое из них сопровождалось ослаблением, но не полной остановкой АМТЦ.

Также выяснилось, что Антарктические ледяные колонки очень слабо или никак не проявляют Д—Э-события [11]. Еще годы исследований. И вот научное сообщество приходит к выводу, что события Д—Э отчетливо проявляются только в Северном полушарии, а события Хайнриха глобальны [12]. Их периодичность 6–10 тыс. лет. Как можно видеть (рис. 5), цикл событий Д—Э во время ослабления завершается пиком события Хайнриха.

Теперь мы знаем, что наряду с «тысячелетними» изменениями климата Д—Э, вызванными обратной связью системы АОС, существуют 7–10-тысячелетние Х-события, которые приводят к резкому изменению термохалинной циркуляции, а именно ее ослаблению или полному закрытию. Особенно важно знать, что все эти события неизменно сопровождаются соответственным ростом или падением СО2 в атмосфере. И как мы ниже увидим, не только в ней.

Итак, в долгосрочной перспективе изменения объема ледников и ледяных щитов, а значит, и климат Земли контролируются астрономическим воздействием солнечной инсоляции. Однако на коротких тысячелетних отрезках времени обратные связи в атмосфере, океане и на суше существенно трансформируют эту взаимосвязь. К счастью, мы учимся ее понимать.

Перед тем, как перейти к рассказу, что происходит с СО2 в атмосфере в геологической перспективе, я сделаю некоторые замечания.

• Революционное открытие Хайнриха и Брокера: небольшое потепление может привести к закрытию глобальной океанической циркуляции и резкому похолоданию.
• Мы не можем остановить тектонический термостат Земли, но мы можем его поломать.
• Углекислый газ не оседает в атмосфере, достигая концентрации на изотерме 270 K, как вода, а продолжает насыщать собою атмосферу, поднимаясь всё выше и достигая уровня 220 К, продолжая отражать инфракрасное излучение от поверхности. Это делает углекислый газ исключительным.

Об этом поговорим в следующий раз.

Irina Delusina

1. Цитата, приписываемая Чарлзу Лайелю .
2. Монин А. Введение в теорию климата. Гидрометеоиздат, Ленинград, 1982.
3. The Köppen climate classification .
4. Archer D. Global Warming. Understanding the Forecast. Wiley, 2012.
5. Dansgaard W., Johnsen SJ, Clausen HB and Gundestrup N., Stable isotope glaciology, Meddel. om Gronland, 197, No. 2, 53. Рp, 1973.
6. Oeschger H., Beer J. , Siegenthaler U., Stauffer B., Dansgaard W. and Langway Jr. CC, Late-glacial climate history from ice cores, Climate Processes and Climate Sensitivity, 288–298, 1983.
7. Broecker WS, Peteet DM, Rind D. Does the ocean-atmosphere system have more than one stable mode of operation? Nature. 1985;315(6014): 21–6.
8. oceanservice.noaa.gov/education/tutorial_currents/05conveyor1.html
9. Heinrich H. Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years. Quaternary Research. 1988;29(2): 142–52.
10. Broecker WS The great ocean conveyor . Oceanography 4 (2), 79–89, 1991 .
    
11. Broecker WS Paleocean circulation during the last deglaciation: a bipolar seesaw? Paleoceanography 13 (2), 119–121, 1998 .
12. Markle BR, Steig EJ, Buizert C., Schoenemann SW, Bitz CM, Fudge TJ et al. Global atmospheric teleconnections during Dansgaard–Oeschger events. Nature Geoscience. 2017;10(1): 36–40.
13. Langmuir Ch., Broecker W. How to Build a Habitable Planet, Princeton University Press, 2012. 718 p.
14. pmel.noaa.gov/co2/story/What+is+Ocean+Acidification%3F
15. Archer D. Near miss: the importance of the natural atmospheric CO2 concentration to human historical evolution. Climatic Change. 2016;138(1):1–11.