The great climatic variations

The great climatic variations

How man has interacted with the great climatic variations

If even non-experts show keen interest in learning about the behavior of living beings such as dinosaurs, mammoths, horses, dogs, etc. following environmental variations, (disappearance of some species or adaptation of their defenses to new situations in order to survive), a reconstruction of the climate in past times, starting at least from the eras in which Homo sapiens was already present in the various continents, must we are more interested in collecting elements to ascertain how Man has reacted to the great climatic variations, adapting to new situations.

In order not to go too far back in time, we have limited the collection and reworking of bibliographic information to the last 12,000 years, that is, starting from an event of capital importance for the Earth and therefore for all living beings: the end of the last ice age that began around 900,000 years ago and lasted until around 18,000 BC

The paleoclimatic reconstructions, for the reasons explained in the previous articles, are based on the deviations of the average temperatures of the various periods compared to the current average temperature. The deviations of the average temperatures of the different eras are taken from historical data (chronicles, literary quotations, artistic reproductions, photos, etc.), fauna and botanical data, geological data, chemical and physical studies, and obviously in the last centuries from instrumental records.

Here we want to recall what has already been repeatedly emphasized in the past months: the trends in the values ​​of the meteorological parameters are not the same as those that characterize climatology, for which variations detectable over several decades are taken into consideration, while meteorology emphasizes the daily variations in a more seasonal context. Climatic variations are accompanied by limited temperature excursions. In fact, it has been found that variations of 1-2 degrees are sufficient to have considerable climatic changes.

For greater clarity, the period of the last 12,000 years has been divided into three parts:

  • THE FIRST 9,000 years that see the greatest change in the climate that occurred in the Quaternary, during which the evolution of Man and Cultures was manifested in all its fullness: from Prehistory to ancient Civilizations;
  • A second period, FROM 1,000 BC. TO 1.000 AD, during which the testimonies on the climate are already more detailed and documented;
  • THE LAST 1,000 YEARS, where human instruments and memory make the collection of data more certain than ever and their interpretation more confused.



With the Neolithic after 850,000 years prehistoric man emerged from the rigors of the Quaternary Ice Age (one of the many glaciations that occurred in the history of the Earth), in particular he entered one of the "hot" variations of glacialism, which is characterized by an alternation of oscillations thermals of only 4-5 degrees corresponding to the glacial and interglacial periods.

In the chart from 10,000 to 750 is observed that the climate trend curve is on the rise until 4,000 BC. where it reaches 1.5 ° as a peak, and then falls towards increasingly relatively cold values, that is towards a glacial phase at the end of the interglacial heat, started 20,000 years ago. The first time the climate average temperature exceeded the Current Average Temperature (T.M.A.) was around the year 8,750 B.C. to reach a positive maximum in 8,500 BC, followed by a negative decline lasting 500 years until the average temperature began to rise with some oscillations to reach the peak of the last 12,000 years in 4,500 BC, with a peak assessed about 1.5 ° with respect to the TMA

Around 3.250 BC and more markedly in 750 BC. there were two periods of cold, separated from each other by a warm phase in 2,000 BC.

With the rise in the average temperature of the climate, the most immediate effect was in correspondence with the vast expanses of ice present at the lowest latitudes, the reduction in thickness and, more conspicuously, the retreat of the glacier fronts, with also significant consequences. in environmental changes.

The reduction of the thickness caused in many areas a lifting of the ground, due to the reduced pressure from huge masses of ice, as happened in the Scandinavian countries where there was an uplift of over 100 m above sea level. In other areas the increase in the volume of the sea invaded ancient beaches, so there were new geographical arrangements that in part still remain: for example around 8,000 BC. Brittany isolated itself from Europe, becoming an island, while the zone of union between America and Asia was invaded by the sea, with all the climatic, anthropological, fauna, botanical consequences, etc. linked to isolation.

The retreat of the glacier front towards the north for hundreds of kilometers led to changes in the distribution of the flora and consequently of the fauna. Plants and animals "migrated" to the north, but some species became extinct because, when environmental conditions changed, they were unable to adapt to them.

Thus a plant association typical of the Tundra consisting of mosses and grasses, then present in our latitudes, with forests on the edges characteristic of cold areas, such as Pines, Hazelnuts, Birches, Polar willows, etc., migrated towards the present Nordic regions, together with a cold climate fauna composed of living species, or totally disappeared, such as Bears, Mammoths, Reindeers, Bisons, etc.

What did Man do during this transition phase from the great frost to milder climates?

When climate change began it was still in the Paleolithic, but the stone tools were already sufficiently perfected for the activity carried out by man represented by hunting and gathering fruit.

The mildness of the climate that was being established allowed man to leave the caves which, for hundreds of millennia had protected him from the cold and from beasts.

He too emigrated to the north to follow the animals and fruits he was accustomed to, until, now in the middle of the Neolithic, there was the great turning point that radically changed the way of life of humanity: the discovery of agriculture and later, the breeding of domestic animals.

The first innovation was the abandonment of the forest, which offered scarce possibilities for nourishment, forcing hunters to travel long distances to search for prey, while the plain made it possible to obtain a product sufficient for the needs of the various communities that were being formed.

As increasingly milder climates took over, the vegetation became more and more luxuriant and the techniques were refined, not only in the cultivation of the various plants, but in the creation and preparation of agricultural land at the expense of forests.

Over time they came to cultivate a considerable number of species of wheat, barley, legumes, vegetables and discovered the properties of flax for weaving.

Neolithic men alternated their activity as farmers and ranchers without having abandoned the previous culture of hunters and gatherers of fruit and wild plants, typical of the Paleolithic. The expansion of the new agricultural activity did not spread as quickly as it may seem: it began in 10,000 BC. in the Middle East, but it took several millennia for it to spread to Europe, so much so that the expansion practically ended in 3,000 BC. with the colonization of the British Isles by the agricultural populations, who in search of new lands were moving towards the west and the north.

In other parts of the world agriculture spread in different ways and with different plants and breeding of domestic animals.

Throughout the period of expansion the tools were exclusively made of stone and terracotta, reaching highly specialized techniques, to the point of representing a real industry in privileged locations for the quality and availability of the raw material, so as to constitute a commercial activity with exports to very distant areas

The discovery of copper was made in 6,000 BC, but for three millennia the use of metals was limited to the production of ornamental objects, preferring stone for hunting tools and for agricultural activity.

If from 10,000 to 3,000 BC the first real revolution took place that humanity faced to establish the two fundamental activities that until today characterize the life of the populations, that is agriculture and livestock breeding, with the help of stone tools created by the men themselves, another much more important revolution, this time industrial, began with 3,000 B.C. that is that of metal, whose importance as an instrumental application was not immediately evaluated, but which was destined to radically change the destiny of humanity.

The discovery of copper, the first metal known to man, occurred around 6,000 BC, thanks to the heating of metalliferous stones randomly present on the place where the food was cooked. No doubt the man was surprised by the ability of the metal to assume the liquid state with the heat and to return to the solid one due to cooling. He is credited with having developed a technique for their processing, but it remains a mystery to have limited the use of metals for the production of ornamental objects for three millennia, preferring stone and wood for hunting tools and for agricultural activity.

As with other major events that occurred on Earth and highlighted by Geology, the warming of the Quaternary climate also took a very long time to settle on decidedly mild values, as clearly represented by the climate trend curve, but one thing to underline is that the large temperature oscillations are the result of many micro-oscillations, which, as we have seen in more recent times, can have an amplitude of a few decades. Unfortunately, for such distant times, there are no means to highlight these micro-oscillations.

So too the "COLONIZATION"on the part of the agricultural population it took a long time, as mentioned above, but the speed of propagation of agriculture was linked to two very specific facts: the speed with which the glaciers retreated further north and the need to acquire new lands from the populations, both for demographic reasons and for the impoverishment of the land due to intense exploitation, without restoring the fertility of the fields.

In the next issue we will start talking about ancient civilizations and how their development was influenced by climatic variations.

Dr. Pio Petrocchi

5 million years of climatic variations: "The climate repeats itself like fractals"

But anthropogenic climate change could "shift" the system too much

When we talk about climate change, we need to look at how the climate was previously, to recognize natural variations and be able to distinguish them from human-induced changes. This is what Danish researchers from the Niels Bohr Institutet of the University of Copenhagen and the University of South China in Guangzhou have done who have analyzed natural climatic variations over the last 12,000 years, during which we have had a hot period interglacial, going back 5 million years to see the main features of the Earth's climate. The study, published on Nature Communications, shows not only that the weather is chaotic, but that the Earth's climate is chaotic and can be difficult to predict.

At the Niels Bohr Institutet they explain that "the Earth's climate system is characterized by complex interactions between the atmosphere, oceans, ice caps, land masses and the biosphere (areas of the world with animal and plant life). Astronomical factors also play a role in relation to major changes such as the transition between ice ages, which typically last around 100,000 years, and interglacial periods, which typically last around 10 - 12,000 years.

According to Peter Ditlevsen, associate professor of climate physics at the Niels Bohr Institutet, “We can look at climate as fractals, that is, patterns or structures that repeat themselves in smaller versions indefinitely. If we talk about centennial storms, then there are 100 years between them? Or does he suddenly discover that there are three of these storms in a short time? If you are talking about very hot summers, when do they happen, every 10 years or every 5 years? How big are the normal variations? Now we have studied it ». In fact, Danish and Chinese researchers have been studying: temperature measurements over the past 150 years. Greenland ice data from the interglacial period 12,000 years ago to the ice age 120,000 years ago, Antarctic ice core data, which is 800,000 years old, as well as data from marine sediments dating back to 5 million years ago.

Ditlevsen recalls that 'We only have about 150 years of direct temperature measurements, so if, for example, we want to estimate how large the variation can be expected for more than 100 years, we look at the temperature data for that period, but it cannot tell us what we can expect for the temperature data over 1000 years ago. But if we can determine the relationship between changes over a given period, then we can make an estimate. These types of estimates are of great importance for the safety assessment for structures and buildings that need to keep well for a very long time, or for structures for which bad weather could pose a safety risk, such as for example drilling platforms or nuclear power plants. Now we have studied all this by analyzing direct and indirect measurements back in time ».

The study shows that natural variations in a given time period depend on the length of the period in a very particular way, which is characteristic of fractals. This knowledge tells us something about how much we can expect a colossal storm that occurs every 1,0000 years and how it relates to a centennial storm and how many centennial thunderstorms to expect in 10 years. The researchers also found that there is a difference in the fractal behavior of the glaciation climate and the current warm interglacial climate.

Ditlevsen. He further explains: “We can see that the climate during an ice age has much greater fluctuations than the climate during an interglacial period. There has been speculation that the reason could be astronomical variations, but now we can rule out this case since in the great fluctuation during the ice age they behaved in the same "fractal" way as in other natural fluctuations around the world ".

The astronomical factors that influence the Earth's climate depend on the influence exerted by the gravity of the other planets in the solar system on Earth and relate to the orbit of the Earth around the Sun, which varies from being almost circular to more elliptical and this affects solar radiation on earth. The gravity of the other planets also affects the rotation of the Earth on its axis. The Earth's axis oscillates between an inclination of 22 degrees and 24 degrees and when the inclination is 24 degrees, there is a greater difference between summer and winter and this has an influence on the violent variations of the climate, ice ages and interglacial periods.

Sudden climate changes during the Ice Age may have been triggered by several mechanisms that have affected the Gulf Stream, which carries hot water from the equator north to the Atlantic, where it is cooled and melts into the cold water of the ocean under the ice and is pushed back south. "This water pump - say the scientists - can be knocked out or weakened by changes in freshwater pressure, by the breaking of the ice layer or by the displacement of sea ice, and the result is increasing climatic variability".

The climate during hot interglacial periods is more stable than the climate of glacial climate. "" In fact - says Ditlevsen - we see that the climate of the ice age is what we call "multifractal", which is a feature that is seen in very chaotic systems, while the interglacial climate is "monofractal". This means that the relationship between the extremes of the climate over different time periods behaves like the relationship between the more normal relationships in different time scales "

This new climate feature will make it easy for climate researchers to distinguish between natural and anthropogenic climate changes, because induced climate change can be predicted not to behave in the same way as natural fluctuations.

Ditlevsen concludes: 'The differences we found between the two climatic states also suggest that if we' shift 'the system too much, we could enter a different system, which could lead to more swings.We have to go far back in the geological history of the Earth to find a warm climate like the one we are heading towards. Even if we don't know in detail the climatic variations so far back in time, we do know that there have been sudden climate changes in the warm climate of that time "


First of all it is necessary to consider the elliptical orbit that the Earth describes around the Sun if only these two celestial bodies existed, this ellipse would be indeformable. However, the Earth is subject to the gravitational attraction exerted by the other planets which perturb its motion by continuously deforming the elliptical trajectory described by the Earth.
The consequences of this deformation are:

  • An oscillation in the value of the eccentricity of this ellipse, which varies progressively between zero and a maximum of 0.06
  • An oscillation of the plane of the orbit, with an amplitude of about 3 degrees
  • A displacement of the perihelion that slowly rotates in the same direction as the earth's motion, completing a full rotation in about 110,000 years.

Influence of motion perturbations on the earth's temperature Modification

Call r the distance of the Earth from the Sun at any time t, it is observed that the amount of heat that the Earth receives in the storm is between t is dt is proportional to dt and inversely proportional to r 2 < displaystyle r ^ <2>> , hence the amount of heat Q that the Earth receives in a year from the sun will be given by

where is it k is a proportionality coefficient. However it is known that the Earth rotates around the Sun following the law of areas, therefore, denoting with the infinitesimal angle of which it rotates in the small time dt the vector ray joining the Earth to the Sun, we have:

where is it c is the constant of the areas. Consequently, formula (1), remembering that in one year the Earth completes a complete rotation around the Sun, i.e. that the angle θ increases by , becomes:

Consequently, whatever the Earth's orbit, the amount of heat it receives in a year is inversely proportional to the area constant, which in turn is proportional to the parameter p = a (1 - e 2) < displaystyle p = a (1-e ^ <2>)> in root, where to is the semi-major axis of the orbit and is is the eccentricity of the ellipse which, as mentioned, varies between 0.06 and zero, which gives for Q a variation of the order of 18 / 10,000.

Ultimately, therefore, the amount of energy that the Earth receives from the Sun in a year can vary by 1/600. But this quantity of energy that the Earth receives, due to the need for thermal equilibrium, is equal to that which the Earth radiates into space, which is proportional to the fourth power of the average temperature expressed in kelvins. Now, the average temperature of the Earth is about 300 K (27 ° C), so the variation in temperature is quantified in about 1 / 10th of a degree. Ultimately, orbital perturbations can produce a variation in the average annual temperature of the Earth of the order of a few tenths of a degree.

But, even if the average variation appears marginal, the distribution of temperature in the various seasons may vary. And this is precisely the object of the study of various scientists, and in particular of Milutin Milanković.
Called for simplicity hot season that in which the day is longer than the night e cold season the other, and defined T 1 < displaystyle T_ <1>> the point of Earth's orbit at the vernal equinox e T 2 < displaystyle T_ <2>> the opposite point, where it is at the autumnal equinox, the length of the warm season for the northern hemisphere will be measured by the time it takes the Earth to go from T 1 < displaystyle T_ <1>> to T 2 < displaystyle T_ <2>> , while the duration of the warm season for the southern hemisphere will be given by the time it takes to go from T 2 < displaystyle T_ <2>> to T 1 < displaystyle T_ <1>> .

But the straight T 1 T 2 < displaystyle T_ <1> T_ <2>> which marks the line of the equinoxes, due to the phenomenon of the precession of the equinoxes, rotates by completing one revolution in about 25,870 years, while also the major axis of the ellipse, due to perturbations, rotates in the opposite direction in about 110,000 years, which leads to establish that the point T 1 < displaystyle T_ <1>> it coincides with perihelion approximately every 21,000 years. Consequently, indicated with C b < displaystyle C_> is C a < displaystyle C_> respectively the duration of the warm season in the northern and southern hemisphere, the difference C b - C a = D < displaystyle C_-C_ = D> it will be a periodic function over time, with a period of the order of twenty thousand years.

Now, due to the perturbations, we have already seen that the eccentricity of the Earth's orbit varies slowly, oscillating between 0.06 and zero. So, for example, about 200,000 years ago, the eccentricity (which is now 0.016) was about 0.057 and the line T 1 T 2 < displaystyle T_ <1> T_ <2>> (which today forms an angle of about 78 ° with the major axis of the ellipse) was then almost normal to it: the difference between the duration of the warm seasons in the two hemispheres must therefore have been much more relevant than today.

There is also another astronomical factor to take into consideration: the inclination of the ecliptic is now 23 ° 27 ', but it varies slowly making oscillations of about 3 ° (from 24 ° 36' to 21 ° 58 '). This influences the distance of the tropics from the equator and that of the polar circles from the poles, both exactly equal to the inclination of the ecliptic: therefore, when this inclination decreases, the two tropics approach the equator and the two polar circles at the poles and the other way around. Therefore, the extensions of the torrid zone, of the two temperate zones and of the two polar caps vary, in a similar way for the two hemispheres, with the effect of attenuating the contrast between summer and winter in the same way in the two hemispheres.

At this point it is necessary to ask whether or not this attenuation of seasonal contrasts is favorable to glacialism: Milanković gave an affirmative answer to this question, while other geophysicists, including especially James Croll, argue the exact opposite.

Upon their appearance, the astronomical theories sparked many hopes in scientific circles, which however were soon disappointed as such theories, while linking the alternation of glaciations and interglacial epochs to the periodic variation of the elements of celestial mechanics, left open many questions and they introduced other new ones, to the point of leading to believe that the mere variation of the astronomical elements was not sufficient to determine important climatic variations, and that its action was only secondary.
It was Milanković's credit for having taken up the problem and, by highlighting the errors of his predecessors, having removed most of the criticisms made against these theories.

Croll's hypothesis Edit

Of the astronomical theories on the origins of the ice ages, that of the Croll, among those prior to Milanković's work, is the best known and the one which, with its shortcomings, has provided the greatest support for criticism.

According to Croll, the origin of the secular climatic variations is to be found in the secular variations of the earth's orbit, combined with the phenomenon of the precession of the equinoxes and the displacement of the perihelion. On the other hand, no importance is attributed to variations in the inclination of the ecliptic. The different length of the seasons depends on the eccentricity of the orbit, while the position of the vernal equinox depends on the precession, and therefore on whether the winter of a given hemisphere will take place with the Earth in perihelion or in aphelion. We have seen that, whatever the eccentricity and position of the equinox, the total amount of solar radiation that one hemisphere receives is equal to the amount received from the opposite hemisphere. It is intuitively understood that, if during one half revolution the Earth is closer to the Sun, and therefore the intensity of the radiation received is greater, during the other half revolution the intensity of the radiation is less, but the duration of the radiation is longer. insolation. However, precisely because the duration of the two winters is not the same, the average daily intensity of the radiation received by the two hemispheres during their respective winters will not be the same. One hemisphere will have a long, cold winter, the other a short, relatively warm winter.

The precession, combined with the perihelion shift, swaps the role of the two hemispheres every 10,500 years approximately each hemisphere should therefore have glaciations interspersed with interglacial epochs, with a full period of 21,000 years. Furthermore, the glaciations of the two hemispheres should be alternating.

Up to this point, Croll's theory does not differ from the earlier one of Joseph-Alphonse Adhémar, who does not take into account variations in eccentricity. Instead, Croll, while accepting the 21,000-year period, observes that the intensity of glaciations depends precisely on the variations in eccentricity, because it is precisely these that determine significant variations in the duration of the seasons. True glaciations will occur only in periods of maximum eccentricity, while in the minimum epochs there will be negligible or even zero glacial phenomena. Since the period of the oscillation of eccentricity is very long (about 91,000 years) and the amplitude is not constant, the ice ages would fall only in correspondence with the most accentuated maximums and would each consist of a short succession of two or three glaciations with a period of about 21,000 years and alternating in the two hemispheres. Based on the theory of secular perturbations of the earth's motion elaborated by Urbain Le Verrier, Croll assigns the dates of the most pronounced relative maximums of the eccentricity of the earth's orbit to the possible great glacial epochs, namely: 100 (e = 0.047), 210 (e = 0.0575), 750 (e = 0.0575), 850 (e = 0.747) and 950 (e = 0.0517) millennia ahead of their time (1850). The dates 750 and 850 are preferred by Charles Lyell, who estimates the first two too recent, while Croll, in agreement with other geologists, indicates the first two as the most probable.

We observe that Croll's theory accorded very well with the ideas of John Tyndall, according to which to have a lot of ice one needed a Improved Capacitor. Now, the terrestrial thermal conditions, in the epochs of maximum eccentricity, would have been precisely those required by Tyndall: one hemisphere, in the glacial phase, had long and cold winters and acted as a capacitor, while the opposite hemisphere, in the interglacial phase, provided the necessary quantity of water vapor. It is true that the hemisphere in the glacial phase would also have had a hot summer, but this would have been short enough and therefore such as not to allow the complete melting of the winter snows.

According to these views, the glacial epochs should correspond to the epochs of maximum annual excursion of the radiation received, and therefore of the temperature, or of maximum seasonal contrast. Instead, in the opinion of some more modern meteorologists, the conditions for an extensive glacial formation would be precisely opposite to those indicated by the Croll. A long and cold winter would not favor the formation of glaciers (indeed, such a winter would oppose it, as for example in Siberia, where there are no permanent glacial formations) but rather a cold summer, which would also favor the descent downstream of the glaciers. As Luigi De Marchi writes, "a decrease in the winter temperature does not necessarily lead to an increase in snowfall in the high mountainous regions (.) On the other hand, an increase in summer temperatures of even a few degrees can significantly accelerate the process of ablation, and therefore the decrease of the glacier. The current periodic expansions of the glaciers correspond to periods of greater rainfall, which are also periods of lower annual temperature range and of lower thermal contrast between continents and oceans, that is to conditions quite opposite to those that would have determined the great glacial expansion in the hypothesis by Croll ".

It must be added that Croll, sensing the insufficiency of his astronomical argument, resorted to the aid of many other physical causes of climatic variation, which, once the glacial phenomenon had begun, in an astronomically favorable period, would help to maintain it. and to enhance its effect. In a certain sense, this work of adapting the theory has had its usefulness, as it has led to the demonstration that the glacial phenomenon tends autonomously to preserve and strengthen itself. But, on the whole, the theory is not acceptable and this has greatly contributed to the belief that astronomical theories are not sufficient to explain the large variations in the earth's climate, playing only a secondary role in the face of other physical and geographical causes.

Milanković's contribution Edit

No other of the numerous authors who, after the Croll and before the Milanković, dealt with the glacial problem from an astronomical point of view has made an essential contribution to the question. With Milanković, on the other hand, astronomical theory takes a decisive step forward. He observes that astronomical and meteorological objections can be raised to the Croll theory.
The former (a certain regular alternation of glaciations, alternation in the two hemispheres, etc.) could easily have been avoided if Croll had set the problem of terrestrial insolation and its secular variations with greater mathematical rigor.
The latter instead (especially the attribution of the ice ages to the periods of maximum seasonal contrasts) derive from the empirical method followed in facing the problem, lacking a preliminary serious attempt to mathematically determine the climate.
Milanković's contribution can be summarized in three points.

  • in having rigorously set up the astronomical problem of terrestrial insolation and its secular variations
  • in having understood the need, to get rid of any empiricism, to place the preliminary study of the mathematical climate at the basis of paleoclimatic research
  • in having given, rather than a new explanation of the ice ages, a method to systematically approach their study.

The secular variability of the terrestrial insolation Edit

The problem must be faced in two successive stages, starting from the study of the terrestrial insolation without atmosphere (or, which is the same, at the outer limit of the atmosphere), then moving on to the study that takes into account the presence of the atmosphere.
The first is a mathematical problem with a relatively easy solution and is very important because it provides the fundamental data of all climatology, that is, the distribution and temporal variation of the solar radiation reaching the upper limit of the atmosphere. These are very regular variations, which are singularly matched by the very irregular ones in the climate, which are a necessary consequence of the former.
The second problem is instead more complex, since it is a question of evaluating the quantity of solar radiation that actually reaches the ground, taking into account both the atmospheric absorption and the dark radiation emitted by the atmosphere itself. Since the current average composition of the atmosphere is sufficiently known, the problem does not present particular difficulties, but the uncertainties about this composition in geological periods add an obvious approximation to the achievable data.
Milanković has faced both of these problems in his work, providing them with very rigorous and comprehensive solutions.

This formula solves with all the desired rigor the problem of calculating the terrestrial insolation in the absence of an atmosphere. In it the place figures through φ is λ (which intervenes through ω), while time figures through ρ, δ is ω. It lends itself very well to the study of the diurnal variation of insolation, since in this case it can be practically posed ρ is δ constant and vary only the hour angle ω of the Sun, but for the study of the annual and secular variations of the insolation it would be very inconvenient.
For this reason it is advisable to replace formulas (4a) and (4b) with others that are less precise but easier to use, and in particular:

This function lends itself very well to the study of the annual variations of the terrestrial insolation, but less well for the secular ones, as it is still too complicated.

  • the average insolation of a parallel in the northern (southern) hemisphere on an average northern (southern) summer day, which we will denote by w e < displaystyle w_>
    ( w ¯ and < displaystyle < overline >_> ), or on a northern (austral) average winter day, which we will refer to as w h < displaystyle w_> ( w ¯ h < displaystyle < overline >_> ). These four quantities always depend on latitude, but as regards the dependence on time, they depend on it only through the secular variations of the earth's motion. They can therefore be replaced very advantageously in the study of secular variations of the climate at w already defined, although with less precision. To calculate them, we indicate with W and < displaystyle W_> , W h < displaystyle W_> ( W ¯ and < displaystyle < overline >_> , W ¯ h < displaystyle < overline >_> ) the quantities of solar radiation that strike the northern (southern) assigned latitude surface unit, and with T and < displaystyle T_> , T h < displaystyle T_> ( T ¯ and < displaystyle < overline >_> , T ¯ h < displaystyle < overline >_> ) the duration of the boreal (austral) summer and winter seasons. Then we can put, with sufficient approximation:

The variables W and < displaystyle W_> , etc. depend on latitude ϕ, and it is shown that they also depend on the inclination ε of the ecliptic and very little from eccentricity is of the Earth's orbit. Instead the variables T and < displaystyle T_> , etc. they only depend on is and longitude π of perihelion counted from the real equinox of the time, which leads us to say that w e < displaystyle w_> is a function of ϕ, is, π ed ε.
Now, calling Δ ε the variation of the inclination of the ecliptic (value of ε at the time minus the present value), and defining with T = T e - T h < displaystyle T = T_-T_> the difference in the duration of the seasons (duration of the hot season minus the duration of the cold season), the fundamental equations for the study of the secular variations of the terrestrial insolation are shown:

The coefficients W and 0 T < displaystyle < frac <>^<0>>>>, W h 0 T < displaystyle < frac <>^<0>>>>, Δ W and 0 T < displaystyle < frac < Delta W_^<0>>>> e Δ W h 0 T < displaystyle < frac < Delta W_^<0>>>> and the variable quantities Δ ε e Δ T are provided by pre-calculated graphs and tables.

The theory according to Milanković Edit

One of the most remarkable results obtained by Milanković is in having shown that the secular variations of the terrestrial insolation deriving from the secular variations of the elements of the motion of the Earth are, if correctly calculated, sufficient to justify the important variations of some climatic factors, in particular of the annual temperature excursion.

But astronomical theory alone is not enough to provide an exhaustive explanation of the climatic variations of the past: in fact, the mere prediction of the annual temperature excursion is not enough to give a sufficiently complete picture of the climate, lacking any mathematically reliable forecast on the data relating to evaporation, circulation and precipitation of water vapor. Until the mathematical climate will be able to reconstruct, at least broadly, the circulation of water vapor in the earth's atmosphere starting from the fundamental data of insolation, all that remains is to take the empirical path of the hypotheses, more or less supported by the data observation today.

Milanković - in agreement with some authors, including De Marchi, and in contrast with others, including Croll - believes that the most favorable conditions for glacial expansion are those of minimum annual excursion, or of minimum seasonal contrasts.
Here we fall back into the field of hypotheses. However, once this hypothesis is accepted, astronomical theory allows us to calculate, with great precision and for each place on earth, the epochs in which these minimum conditions occurred. However, it should be noted that Milanković did not fully exploit all the possibilities of his theory, limiting himself to a generic discussion and for a single latitude, content with having provided an instrument of investigation rather than a complete theory, while it would not have been difficult to elaborate a framework complete with the secular trend of seasonal contrasts across the globe.

This first is followed by a second period in the northern hemisphere during which the seasonal contrasts change only in a not very significant way and which lasted until about 60,000 years ago. Starting from this epoch, and always proceeding towards the past, a second climatic wave begins for the northern hemisphere which presents, around 72,000 years ago, a new epoch of minimal seasonal contrasts. Moving further towards the past, the epochs of minimal seasonal contrasts in which, according to Milanković, glacial phenomena could have taken place, are the following (in thousands of years ago):

  • for the northern hemisphere: 23 (doubtful), 72, 116, 188, 230, 475
  • for the southern hemisphere: 106, 197, 313, 465.

It is very likely that the maximum glaciation took place for the northern hemisphere around 188,000 years ago. Since the glacial one is a phenomenon that tends to strengthen and preserve itself, we can think of a great glacial period from the year -235,000 to the year -180,000 another glacial age could have occurred around the year -475,000, with a long period interglacial from this date to the year -235,000. Finally, other less important and more recent glaciations could have taken place around the years -116,000, -72,000 and, perhaps, -23,000.

These results, which Milanković arrives at, are supported by some considerations deriving from the theory of the mathematical climate, which is beyond the scope of this discussion. However, it is sufficient to consider that the climatic conditions around the year -188,000, with the duration of the hot season exceeding that of the cold season by about 20 days and with a relatively small inclination of the ecliptic, are very similar to those that the theory of mathematical climate indicates for the so-called State III: in these conditions, according to the mathematical climate theory, in correspondence to the latitude + 48 ° the insolation of the State III is equivalent to that of the current state relative to a latitude of about 8 ° further north this in practice means that at the time the Alpine massif, which has an average latitude of about 46 °, was in the climatic situation that currently have the coasts of Baltic Sea. An Alpine glaciation therefore seems possible around that time.

Climatic variations

Climatic variations

Research on v. c. are systematically collected by the IPCC (Intergovernmental Panel on Climate Change), which has been established since 1988 by two UN agencies, namely the WMO (World Meteorological Organization) and UNEP (United Nations Environment Program).

The IPCC takes up research on the climate carried out in every part of the globe by researchers from individual countries and draws up guiding documents, mainly with the aim of offering an indication to national and supranational political authorities to direct them towards those economic and industrial activities that have a reduced impact on the processes of continuous climate change. At the end of each new collection of information, reports are drawn up whose purpose is to guide the strategies of all countries in order to tackle climate change in order to try to mitigate its consequences, especially in the poorest ones, the most vulnerable to events. meteorological which are induced by climatic variations.

The IPCC is divided into four specialized subgroups and regularly produces scientific, technical and socio-economic reports aimed at understanding the ways in which climate change occurs, its potential consequences and possible indications for an adaptation and reduction of the effects on environment. The first subgroup investigates the scientific aspects of the climate system and the origins of climate change, in an attempt to distinguish natural changes from those due to human activities. The second evaluates the sensitivity and vulnerability of both natural and human socio-economic systems in relation to climatic variations. The third subgroup studies the possible actions aimed at reducing greenhouse gas emissions as a mechanism for limiting and containing climate change. The fourth subgroup, then, The task force on national greenhouse gas inventories, collects the information for the compilation of a catalog of those gases which, once released into the atmosphere, increase the greenhouse effect. The IPCC produced its fourth report in 2007.

In a simplification of the climate and its variations, an effective thermodynamic model on a global scale sees the energy balance of the Earth through the flow of radiant energy. The balance is the algebraic sum of the incoming radiation, mainly of solar origin, and of the outgoing radiation towards sidereal space, emitted both by reflection of the solar radiation and by thermal effect as black body radiation. The result of a model of this type is the equilibrium thermodynamic temperature of the atmosphere as obtained from the set of meteorological observations. The percentage and absolute variations in the atmospheric concentration of greenhouse gases, aerosols, solar radiation and the properties of the earth's surface alter the thermodynamic equilibrium on which the climate system rests. Atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrogen dioxide (NO2), which have increased as a result of human activities since the beginning of the industrial era, today have reached values ​​that are much higher than those of the pre-industrial age, measured through the analysis of the Antarctic and Greenland paleo-ice. The concentration of CO2 went from a pre-industrial stage value of around 280 ppm (parts per million) to 379 ppm in 2005. The historical variability range of this concentration, obtained from paleoghiacci, goes from 170 to 300 ppm during 650,000 years. The annual growth of carbon dioxide during the decade 1995 - 2005 was 1.9 ppm / year, higher than the average growth observed since the start of CO measurements.2. From 1960 to 2005, the average growth was 1.4 ppm per year. The increase is mainly due to the use of fossil fuels and, to a lesser extent, to the reduction of forests that absorb CO2 atmospheric. The total emission of CO2 it went from 23.5 ± 1.5 Gt / year in the 1990s to 26.4 ± 1.1 in 2005. Atmospheric methane also had a very strong increase due to human activities mainly agricultural and livestock, reaching a concentration of 1732 ppb (parts per billion) in 2005 against a range from 320 to 790 ppb over the 650,000 years of surveys. deduced from the polar paleo-ice.

The 2007 IPCC report attributes a very high probability to the increase in the concentration of greenhouse gases as the main cause of the increase in the temperature of the atmosphere due to the decrease in radiation emitted from the Earth towards space. It is believed that the combination of the different gases gives a radiative contribution to global warming of 2.3 ± 0.23 W / m 2, higher than estimated for the last 10,000 years. The increase due to carbon dioxide was about 20% from 1995 to 2005, and was the largest for a single decade in the past 200 years. The v. c. in progress it presents an unequivocal warming, which becomes more and more evident from the observations of an increase in the temperature of the atmosphere and of the temperatures detected at the surface of the oceans, from a generalized phenomenon of melting of ice and perennial snow and from the elevation of the sea level.

Of the twelve years between 1995 and 2006, eleven belong to the series of the twelve warmest years since 1850, which marks the beginning of instrumental measurements of air temperature. The rapidity of the increase in temperature in the second half of the last century doubled compared to the first half of the same. The feared effect of urban heat islands appears to have negligible effect on global warming (to the extent of only six thousandths of a degree centigrade on land and zero contribution to the oceans). The increase in water vapor in the atmosphere is compatible with the increased containment capacity due to the increase in air temperature. Glaciers and perennial snows are decreasing in both hemispheres and are believed to be responsible for the rise in sea levels over the period 1993-2003. The global level of the oceans increased by 1.8 ± 0.5 mm per year from 1961 to 2003 but from 1993 to 2003 the height of the seas grew by 3.1 ± 0.7 mm per year. It is estimated that the raising process began in the 19th century. and that the average increase in the 20th was 0.17 meters. The new and most recent values ​​were derived from observations of geophysical satellites. Numerous long-term changes have been observed in the climate of continental and regional areas, and around basins such as the Mediterranean: among these, the reduction in the extension of the Arctic ice cap, the different distribution of rainfall in quantity and type, the variation in the circulation of winds, the intervention of arid periods and flood events with increasing frequency.

To try to distinguish the anthropogenic component of v. c. from its own natural one, the IPCC, in its fourth 2007 report, also examined paleoclimatic research. Paleoclimatic information confirms the hypothesis that the warming observed in the second half of the last century is anomalous in relation to the last 1300 years. The last time that the polar regions had a temperature higher than the current one dates back to about 125,000 years ago, that is, in an interglacial period, particularly different from the present, in which the sea level was from 4 to 6 m above sea level. current and the Earth's orbit was slightly different. The conclusion of the fourth report of the IPCC pays much attention to the fact that the responsibility for the acceleration of v. c. in progress is attributable, with great probability, to human activities of variation of the terrestrial thermodynamic balance, in the sense of an increase in the radiation retained by the planet. Recommendations addressed to political authorities converge on the need to reduce the impact of activities on the climate.

The vulnerability of systems, natural and human

The new knowledge acquired allows us to evaluate some of the consequences of v. c. on natural ecosystems and human activities. The rise in temperature may be followed by various effects, including increases in the surface of glacial lakes, the instability of soils subject to permafrost, the frequency of avalanches.

In the Arctic and Antarctic polar regions, variations in temperature and frozen surface will change ecosystems with consequences that will certainly modify the biomes (i.e. the living complexes in relation to environmental conditions) in the water-ice transition zone, putting the super predators of the chain at risk. food, such as, for example, polar bears and killer whales. The variation in temperature will lead to a different distribution of precipitation by location, intensity and quantity. In agriculture, particularly in advanced agriculture where products depend on irrigation, there may be a need to find water resources in other places, which have become scarce in absolute terms or in any case distributed differently over the seasons. In poor countries, then, agriculture could suffer disastrous consequences, when the expected seasonal rains do not occur, or occur in violent and unusable forms. The rise in sea level will directly affect settlements and indirectly affect coastal aquifers in relation to the effect of saline penetration, which will increase accordingly.

IPCC, Fourth assessment report "Climate change 2007", Genève 2007.

Climate change: the solutions

L'Paris Agreement, signed in 2015 by the representatives of 196 governments, has set the objectives to be achieved to combat climate change through a series of initiatives, some on a voluntary basis and others not.

What are these goals? To maintain within 2 degrees centigrade the world average temperature increase up to the end of the century, compared to the pre-industrial one, e reduce greenhouse gas emissions by 55% by 2050 and reaching zero by 2060-2075.

The UN secretariat that deals with compliance with the parameters decided in Paris is called the United Nations Framework Convention on Climate Change.

Every year all the parties come together as one Cop (conference of the parties), to have a discussion on what has been done and what still needs to be implemented.

But managing these solutions to combat climate change is complex and the commitments made so far are not yet sufficient to reduce greenhouse gases.

Furthermore, the United States has effectively withdrawn and has begun the process to formally exit the Paris agreement, as announced by Donald Trump in 2017.

Only this year's presidential elections could turn the tables if Trump is not reconfirmed.

The latest report from the IPCC (The Intergovernmental Panel on Climate Change, the UN's scientific committee on climate) in October 2018 published an alarming report on the climate: if the world does not immediately reduce the emission of greenhouse gases, as early as 2030 global warming could exceed the threshold of +1.5 degrees from pre-industrial levels, with disastrous consequences.

A few examples? The further rising of the seas, periods of intense drought, floods, increased storms and hurricanes, with serious repercussions on millions of people.

The current level of greenhouse gases does not allow us to wait any longer.

The peaceful battle that the Swedish activist Greta Thunberg is carrying out, together with all the young people who have decided to support and follow her example, is focused precisely on the need to act urgently, with decision and concrete actions.

The future "is the only thing we need," says Greta.

COP25 was held in Madrid from 2 to 13 December 2019, which ended without a clear agreement on CO2 emissions.

All postponed to Cop26 in Glasgow which, due to the coronavirus pandemic, has been postponed from November 2020 to 2021.

By the end of the year, the various states will have to present new national plans not to exceed the threshold of 2 ° above the pre-industrial average temperature of the earth, to be lowered to 1.5 °, according to scientific reports.

With current plans, it would come to + 3.2 ° by the end of the century.

We would be at a point of no return: a climate catastrophe for the planet, but also for humanity.

The great climate changes of the past (and why it's different today)

Orbital oscillations, plate tectonics, evolutionary changes and other factors have caused the planet to enter and exit a series of ice ages (© Yadid Levy / AGF)

At different times in its past, the Earth has been both a snowball and a red-hot greenhouse. But if the climate changed before the arrival of humans, how can we be sure that the responsibility for the current dramatic warming is ours?

In part it depends on the fact that we can unequivocally demonstrate the causal link between carbon dioxide emissions due to human activities and the increase of 1.28 ° C (and more) in global temperatures compared to the pre-industrial era. The molecules of CO2 they absorb infrared rays, so if there are more of them in the atmosphere, they trap a greater amount of the heat radiated from the earth's surface.

But paleoclimatologists have also made huge strides in understanding the processes behind past Earth's climate change. The following is an overview of ten ways in which the climate changes due to natural causes and a comparison between each of them and what is happening today.

Artist's impression of the "snowball" Earth (© SPL / AGF) Solar cycles
Magnitude: 0.1 ° C to 0.3 ° C cooling
Time scale: episodes of slowdown in solar activity that last 30-160 years, centuries apart from each other

Every 11 years the Sun's magnetic field reverses and in doing so sets the pace for an eleven-year cycle in which solar activity increases and then decreases. However, this variation in solar activity is modest and has a negligible impact on the earth's climate. Most significant are the "great solar minima," periods of reduced solar activity lasting decades and occurring 25 times in the past 11,000 years.

A recent example, the Maunder Minimum, which took place between 1645 and 1715, saw the energy of the Sun drop between 0.04 and 0.08 percent below the modern average. Scientists have long believed that the Maunder Minimum may have caused the 'Little Ice Age', a cold period that lasted from the 15th to the 19th centuries more recently, however, it has been shown that that minimum was too small and did not happen at the time. just to explain the cooling, which was probably more due to volcanic activity. In the last fifty years the Sun has reduced its activity while the Earth has progressively warmed, so global warming cannot be attributed to the Sun.

Sulfur of volcanic origin
Magnitude: cooling from 0.6 ° C to 2 ° C
Time scale: from 1 to 20 years

In the year 539 or 540 AD, the Ilopango volcano in El Salvador exploded in an eruption so violent that the eruptive column reached very high in the stratosphere. Cold summers, droughts, famines and epidemics devastated societies around the world. Eruptions such as that of Ilopango inject droplets of reflective sulfuric acid into the atmosphere, which block sunlight and cause a cooling of the climate. As a result, the ice pack can become larger, reflect more sunlight back into space, and thus prolong global cooling.

The Ilopango caused a drop of about 2 ° C that lasted twenty years. More recently, the 1991 eruption of Pinatubo in the Philippines cooled the global climate by 0.6 ° C for 15 months. The presence of sulfur of volcanic origin in the atmosphere can cause perturbations, but on the scale of terrestrial history it is a tiny and passing phenomenon.

The fungus of the Pinatubo eruption (© USGS) Short-term climatic changes
Magnitude: up to 0.15 ° C
Time scale: from 2 to 7 years

In addition to the weather patterns that are repeated over the seasons, there are other short-term cycles that have an effect on rainfall and temperatures. The most significant, El Niño-Southern Oscillation, involves changes in the atmospheric circulation in the tropical zone of the Pacific Ocean, which occur every 2-7 years and have a very strong influence on precipitation in North America. The North Atlantic Oscillation and the Indian Ocean Dipole are two other phenomena with strong regional effects and both interact with the El Niño-Southern Oscillation.

In the past, the interconnections between these cycles made it difficult to demonstrate that human-induced climate change had statistical significance and were not simply another example of natural variation. But more recently, anthropogenic climate change has far outstripped natural variation in terms of seasonal climate and temperatures. The 2017 edition of the U.S. National Climate Assessment concluded that "there is no convincing evidence in the observations that natural cycles can explain the observed changes in climate."

Orbital oscillations
Magnitude: about 6 ° C in the last 100,000-year cycle varies over the geological ages
Time scale: regular and superimposed cycles of 23,000, 41,000, 100,000, 405,000 and 2,400,000 years

The orbit of the Earth oscillates when the Sun, the Moon and the other planets change their relative position. These cyclical oscillations, called Milankovitch cycles, cause a variation of up to 25 percent in the amount of sunlight that reaches the mid-latitudes and therefore lead to an oscillation of the climate. These cycles have always existed and have given rise to the alternating layers of sediments that are seen on cliffs or rock walls.

During the Pleistocene, which ended about 11,700 years ago, the Milankovitch cycles made our planet go through several ice ages. When Earth's orbit made summers warmer than average in the Northern Hemisphere, the huge glaciers in North America, Europe and Asia melted as the orbit cooled off the Northern Summers, the glaciers began to grow again. Since the oceans absorb less CO2 when they are warmer, the levels of carbon dioxide in the atmosphere rose and fell at the rate of these orbital oscillations, amplifying their effects.

Today the Earth is approaching another minimum point of insolation in the northern hemisphere, therefore without human emissions of CO2 we would be heading into another ice age within the next 1500 years or so.

When the day lasted eighteen hours

A young and weak Sun
Magnitude: no net effect on temperature
Time scale: constant

Although the Sun's brightness fluctuates on a smaller time scale, overall it increases by 0.009 percent every million years and has increased by 48 percent since the solar system's birth 4.5 billion years ago.

It therefore follows that, with a young and still weak Sun, the Earth should have been entirely covered with ice for the first half of its existence. Instead, paradoxically, geologists have found 3.4 billion-year-old rocks that formed in rippled water. It is likely that the Earth's unexpectedly warm primordial climate is explained by a combination of factors such as less earth erosion, clearer skies, shorter length of day and the peculiar composition of the earth's atmosphere, which was not yet rich in oxygen.

The mild conditions in the second half of the Earth's existence, despite the increase in brightness of the Sun, do not create a paradox: the planet has a natural thermostat, linked to the weathering process, which counterbalances the effects of the extra light, stabilizing the Earth temperature (see below).

Carbon dioxide and natural thermostat
Magnitude: counterbalances other changes
Time scale: 100,000 years or more

The main selector for regulating the earth's climate since its origins has always been the level of carbon dioxide in the atmosphere, since carbon dioxide is a persistent greenhouse gas that traps the heat that tries to escape from the planet.

To emit CO2 in the atmosphere are volcanoes, metamorphic rocks and the oxidation of carbon in eroded sediments, while chemical reactions with silicates remove carbon dioxide from the atmosphere and bury it in the form of limestone. The balance between these processes works as a thermostat, because when the climate becomes warmer, the chemical reactions become more efficient in removing CO2 from the atmosphere, thus curbing heating. As the climate cools, reactions become less efficient, slowing cooling. As a result, the Earth's climate has remained relatively stable over a longer time scale, resulting in a habitable environment. In particular, the average levels of carbon dioxide have constantly dropped in response to the increase in brightness of the Sun.

However, this natural thermostat linked to the weathering process needs hundreds of thousands of years to react to changes in CO levels.2 in the atmosphere. Oceans are a bit faster in absorbing and removing excess carbon, but even this process takes millennia and can be overwhelmed, which leads to ocean acidification. Every year, the burning of fossil fuels emits about 100 times more carbon dioxide than volcanoes, an amount too large and too fast for the oceans and the natural thermostat of the weather to neutralize it and that is why the climate is overheating and the oceans are acidifying.

© Jed Share / Kaoru Share Plate tectonics
Magnitude: around 30 ° C in the last 500 million years
Time scale: millions of years

The shifting of continental masses on the earth's crust can slowly reset the natural thermostat of weathering to a new level of equilibrium.

In general, the planet has cooled in the last 50 million years, as collisions between tectonic plates have brought to the surface chemically reactive rocks such as basalts and volcanic ash in the hot and humid environment of the tropics, increasing the rate of reactions that they remove carbon dioxide from the atmosphere. Furthermore, in the last 20 million years, the formation of the Himalayas, the Andes, the Alps and other mountains has more than doubled the rate of erosion phenomena, increasing the meteorisation. Also contributing to the cooling trend was the removal of South America and Tasmania from Antarctica 35.7 million years ago, which gave rise to a new ocean current around Antarctica. The new current gave renewed vigor to ocean circulation and plankton that consumes carbon dioxide as a result of the Antarctic glaciers significantly increased in size.

Before then, in the Jurassic and Cretaceous, there were dinosaurs in Antarctica, because the increased volcanic activity, in the absence of those mountain ranges, supported CO levels2 in the atmosphere equal to about 1000 parts per million compared to 415 ppm today. The average temperature of that ice-free world was 5-9 ° C warmer than today, and the sea level was about 75 meters higher.

Impacts of asteroids
Magnitude: about 20 ° C cooling followed by 5 ° C heating (Chicxulub)
Time scale: centuries of cooling, 100,000 years of warming (Chicxulub)

The Earth Impact Database recognizes 190 confirmed impact craters on Earth so far. None of them have had any perceptible effect on Earth's climate except the Chicxulub impact, which pulverized part of Mexico 66 million years ago and exterminated the dinosaurs. Computer models suggest that the Chicxulub impact fired enough dust and sulfur into the upper atmosphere to reduce sunlight and cool the Earth by more than 20 ° C, as well as acidifying the oceans.

It took the planet centuries to return to the temperature before the impact, and then warmed up by another 5 ° C due to the carbon dioxide released into the atmosphere by the Mexican limestone that had been pulverized. It remains unclear whether volcanic activity in India at the same time as that impact may have exacerbated climate change and mass extinction.

Evolutionary changes
Magnitude: different depending on the event about 5 ° C of cooling in the late Ordovician (445 million years ago)
Time scale: millions of years

It has occasionally happened that the evolution of new life forms has reset the earth's thermostat. For example, the photosynthesizing cyanobacteria that emerged about 3 billion years ago began to terraform the planet with the release of oxygen. With their proliferation 2.4 billion years ago, the amount of oxygen in the atmosphere eventually increased, while carbon dioxide levels plummeted. This plunged the Earth into a series of "snowball" phases for 200 million years. The evolution of marine life larger than microorganisms in size started another series of snowball stages 717 million years ago, in this case because those organisms began dropping debris to the ocean floor, removing carbon. from the atmosphere to end up in the abyss and finally bury it.

When the first terrestrial plants originated (about 230 million years later, in the Ordovician), they began to form the terrestrial biosphere, burying carbon in the continental masses and extracting nutrients from the soil that then flowed into the oceans, favoring here too the flowering of life. It is likely that these changes triggered the ice age that began about 445 million years ago. Later, in the Devonian, the evolution of trees further reduced carbon dioxide and temperatures, joining the formation of mountains to kick off the Paleozoic Ice Age.

Great igneous provinces
Magnitude: about 3-9 ° C of heating
Time scale: hundreds of thousands of years

The great igneous provinces, huge lava flows and subterranean magma on a continental scale, triggered many of the mass extinctions on Earth. These volcanic events brought with them a whole arsenal of deadly tools (including acid rain, acid fog, mercury poisoning and destruction of the ozone layer, while at the same time causing the planet to warm by releasing enormous quantities of methane and anhydride into the atmosphere carbon dioxide at faster rates than the natural weathering thermostat could have coped with.

The cause of the greatest environmental catastrophe

In the event that took place at the end of the Permian, 252 million years ago, which caused 81 percent of marine species to disappear, underground magma ignited coal in Siberia, brought the level of carbon dioxide in the atmosphere to 8,000 parts per million. and raised the temperature by 5-9 ° C. The Paleocene-Eocene thermal maximum, a smaller event that occurred 56 million years ago, cooked methane in North Atlantic oil deposits and injected it into the atmosphere, heating the planet by 5 ° C and acidifying the oceans in the following period alligators and palms thrived on the Arctic coasts. Similar releases of fossil carbon deposits occurred at the end of the Triassic and at the beginning of the Jurassic and had the consequences of global warming, the presence of dead zones in the oceans and the acidification of the oceans themselves.

If these things seem familiar to you, it is because human activity today has similar effects. As a research group studying the event at the end of the Triassic wrote in April in "Nature Communications": "Our estimates suggest that the amount of CO2 injected into the Triassic atmosphere by each magmatic impulse is comparable to the anthropogenic emissions projected for the 21st century ".

(The original of this article was published on 21 July 2020 on QuantaMagazine, an independent online editorial publication promoted by the Simons Foundation to improve public understanding of science. Translation by Alfredo Tutino, edited by Le Scienze. Reproduction authorized, all all rights reserved)

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Can climate variations influence the evolution of a civilization?

For about 4 billion years, the climate and life on our planet have evolved in a mutual intertwining of geological, climatic and biological events culminating in the environment in which we live today. Large-scale climate changes throughout Earth's history have been accompanied by rapid evolutionary shifts. Extinctions, speciation, the emergence of new organisms are the events that occurred to adapt to changed environmental conditions.

Changes in the climate of the past

The evolutionary explosion of Cambrian it happened, for example, about 541 million years ago. He saw the proliferation of complex life forms in the oceans, probably due to an increase in the amount of oxygen present in sea water.

The extinction of the Permian, 252 million years ago, was instead triggered by intense volcanic activity lasting thousands of years in the region of present-day Siberia. This last event led to an increase in the emission of carbon dioxide (CO 2) into the atmosphere which wiped out about 90% of marine species and 70% of terrestrial ones.

The reconstructions of the climate of the last millennia suggest a continuous interaction between climate variations and changes in human societies. In particular, some research carried out in the Mediterranean basin and in central Europe, have identified close links between variations in the climate and the evolution of civilizations and empires.

In particular, a set of marine and continental paleoclimatic indicators testifies that the climatic conditions during the existence of the Roman Empire changed significantly.

What do paleoclimatic indicators tell us?

A group of scholars, analyzing the shell of marine microorganisms (planktonic foraminifera) was able to make estimates of the surface temperature of the Mediterranean sea of ​​the last 5000 years. The samples analyzed come from sedimentary sequences sampled in the Aegean Sea, the Sicilian Channel, the Alboran Sea and the Menorca basin. Thanks to these observations, the hottest period in the last 2,000 years appears to be the one that includes the first 500 years of the Common Era. In this period, the surface temperatures of the sea are about 2 ° C higher than the average calculated over the entire period.

This climatic phase, also known as Roman Climatic Optimum, roughly coincided with the birth and flourishing of the Roman Empire, probably favoring its expansion.

With the same goal, a different study has reconstructed the rainfall and summer temperatures by analyzing the growth rings of trees in central Europe. Also in this case, the team of scholars found correspondences between hot and rainy summers and periods of prosperity of the Empire (between 100 BC and 250 AD).

In the same study, between 250 and 600 AD, it emerged that rapid climate changes corresponded to periods of drought, floods, cold, often associated with epidemics.

The case of the Roman Empire

The birth of the Roman Empire dates back to 27 BC, when the emperor Gaius Julius Caesar Augustus better known as Octavian, He took power. The deposition of the last Western Roman emperor, Romolo Augustolo, in 476 A.D. definitively sanctions its end.

The period of birth and development of the Empire thus coincides precisely with that period already mentioned, to which scholars attribute mild temperatures and stable climatic conditions.

In the first part of what covers a period of about 600 years, from 27 BC around 300 A.D., the climate did not undergo particular variations. In particular between 100 A.D. and 250 A.D. unusually favorable conditions for agriculture occurred, favoring prosperity and development at least until about 300 AD. The conditions were also such as to facilitate the overcoming of the mountain ranges allowing the expansion of the Romans beyond the Alps, beyond the English Channel and towards Orient.

In this same period also theEgypt lived a period of prosperity. Together with Sicily, it earned the title of granary of the empire, producing large quantities of cereals. In the years preceding this, as many as seven of the nine floods of the Nile in the first century BC. in fact, they were not adequate to fertilize a sufficiently large area. The subsequent 329 years, on the other hand, were characterized by an increase in the frequency of favorable floods, ensuring a continuous supply of wheat, barley and spelled towards Rome and other regions of the Empire.

The arrival of rapid climate change

Starting from about the 300 A.D. rapid climatic fluctuations began to appear which probably heavily interfered with the fate of the European peoples. The changes affected different regions of the empire, but in different ways and times. Although physical and historical data do not coincide perfectly, they do exist in any case surprising convergences on the sequences of the best known events.

The period between 250 and 600 A.D. was characterized by rapid climate change on an annual to ten-year scale. The phenomenon was caused by several factors, the main ones being:

  • variations ofsolar activity
  • fluctuations in climatic indices which regulate rainfall on continents (such as El Niño of the Central-Southern Pacific Ocean and the NAO of the North Atlantic Ocean)
  • resumption of volcanic activity in different areas of the planet.

Droughts, floods, low temperatures occurred during this period and, combined with frequent epidemics, put a strain on the population, destroying the ability to produce food of the agricultural societies of the time. This was a period of political turmoil, cultural changes, barbarian invasions and economic and social instability in various provinces of the Empire. Events that over time, through alternating phases of crisis and resumption, caused its definitive collapse.

Changes in the climate and the barbarian invasions

Historians and archaeologists have always debated whether the climatic conditions also had to do with the barbarian invasions. Beyond the eastern Roman borders, in Central Asia, there lived various nomadic populations. Their expansion had interfered and interfered with the life of the settled empires of Eurasia. Pastoralism, an important part of their economy, made them particularly sensitive to fluctuations in rainfall and climate.

A decisive event for the descent of these populations towards the borders of the Empire was the grave drought of the fourth century which lasted almost 40 years. It was one of the worst of the last 2000 years, begun in 338 AD. and ended in 377. The dynamics of the event played a crucial role in guiding the nomadic populations, the Huns, looking for new pastures and places to plunder. Historical sources indicate that these reached the banks of the Don in 370 AD. and they crossed it five years later. Their attacks in the area north of the Black Sea pushed local populations, i Goths, to seek asylum and enter the Roman Empire. Subsequently, in 378, the latter also attacked him, defeating the Eastern Emperor Valente in Adrianople.

Learn from the past

From this brief analysis it therefore seems clear how the social and economic well-being of a society can be linked to rapid changes in the climate. Examples from the past may suggest what can be expected in the near future as an effect of the rapid climate change we are experiencing.

Today we are potentially less vulnerable to this type of phenomena than our ancestors. However, we cannot consider ourselves immune to the effects of the climatic conditions foreseen for the near future. We must not and cannot continue to believe that we are isolated and alien to the variations of the natural environment that surrounds us.

On the cover: the Roman Forum, photo by L. Cafarella

Climatic variations of the Earth

Millennial motions of the Earth and climatic variations

The duration of each astronomical season depends on the speed with which the Earth travels the corresponding stretch of orbit. Since the precession of the equinoxes changes the position of the seasons on the orbit, over the millennia there is also a variability of their duration (see figure).

Currently the autumn-winter semester falls for our northern hemisphere in the sector of the earth's orbit closest to perihelion, where the Earth has a higher revolution speed, and therefore it lasts about 7 days and 6 hours less than the spring-summer semester. , which corresponds to the stretch of orbit closest to the aphelion (where the speed of the Earth is slower). But in about 10 500 years the situation will be the opposite.

To this it must be added that the Earth-Sun distance, which varies over the course of the year, also affects warming, although not to a considerable extent. If a hemisphere has its winter when the Earth is at its closest distance from the Sun - as is currently the case for the northern hemisphere - it will be less cold and the summer season will be less hot because it is further away from the Sun. When this occurs in one hemisphere , in the other there is the opposite situation. Due to the precession of the equinoxes there is a continuous change of this state of affairs: the two hemispheres alternately pass from one situation to the other.

The oscillation of the annual caloric excursion (i.e. the difference in heating between the two extreme seasons) produced by the precession of the equinoxes varies in intensity with time, as a consequence of another millennial motion of our planet, namely the variation of eccentricity of the orbit since the difference between the Sun-aphelion and the Sun-perihelion distances varies with it.

The change in the inclination of the Earth's axis also has important effects on the climatic conditions of our planet. In fact, when the inclination of the axis - with respect to the perpendicular to the plane of the orbit - takes on greater values, the seasonal contrast becomes more marked in the opposite case, this contrast is reduced. Given the slowness of these movements, the climatic variations they induce are extremely gradual and therefore escape direct observation. However, we know that they are one of the main causes of glaciations.

(Top in figure) Precession of the equinoxes. The double-conical motion of the earth's axis changes the arrangement in space of the celestial equatorial plane (which is perpendicular to the axis) and therefore determines the clockwise rotation of the intersection between this plane and the plane of the ecliptic, i.e. line of the equinoxes (precession).

The glaciations

The fact that the three millennial movements described do not substantially alter the total insolation of the Earth, but only its distribution at different latitudes and throughout the year, could suggest that their effects on the climate are modest. On the other hand, the hypothesis - proposed by the astronomer M. Milankovitch in the first half of the twentieth century - is now accepted, according to which the glaciations that have probably occurred five times over the last 2 million years are to be related to the main movements millennia of our planet.

One of the determining causes of the alternation of glacial and interglacial ages is not so much the variation of the global insolation of the Earth, but rather the summer insolation, which at high latitudes can vary by up to 20%, that is much more than the total insolation, as a consequence of the millenary motions of the Earth (see figure). In fact, in the areas located at high latitudes (and also on the mountainous reliefs that reach the highest altitudes) snow accumulates easily even during a relatively mild winter, as is what occurs when the Earth is near the perihelion. But it is important to consider how much of the snow fallen in winter can be preserved during the summer: this essentially depends on the extent of summer sunshine.

(Top in the figure) The variation of the eccentricity of the orbit modifies the intensity of the climatic oscillations due to the precession of the equinoxes.

TO The most favorable situation for the development of the ice sheets in the northern hemisphere. Winter falls near the perihelion and is therefore milder, while summer falls near the aphelion and is therefore cooler at the same time the maximum eccentricity of the orbit decreases the Sun-perihelion distance and increases the Sun-aphelion mitigating the winter even more and making the summer even cooler.

B. For the northern hemisphere this is the most unfavorable situation for glacial expansion. In fact, summer is particularly hot, both because it occurs in peri and I, and because the latter is located at its shortest distance from the sun.

C.., D.. In these situations there are intermediate climatic conditions compared to the previous two.

If the solar radiation that reaches high latitudes during the summer season is lower than normal, as happens when summer occurs in aphelion, the snows that fall during the cold season cannot completely melt, but are accumulating from year to year. and slowly turn to ice. In this way the ice caps begin to expand and our planet enters an ice age. If, on the other hand, during the summer the insolation increases, as happens when this season occurs at perihelion, a greater quantity of ice melts than that which can be replaced by the winter snows, therefore the glaciers tend to retreat and a climate is established on the Earth. warmer (interglacial).

Among the factors that contribute to making the Earth's climate difficult to interpret - and therefore to predict - are the so-called feedback processes: these are mechanisms that, triggered by heating (or cooling), can further reinforce the heating effect. (or the cooling one), in which case they are called positive, or they can counteract it, in which case they are called negative (see figure). Scientific research continues to discover unpublished but let's see the main ones known to date.

Albedo is the relationship between the energy reflected by an object and that incident on it. Overall, currently the albedo of the Earth-atmosphere system has a value of 0.35 (so just over a third of the incident solar energy is reflected). Any factor capable of modifying this value can cause the energy absorbed by the planet to vary. For example, the melting of ice at high latitudes due to global warming could have the effect of decreasing the albedo of these areas, with the result of increasing the energy absorbed by the Earth-atmosphere system (positive feedback).

(Top of figure) Feedback processes are mechanisms by which a change is followed by a reaction that can reinforce or counter that change.

The trend of ocean currents is linked to winds and above all to thermohaline circulation (which depends on the temperature and salinity of the oceans). If the thermohaline circulation were to be blocked due to global warming, there could be a decrease in the average temperature of large areas in Europe (negative feedback).

Biochemical processes in the soil, by bacteria and other microorganisms, lead to the emission of CO2. If the effect of heating increases the microbial activity in the soil, and greenhouse gases stored in the permafrost are released, it is possible that the emissions of these gases grow faster (positive feedback).

According to some forecasts, the rising temperature of our planet should also cause an increase in water vapor in the atmosphere, which - condensing in the clouds - could shield the incoming sun rays (negative feedback). But clouds can also absorb additional solar radiation and contribute to atmospheric warming (positive feedback). It remains to be established which of the two effects is prevalent.

I chose to include this information (taken from the following book) because you often read comments in the various blogs, of people who seem to have no clear ideas about certain terrestrial phenomena such as global warming (some say it is a hoax), the inversion of the poles, the variation of the earth's inclination, etc. Well, in this book everything is explained well and in a "very understandable" way since it is a text intended for high schools. I think that many people would need to read a book of this type, among other things it is very interesting because it deals with different topics, and is well structured, but above all "updated".

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