Download main factors influencing climate change: a review

Доклади на Българската академия на науките
Comptes rendus de l’Académie bulgare des Sciences
Tome 67, No 11, 2014
SCIENCES DE LA TERRE,
L’ATMOSPHERE ET L’ESPACE
Climatologie
MAIN FACTORS INFLUENCING CLIMATE CHANGE:
A REVIEW
Todor Nikolov, Nikola Petrov∗
Mini Review
Written upon invitation of the Editorial Board
Abstract
This article is an overview of topics related to the impact of main factors
and especially of the Sun on climate changes on Earth which have always had
significance for the life on our planet. Climate system of the Earth is very
complex and is characterized by chaotic dynamics. It has been formed and
is under the constant influence of several key factors: (1) variations in solar
radiation, driven by dynamic processes of the Sun; (2) changes in the orbital
parameters of the Earth due to its movement around the Sun; (3) changes in
the intensity of galactic cosmic rays that alter the Earth’s cloudiness; (4) geophysical and geological (tectonic) processes that generate the internal structure
of the Earth, the structure and movement of lithospheric plates, formation of
mountain systems, the opening and/or closing of oceans and formation of the
main geomorphological features of the planet; (5) the strong impact of human
activity, its growing importance since the Early Holocene. These factors can be
divided into three groups: external (astronomical and orbital), internal (Earth –
geophysical, geological and geographical) and anthropogenic. Climate changes
are caused by the combined effect of these various factors, among which the
orbital effects are of paramount importance. The role of the Sun as the primary
energy source for the Earth and a driver of global climate change is particularly
important. We also pointed many controversial issues about the exact physical
processes that cause climate change. Some major problems are related to clarification of the relationship between solar variability and solar forcing, and also
to the insufficient reliable statistical data disallowing more accurate physical
models leading to inability to predict the long term climatic events [2, 4–6 ].
Key words:: climate changes, solar influence on climate, orbital forcing
phenomena, human impact on climate, climate in Earth’s history
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Introduction. Climate changes and global warming of our planet are one
of the most debated topics in Earth, Atmospheric and Space sciences in the last
30 years. In 1975 Broecker [1 ] expressed the idea of global warming and it
was accepted by many scientists. Some of them believe that climate change
is caused by human activity and the increasing greenhouse gases – particularly
CO2 . Others argue that the Earth’s climate depends on the influence of natural
astronomical, physical and geodynamic factors among which the complex solar
impact on climate stands out as well as the impact of changes in Earth’s orbit.
The tendency of global warming determined by natural factors is combined with
the impact of greenhouse gases – especially CO2 and the role of man is crucial in
this phenomenon. Definitely, the galactic cosmic rays have an impact, too.
Here we present an overview of problems related to the impact of main factors
and especially of the Sun on climate changes on Earth which have always had
significance for life on our planet.
It is known that the measurement of temperatures on Earth began in 1856,
when the British Meteorological Society began collecting temperature data worldwide. The climate in the Earth’s history before that date is characterized on the
basis of data from the geological record – proxy data, which are a key to temperatures in the geological past. Such are tree rings, ice cores with gas bubbles
trapped in ice, coral epitheca, pollen spectra, and sediments, etc., which allow
the establishment of climate variability.
Astronomical and orbital factors are crucial to the Earth’s climate. Global
climate changes may be indirectly due to gravitational resonances generated by
the big planets in the solar system and the Sun, or to passing of the solar system
through the surface of the Milky Way. These factors are also related to the
luminosity of the Sun; the position of the Earth in the solar system; the Earth’s
rotation around its axis and around the Sun; the rotation of the solar system
around the galactic centre; the interaction of systems Earth-Sun and Earth-Moon;
interaction with other planets in the solar system and peculiarities in the orbital
motion of the Earth. They influence directly or indirectly the evolutionary process
on Earth: internal dynamics, dynamics of the crust, geoidal eustasy, gravity and
magnetic potentials, climate’s dynamics, eustatic fluctuations in the sea level,
evolution of the biosphere, etc. The most common feature of astronomical and
orbital impacts on the Earth is the cyclical nature of the main geological processes
(including climate) that shape the Earth. As a result of these effects, cycles of
varying lengths are formed.
The long-term 2 orbital cycles (over 220 Ma1 ) are connected with the rotation
of the solar system around the centre of the galaxy, while the short-term cycles
are determined by orbital effects and the interaction of the Earth with other
planets and especially by the influence in the system Sun-Earth-Moon. Against
1
1 Ma = 1 million years.
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T. Nikolov, N. Petrov
the background of these regularities global changes in climate in Earth’s history
must be considered as part of geological cycles [2, 3 ].
The role of the Sun in climate. The influence of the Sun on the Earth’s
climate system is a primary one, and this is a reason to accept the opinion of
Beer et al. [4 ] that “the Sun is the engine that drives the climate system”, i.e.
the climate of our planet is formed by complex compound of interacting factors,
with priority to the role of the Sun [4–6 ]. As noted by Beer et al. [4 ], however,
little is known about how variable is this effect in different time frames, ranging
from minutes to millennia, and how the climate system responds to changes in
this respect.
Variations in global insolation2 . The climate system of the Earth depends mostly on the Sun, which is a major source of electromagnetic energy for
our planet. The flow of radiant energy that the Earth receives from the Sun in
the upper layers of the atmosphere is about 1365 W/m−2 . The mass of the Sun
is 99.8% of the total mass of the entire solar system. It is, in its turn, a part of
the Milky Way galaxy, which is a spiral galaxy with a diameter of about 100 000
light-years and contains about 200 billion stars. The solar system is located in
one of the arms of the Galaxy, and it is located at between 25 000 and 28 000
light-years from the galactic centre. It moves at a speed of 220 km/s on its orbit
around it, and performs one complete rotation for average of 226 million years
(with variations between 220–250 million years). This interval is defined as a
galactic year or a cosmic aeon. In its motion around the galactic centre the whole
solar system performs undulation like a dolphin diving in the sea. Over a period of about 25–30 million years, it is under the galactic plane or goes above it
(Fig. 1).
Supercontinental cycles, which are related to the defragmentation of megacontinents such as Pangaea and the drift of lithospheric plates, correspond in
duration to one cosmic aeon. They are also associated with the largest glacial
periods in Earth’s history (Cryogenian – 850–635 Ma BP; Ordovician – 440 Ma
BP; Permian – 250 Ma BP; Pleistocene – 1.6 Ma BP). The periods of increased
radioactivity on Earth also coincide with the cosmic eons [9 ].
The Sun provides a continuous flow of radiant energy (solar radiation), which
is the main source of light and heat on Earth. The energy emitted by the Sun is a
result of thermonuclear fusion reactions. Every second in the core of the Sun (at
about 14 million K)3 1038 proton-proton reactions are performed that convert 700
million tons of hydrogen into 695 million tons of helium. The additional 5 million
tons are emitted in the form of radiative energy. This enormous amount of energy
is carried to the Earth in the form of electromagnetic waves from all areas of the
spectrum. Beer et al. [4 ] noted that global insolation is a function not only of
2
The term “global insolation” refers to the total electromagnetic solar radiation in the upper
part of the atmosphere, determined by the W/m2 .
3
K – Kelvin degrees, the so called absolute degree, which in size is equal to Celsius degree.
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
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Fig. 1. Schematic representation of our solar system in the Milky Way. The ecliptic plane
is inclined to the galactic plane at an angle of 60◦ . The solar system performs undulating
movements on its orbit around the galactic centre with a period of about 28 million years. The
last 3 million years we are moving away from the galactic plane, and are “above” it (arrow
“now”). The scheme is composed by numerical data of Thaddeus and Chanan [7 ] and Bahcall
and Bahcall [8 ]
the solar dynamics but also of the changing transmission conditions between the
Sun and Earth, including the Sun-Earth distance, especially related to changes
in the eccentricity (ellipticity or flattening). The eccentricity of the Earth’s orbit
varies between extremes zero (circular) and 0.06 in a cycle of 96 600 years. The
present value of ellipticity of the orbit is 0.0167. The last maximum ellipticity of
the Earth’s orbit (0.019) was about 10 000 years ago, and the previous minimum
(0.010) was about 40 000 years ago.
On its way to the Earth, a part of the solar energy is permanently absorbed
and re-emits at increasingly lower temperatures, and another part of it (about
10%) is reflected by the Earth’s atmosphere back into space. Significant scattering
happens in the Earth’s atmosphere itself, especially when the air is dustier and
more humid. Rays with a wavelength from 0.3 to 3 µm (ultra-violet, visible, and
infrared) reach the Earth’s surface. For each geographical area, the spread of solar
power depends on the slope at which the Sun’s rays fall on the Earth’s surface.
Upon reaching the Earth’s surface, the solar energy is primarily visible light.
Another part of the solar radiation (about 30%) is retained in the atmosphere,
heating its upper layers. Much of the solar energy (about 37%) is taken by
the ocean, which becomes the main heat accumulator of the Earth, and it mainly
influences the climate. Biosphere takes only 0.08% of the solar radiation. Changes
in the intensity of solar radiation through the seasons are small and they are about
3.5%.
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It is assumed that the solar radiation that reaches the Earth’s atmosphere
was relatively constant and did not experience significant fluctuations in long
periods of Earth’s history. A number of authors [4, 10 ] indicate that main statistic
data about the stars, as well as the modern astronomical theories give grounds
to accept the conclusion for stability of solar radiation (luminosity of the Sun)
at intervals of hundreds of millions of years. This is also expressed by the socalled solar constant, which represents a flow of solar electromagnetic radiation,
reaching the Earth for unit time in a given area outside the atmosphere, measured
in a plane perpendicular to the rays. It ranges from about 1365 to 1368 W/m2 .
The solar constant includes all types of solar radiation, not only the visible light.
According to data from measurements made by satellites, the solar insolation
fluctuates with about 6.9% over the year – from 1412 W/m2 in the beginning of
January to 1321 W/m2 in early July, depending on the variations in the Earth-Sun
distance. Thus, the total power of solar radiation for the Earth is 1.740 × 1017 W
(±3.5%) [11 ].
Luminosity depends mainly on the mass of the star and therefore it changes
slowly. When some astronomers say that “solar constant is not constant” it should
be borne in mind that they consider the solar luminosity over relatively short (in
geological terms) intervals, usually several years up to a century in which both
the appearance of sunspots and the intensity of sun’s luminosity show variations.
In the long periods of tens or even hundreds millions of years the solar luminosity
has changed slightly, i.e. it is equal enough for large geological intervals [9 ].
Like all stars the Sun’s luminosity increases over time so that since the formation of the solar system till today the Sun has “flared” and now it emits more
energy [9, 11–13 ]. According to Schwarzschield [12 ] since its emergence about
5 billion years ago till now the Sun has higher luminosity with 60%. Aller [14 ]
assumes that the luminosity of the Sun today is 25% higher than 4.5 to 4.6 billion
years ago, when the formation of the Earth began. In such a regularity it can
be assumed that after the appearance of life on Earth (3.8 to 3.6 billion years
ago) and especially after the acceleration of biological evolution (about 2 billion
years BP) the intensity of solar luminosity has increased negligibly and the temperature of the Earth’s surface has stabilized and remained relatively constant
in subsequent geological periods, with some variations in some ages from 10 to
25 ◦ C [15 ]. The decrease in the intensity of solar radiation, however, only with
parts of the percent causes global cooling and respectively glacial periods on the
Earth [16 ].
Sunspot cycles. The most striking manifestations of solar activity are the
sunspots, prominences, the frequency and power of solar eruptions, which show a
certain cycle. Sunspots can reach to more than 100 000 km in diameter and they
are caused by complex but so far not well explained changes in the magnetic field
of the Sun. They occur in a period from 9 to 14 years (average 11.2 years). This is
defined as the 11-year cycle of solar active (or sunspot-cycle). The duration of this
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
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cycle is not strictly constant – for example, the sunspot-cycle in the 20th century
was about 10 years, and in the previous 300 years it ranged from 9 to 14 years.
The last time when the Sun activity reached a peak was in 1990–91, in 2000 and
in January 2012–2013. The last maximum of solar activity is characterized by a
double peak regarding the number of sunspots. Non-periodic fluctuations in the
solar activity are also established but they have not been explained yet. Moreover,
the reasons for cyclical solar activity are also still a controversial issue. In this
connection, the announcement by Beer et al. [4 ] that the satellite measurements
have shown for more than two decades a clear link between solar radiation and
11 year cycle of sunspots is of considerable interest. We share the opinion of
Beer et al. [4 ] that “the response of the climate system to solar forcing depends
not only on the amount of radiation, but also on its spectral composition (e.g.,
UV contribution), seasonal distribution over the globe, and feedback mechanisms
connected with clouds, water vapour, ice cover, atmospheric and oceanic transport
and other terrestrial processes. Therefore, it is difficult to establish a quantitative
relationship between observed climate changes in the past and reconstructed solar
variability. However, there is growing evidence that periods of low solar activity
(so called minima) coincide with advances of glaciers, changes in lake levels, and
sudden changes of climatic conditions”.
The clarification of the variations in solar activity or what is defined as solar
impact on the climate (solar forcing), is very important for the explanation of
climate variability. The sunspots characterize best the solar activity. The number
and size of the spots grow rapidly upon active Sun (up to 100–200 spots (Wolf
number) with area up to 16 billion km2 ) and they mark the maximum of the solar
activity (Fig. 2) and at a reduced solar activity – they are few and on limited
areas or disappear completely (solar minimum) [13, 17 ].
An important property of sunspots is their magnetic field whose voltage
determines the size of the spots themselves4 . It is interesting to be noted that the
eruptions of Sun happen most often near the sunspots and it is likely that they
draw their energy from the strong magnetic fields of the spots. The analysis of
these magnetic fields shows that there is a magnetic cycle, which includes two 11sunspot-year cycles. In these heliomagnetic transitions the magnetic field of the
Sun returns to its original position after two 11-year intervals. This is indicative
of the relation of the sunspots with the magnetism of the Sun and it defines the
22-annual cycle as the main interval in the solar activity [18 ].
Sunspot cycles are related to the Earth’s climate, although the mechanisms
of this influence have not been clarified yet. In this connection it is interesting
to note that the thickness of the growth rings in some trees have 11-year cycle,
and that 11-year cyclical fluctuations of some rivers’ levels are also observed [19 ].
4
The tension of the magnetic fields of the small sunspots is about 100 gauss, while for the
large ones it reaches up to 4000 gauss (in comparison, the tension of the magnetic field of the
Earth’s magnetic poles is about 0.5 gauss).
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T. Nikolov, N. Petrov
Fig. 2. Solar disk images from SOHO (NASA/ESA) spacecraft, operated by Extreme Ultraviolet
Imaging Telescope (in the light of 304 and 171Å wavelength). The images show different levels of
solar activity for different time, in accordance with the 11th year of solar activity cycle. Bottom
part of the figure shows the number of sunspots (Wolf number) for the period January 1749 –
December 2013 (SILSO data/image, Royal Observatory of Belgium, Brussels)
Fig. 5. Climates in Earth’s history a) International stratigraphic chart, v. 2013/01, after
International Commission on Stratigraphy – www.stratigraphy.org; b) Global climate changes
through time (after Scotese, 2012 – http://www.scotese.com/climate.htm
There are also longer cycles of solar activity lasting 80 to 100 and 200 years; they
are called Secular cycles and cycles with a period of occurrence of thousands of
years.
The fluctuations of the solar constant, depending on the level of solar activity,
do not exceed 1.5%, i.e. they are within the tolerances of its defining. Ruddiman [13 ] notes that changes by 0.15% in total solar activity (TSA) can change
the average temperature of the Earth with 0.2 ◦ C if they act over a longer period.
However, it is difficult to be evaluated for the shorter 11-year cycle with its 5.5
years’ interval between the minimum and maximum.
Current options for measurements of TSA are considerably larger. Data from
satellite studies show that the strength of solar radiation is correlated with 11-year
sunspot cycles. Therefore, researchers continue to look for a relationship between
solar activity and climate change on Earth. It is still not specified how climatic
cycles coincide with the 11-year cycles of solar activity. Although individual cases
of coincidence are identified, there are no reliable statistical data yet for defining a
specific regularity. According to Ruddiman [13 ], the average surface temperature
in the last 100 years follows a trend similar to the trend of the sunspot cycles.
In this case the long-term values of the sunspot maximums are averaged over
several decades that correlate with climate changes. The above shows that there
are differences among scientists about the direct impact of solar luminosity on
the climate.
It is obvious that the climate system of the Earth has always been dependent on the intensity of solar radiation, periodically strengthened or reduced by
changes in the orbital parameters of the Earth and its evolutionary change. The
solar radiation itself is not one of the direct primary definitive factors of the climate, especially for large climatic cycles. There are also cycles of the so-called
decade group as well as Secular cycles that are driven by variations in solar
radiation. As it has already been noted, such cycles are 11-year cycles in the
appearance of sunspots, as well as 22-year cycles in the inversion of polarity of
the heliomagnetic field. A correlation is determined between the cyclic decreases
of temperatures and cold spells in the last 500 years, and deep sunspot cycles.
In most cases, however, the influence of 11-year variations in the solar luminosity
is weak, probably due to the impact of the Thermal inertia of the oceans. As a
rule, the Earth’s climate system corresponds to relatively short-term variations of
solar radiation with variations of the surface temperatures of less than 0.1 ◦ C, but
without a clear long-term effect [13 ]. The response of the system, upon changes
in the orbital parameters, is always dramatic and it is associated with marked
fluctuations (cool or warm).
Some climatologists suggest that sunspot cycles have occurred in the distant
past in larger intervals of enhanced solar activity (one- and two-centuries cycles).
Moreover, in some of these periods of the Sun, spots have not appeared. This is
the case of the well-known Maunder minimum (between 1645 and 1715 AD) and
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Table
1
Events of solar activity with approximate dates of beginning and end
Events
Oort minimum
Oort minimum (see Medieval Warm Period)
Medieval maximum (see Medieval Warm Period)
Wolf minimum
Spörer minimum
Maunder minimum
Dalton minimum
Modern maximum
Little Ice Age
Beginning
1010
1040
1100
1280
1460
1645
1790
1950
1350
End
1050
1080
1250
1350
1550
1715
1820
and now
1850
Spörer minimum (from 1460 to 1550 AD) during which the Sun’s emissions were
unusually low (see Table 1) [20 ].
Before the sunspots could be observed directly, these minimums of solar
activity (Fig. 3) were determined by analysis of the ratio of carbon-14 in tree
rings, ice cores and cave formations. During the so-called Little Ice Age (1350–
1850), there was a significant cooling of the climate in Europe, North America
and Greenland when the river Thames in London, the Seine in Paris, the canals
of the Netherlands and others were ice-bound; entertainments were arranged on
the frozen Moscow River. Many villages were abandoned in Greenland.
The secular variations in the solar luminosity have a direct impact on climate,
although some astronomers claim that they lag 10–15 years because of the thermal
inertia of our planet. Moreover, some authors suggest there are such parallel
changes on Mars as well due to secular variations in the solar radiation. This
is an evidence for the fundamental impact of the Sun not only on the Earth’s
climate, but also on other planets in the solar system. Other opinions affirm that
“up to the Quaternary time (i.e. more than 2 million years BP) astronomical
Fig. 3. Solar activity events recorded in 14 C. Present period is on the right. Values since 1900
not shown. (After Leland McInnes at http://en.wikipedia.org/wiki/Solar_variation)
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factors have not had a big impact on the change of climatic conditions” on the
Earth [21 , p. 168]5 Such statements are unfounded because the geological record
provides many examples of the influence of these factors on the climates of our
planet. Moreover, no one could explain what “triggered” the astronomical factors
in order to start affecting the climates during the Quaternary Period, because
during the previous 4.5 billion years they were “locked”. And they were not
“locked”, but they have acted throughout the whole Earth’s history [9 ].
Dendrochronological data. Our dendrochronological data, specifically for
the growth rate of the tree species, recorded in the tree rings, are different for
different species. In some cases, humidity is a determining factor of growth, and
the width of the tree rings is a function of rainfall. In other cases, the temperature
plays more important role for growth of new wood. The most favourable one is
the combination of heat and rain.
The analysis of changes in tree growth rings in different species allows to
determine the dependence of the width of tree rings by temperature, rainfall
and other climatic indicators, and the main features of Paleoclimate to be determined [13 ].
Climate variations related to the orbital parameters of the Earth.
Besides astronomical factors, related mainly to the activity of the Sun and the
power of its luminosity, orbital variables have a strong effect on various processes
of the Earth and special definitive impact on the climate system since they cause
successive cyclic changes in climate and various geochemical, sedimentation and
biological systems.
Climate models show that the combination of solar activity with orbital and
Earth factors define the major climatic cycles of the Earth and their fluctuations.
The effects of astronomical and Earth factors are intertwined, that is why they
cannot always be clearly distinguished. For example, changes in the tilt of the
Earth’s axis of rotation depends on both the geological and astronomical factors; the atmosphere is influenced by different Earth’s processes and especially
volcanism, but definitely the ultraviolet radiation of the Sun has impact on it.
The Earth performs various movements. The most important ones are the
rotation around its axis; rotation around the Sun; rotation of the Earth together
with the Solar system around the centre of the Galaxy, when the whole system
periodically passes through clouds of stardust with heterogeneous density. A fundamental fact derives from these basics: due to periodic changes in the parameters
in the Earth’s orbit there are continuous cyclical changes in the climates of the
planet which are established in its long history. While changes in solar radiation
cause short-term variations in climate, the orbital factors determine the climatic
fluctuations in long-term intervals, usually over 23 ka [4, 6, 9, 13 ].
5
“In the pre-Quaternary period astronomical factors do not have a large impact on climate
change conditions” [21 , p. 168].
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1463
Fig. 4. Astronomical variables influencing the climate on Earth
The astronomical influence on the Earth depends strongly on the changing
position of Earth’s axis as it moves around the Sun (Fig. 4). In its turn, the tilt of
the Earth’s axis also changes with respect to the plane of the ecliptic, and it is not
strictly constant. The precession axial inclination and eccentricity of the Earth’s
orbit have a determining impact. One precessional cycle is 19 ka to 23 ka with
deviations 14 ka and 28 ka [22 ]. The direction of the Earth’s axis also changes in
space. Because of the precession it describes a cone in the space for 23 000 years.
It is added to the precession and the smaller fluctuations of the deviation of the
axis of rotation (nutation), following gravity effects of the Sun and the Moon.
Boer and Smith [2 ] emphasize that these variations depend on the gravitational
forces in the rotation system Sun-Earth-Moon, as well as on the influence of the
other planets of the solar system.
Inclination reflects the tilt of the Earth’s axis towards the Ecliptic. It ranges
in about 41 ka between 22◦ 020 3300 and 24◦ 300 1600 , and affects the seasons, particularly in the high latitudes [2, 23 ].
It is known that the Earth’s orbit has the shape of an ellipse, the Sun being
situated in one of the foci. The variations are defined as eccentricity that varies
in about 100 ka, but there are also cycles with longer duration – 400 ka, 1300 ka
and 2 Ma [24 ].
The orientation of the axis remains the same in different parts of the Earth’s
orbit – an effect which determines the Earth’s seasons. The precession of the
Earth’s axis causes the slow changing in the beginning of the seasons to the
position of the Earth in its orbit.
Many meteorologists and climatologists have tried to explain the influence
of separate factors on the distribution of solar energy on Earth, moreover in
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the course of Earth’s history they could be modified in certain and sometimes
significant limits. This applies to both the slope of the Earth’s axis and the
precession and the eccentricity of Earth’s orbit. Therefore, reasonably accurate
characteristics cannot be obtained for the amount of solar energy that the Earth
receives per unit time. A successful attempt to solve this problem was made by the
Serbian scientist Milutin Milanković (1879–1958). His attention was drawn by the
causes of ancient ice ages. In 1924 he published his basic ideas for his astronomical
theory of climate changes, which he accomplished in 1941 [25 ]. According to his
theory, the main climatic changes on Earth are determined by the magnitude of
solar radiation (insolation) and the cyclical variations in the Earth’s orbit. These
astronomical variables (also called Milanković’s variables) with the gravitational
effects of the Sun and Moon on the Earth have different duration and timing.
For decades, the theory of Milanković attracted the attention of scientists and
was a subject of great discussions, often met with complete denial. Only after
1976, thanks to the widespread application of radiometric methods for absolute
dating, this theory is assessed on its merits. “World Science made a quick and
decisive turn with the recognition of the theory of Milanković, whose basic principle was: all great changes in the global climate are due to orbital related changes
in the radiation balance of the planet” [16 ]. The main cycles in accordance with
the theory of Milanković are connected with the orbital movements of the Earth
and they are mainly in three orders: 1) 19–23 curve (average 21 ka); 2) 41 ka;
3) 54, 100, and 410 ka. The cycles from the first order are associated with the
precession, those from the second order – with the change of the inclination of the
Earth’s axis, and the cycles of 54, 100 and 410 ka are determined by the changes
in the eccentricity of the Earth’s orbit. The cycles following in significance are
of 400 ka, 1.23 Ma, 2.04 Ma and 3.4 Ma. In short, according to the theory of
Milanković, due to periodical changes in the parameters of the Earth’s orbit, the
climate on our planet shows cyclical fluctuations with periods of ice, followed by
periods of global warming. These fluctuations are called Milanković cycles.
After some time of oblivion and denial by some authors (between 1940 and
1960) this theory is now revived and enriched by contemporary scholars, remaining the greatest work of the Serbian scientist Milutin Milanković. Indeed Milanković treated calmly the criticism of his theory, noting the following: “It is not
my duty to eradicate someone’s ignorance and I do not force anybody to admit
my theory, to which no one can oppose so far.”
About cosmoclimatology. Recently Svensmark [26 ] presented a new idea
for the climate changes, according to which changes in the intensity of galactic
cosmic rays (CRF – Cosmic Ray Flux) influence the formation of clouds. He
described the idea as a new paradigm (model) for climate changes, which he
called Cosmoclimatology.
Comparisons of data, obtained by satellites with measurements from ground
stations, give reason to assume that the increase in the flow of galactic cosmic rays
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strongly influences cloudiness. According to Svensmark (loc. cit.) the variations
in CRF are within the range of decade, centuries and millennia cycles in climate
changes. In addition, this author stressed: “Data on cloud cover from satellites,
compared with counts of galactic cosmic rays from a ground station, suggested
that an increase in cosmic rays makes the world colder. This empirical finding introduced a novel connection between astronomical and terrestrial events, making
weather on Earth subject to the cosmic-ray accelerators of supernova remnants
in the Milky Way” [26 ].
In fact, the idea of the influence of galactic cosmic rays on climate changes
is analyzed by other authors as well – for example, Shaviv and Veizer [27 ], who
present many facts to support the important influence of CRF on climate changes.
However, there is insufficient evidence for a correlation between galactic cosmic
radiation and changes in global temperatures on Earth, at least in the last few
decades.
Celestial drivers vs. CO2 . Due to the fundamental importance of the
ideas presented by Shaviv and Veizer [27 ] we quote a summary of their comments:
“Atmospheric levels of CO2 are commonly assumed to be a main driver of global
climate. Independent empirical evidence suggests that the galactic cosmic ray flux
(CRF) is linked to climate variability. Both drivers are presently discussed in the
context of daily to millennial variations, although they should also operate over
geological time scales. Here we analyze the reconstructed seawater paleotemperature record for the Phanerozoic (past 542 Myr), and compare it with the variable
CRF reaching Earth and with the reconstructed partial pressure of atmospheric
CO2 (p2). We find that at least 66% of the variance in the paleotemperature
trend could be attributed to CRF variations likely due to solar system passages
through the spiral arms of the galaxy. Assuming that the entire residual variance
in temperature is due solely to the CO2 greenhouse effect, we propose a tentative
upper limit to the long-term “equilibrium” warming effect of CO2 , one which is
potentially lower than that based on general circulation models” [27 , p. 4].
According to contemporary supporters of global warming theory the cause
(blame) is the increasing amount of greenhouse gases and especially CO2 determined by the economic activity of man.
The main greenhouse gases in the Earth’s atmosphere are: water vapour,
carbon dioxide, methane, nitrous oxide and ozone. They keep heat near Earth’s
surface, and strongly influence the Earth’s temperature. Water vapour is the
most powerful greenhouse gas, but CO2 exists much longer in the atmosphere,
and therefore it is assumed that this gas is a major cause for about 80% of today’s
global warming [28 ].
The role of CO2 in the greenhouse effect has always been great, but it should
be borne in mind that long-term cycles of global climate change in the history
of Earth are associated with orbital factors. For example, the end of the Ice
Age of Pleistocene and early warming in the early Holocene (11.7 to 8 ka BP)
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are causally determined by changes in the Earth’s orbit, rather than increasing
of carbon dioxide (CO2 ) [9 ]. The modern cycle of global warming really is from
the early Holocene about 11 ka ago, rather than the industrial era around 1850.
In the history of Earth the rise in CO2 in the atmosphere usually follows a preceeding warming, determined by astronomical and orbital factors. It is known
that the oceans contain more carbon dioxide (CO2 ) than the atmosphere and
they absorb this gas from the atmosphere. When carbon dioxide from the oceans
starts moving back into the atmosphere, then it accelerates (stimulates) warming. Therefore, in warming caused by orbital factors, CO2 moves back into the
atmosphere, intensifies warming and becomes a primary factor in the increase of
temperatures.
Almost 40 years have passed since the announcement of the idea of global
warming [1 ] and today many scientists agree that human activity after the Industrial Revolution led to a significant increase in the level of greenhouse gases
in Earth’s atmosphere. A report from 2007 of the Intergovernmental Panel on
climate change (IPCC) [29 ] says: “Changes in the atmospheric concentrations of
GHGs and aerosols, land cover and solar radiation alter the energy balance of
the climate system and are drivers of climate change. . . ” and “There is very
high confidence that the global average net effect of human activities since 1750
has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W/m2
(Fig. 2.4)” (IPCC, 2007, 2.2 Drivers of climate change, p. 37). In Figure 2.4 authors of the IPCC report of 2007 noted the influence of the Sun on climate as the
weakest factor marked as enigmatic “level of scientific understanding (LOSU)”.
Beer et al. [4 ] noted that “a non-linear regression model to separate natural and
anthropogenic forcing since 1850 is consistent with a solar contribution of about
40% to the global warming during the last 140 years”.
It is unclear why the supporters of the idea of global warming ignore that
fluctuations in the climate system depend on many related factors, including the
fact that astronomical and orbital effects and galactic cosmic rays have general
deterministic importance. The influence of the Earth’s dynamics is put on them,
Plate-tectonic processes, which are related to the movements of lithospheric plates
and changes in location and configuration of the continents and oceans hot points
in the lithosphere, greenhouse gases, aerosols from volcanic eruptions, El Niño and
La Niña, which regularly occur in tropical areas of the Pacific, but actually shake
the whole Earth atmosphere.
Data from the geological record – a retrospective view or remembrance of things past6 . The post-Archaen history of Earth climates shows
cyclical fluctuations in certain limits of temperature of ground surface (12 to
25 ◦ C or even more extreme 7 to 27 ◦ C) [15 ].
6
2
A reminder of Crowley [30 ].
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
1467
Early in the history of our planet (Fig. 5), the main factors that influence
the climate start operating, among which the astronomical and orbital effects
should be mentioned. As for the other factors that affect the climate, it must
be emphasized that at the end of the Proterozoic, the role of greenhouse gases
began to be felt and the greenhouse effect is included as an important factor
in the climate system of the Earth. The impact of astronomical and orbital
factors determines the long-term changes of climate while greenhouse effect has
significant influence on the short-term intervals of climate changes.
The main factors exerting impact on climate have been triggered even in the
very early stages of our planet’s history. The assumption made by some scientists
that during the Archean and at the beginning of the Proterozoic solar activity
was about 20% lower than at present has not yet been supported by any data.
Towards the end of the Proterozoic, the role of greenhouse gases on climate
had become noticeable and the greenhouse effect had been incorporated as an
important factor in the Earth’s climate system. While astronomical and orbital
factors play a central role in long-term climate change, greenhouse gases play a
decisive role in shorter-term climate change.
Overview on global climate change in the geological history of Earth over the
past 1.5 billion years shows continuous cyclical variations in climate change with
extensive glaciations in the late Proterozoic and also in the Phanerozoic. There
was a particularly strong warming in the late Proterozoic, in Cambrian and in
the first half of Ordovician, in Devonian, Permian, in almost all Cretaceous and
in a significant part of the Paleogene (Paleocene and Eocene). In these warm
intervals, the sea level rose significantly and the highest known values for the
Cambrian-Ordovician interval are (almost 400 m above modern sea level) and in
the second half of the Cretaceous (about 250 m above the current level) [9 ].
Several large and long lasting glaciations are a characteristic of the Proterozoic (925, 800, 680 and 450 Ma BP) which give reason to a number of scientists
to develop a hypothesis of Snowball Earth. According to the supporters of this
hypothesis, glaciations stretched to the equatorial areas. Since 635 Ma BP a
climate warming started, which activated the biological evolution, stimulated diversification of evolutionary lineages and brought great biodiversity on the planet.
These events prepare the so-called Cambrian evolutionary explosion that marks
the beginning of Phanerozoic aeon.
Within the context of a dynamic terrestrial climate system, at least in the
last 542 million years, i.e. during the Phanerozoic, the average temperature of
the Earth’s surface has never been 8 ◦ C lower or 10 ◦ C higher than present day
values, i.e. temperature variations in the Phanerozoic were within the range of
+10 to +25 ◦ C [15 ].
During the Phanerozoic (the last 542 Ma), the dynamics of the climate system is more intense, the cycles of change are relatively short and the ages of
climatic optimum were more frequent. The climate optimums were more se1468
T. Nikolov, N. Petrov
vere in the Cretaceous and around the Paleocene-Eocene boundary. Globally
four stages of climate cooling were developed in the Phanerozoic: in the interval
Ordovician-Silurian, late Carboniferus-Permian, partly in the Jurassic, and sustainable cooling after the Cretaceous Period – in the Cenozoic. These intervals are
separated by periods of warm climates. Transitions between cycles cold-warmcold-warm climate created crises in the evolution of the organic world and caused
mass extinctions of whole species groups.
The general regularities in the climates of the Paleozoic Era show a number
of cyclical fluctuations with extreme conditions of global warming, and of global
cooling with significant glaciation. During the Early Paleozoic climates were
isothermal for the whole Earth while in the Late Paleozoic, especially in the
second half of the Carboniferous period and in the Permian, broad temperature
zones developed with tropical, subtropical and boreal areas. Therefore the early
Paleozoic as well as the Devonian climatic fluctuations were related to a significant
degree to variations in atmospheric humidity, with both humid and arid phases.
Climates in significant intervals of the Paleozoic Era were greatly influenced
by the high content of water vapour and carbon dioxide in the atmosphere. This
determines the prevalence of the greenhouse effect in the Paleozoic atmospheres,
which was significantly higher than in the Mesozoic and Cenozoic Era. This
condition lasted until the end of the Early Permian, after which the sustainable
arid climate prevailed in the Late Permian and throughout much of the Triassic
period.
Glaciations in the Paleozoic Era occur mainly in the southern continents,
which are integrated into Gondwana supercontinent, located in the South Pole
area.
Mesozoic climates were warmer and less contrasting in comparison to those
in the Palaeozoic. Actually the warm Mesozoic Era was included between two
phases of strong frosts – on the one hand, these are late Paleozonic glaciations and
on the other hand – in the Cenozoic Era. Between the Middle Triassic and mid
Cretaceous, climates were characterized by a significantly higher global average
temperatures (at least over 10–12 ◦ C) compared to today.
Climate in the Cenozoic Era marked the transition from warm to cold oceans.
In general, the Cenozoic has very different paleogeographic and paleoclimatic
characteristics compared to the Mesozoic Era.
During the Cenozoic there were significant climatic fluctuations that cover
different time intervals. These fluctuations were gradually enhanced and became
more frequent during the Pliocene and Pleistocene. Palaeogene and Neogene
correspond to two main climatic cycles, typical with their internal fluctuations in
each stage. Furthermore, Paleogene climates performed the transition between
the warm and practically iceless Cretaceous Period to the cold Neogene. The
continuous drift of continents changed the nature and configuration of the ocean
currents, which affected the climate.
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
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The whole Cenozoic paleoclimate evolution was characterized by a gradual
decrease in temperature very clearly expressed in high latitudes. The cooling is
not uniform; it is separated by shorter intervals of relative warming.
After cooling, which occurred in the earliest Paleocene (Danian), a tendency
towards gradual global warming for the rest of the Paleocene to mid-Eocene was
quite clear; there were some remarkable temperature maxima in the curve of this
trend.
The numerous cyclical variations were the most important feature of climate
in the Pliocene which were particularly strong in the interval between 5.2 and
3.3 Ma BP. Pliocene covered an interval during which the planet passed from the
relatively warm Miocene climates to global cold climates of the Pleistocene.
The last 5 million years are characterized by frequent ice (glacial) and interice
(interglacial) ages that show significant dynamics. The cycles lasting 40 ka and
100 ka, known as Milanković cycles, are most clearly manifested. Approximately
in the range 11–10 ka BP, a pretty steady trend towards climate warming started
and the Earth today is in an interglacial stage. At this stage economic human
activity is obviously included as an important accelerating factor that influences
climate.
The Quaternary is one of the most dramatic periods in the history of our
planet during which frequent and extreme weather fluctuations occurred, causing
unusual global changes in natural landscapes. In Quaternary times for about
2.5 million years several glacial episodes occurred on Earth, each one being expressed by formation of glaciers in the high and middle latitudes of the Northern
Hemisphere.
Perry and Hsu [31 ] published a remarkable analysis of geophysical, archaeological and historical data in support of “a solar-output model for climate change”
for the interval from 40 ka BP to today with forecast to 10 ka AD. Their approaches are graphically illustrated in Fig. 6.
After the last major late Pleistocene glaciations (11 000–10 000 years BP)
from the beginning of the Holocene a trend for global warming emerged which
was interrupted by short cooling episodes. According to different authors temperatures have increased by 4 to 5 ◦ C in the past 13 000 years and this marks the
release of Earth from the icy embrace of the Pleistocene. The last big glaciers in
the Northern Hemisphere disappeared in the period 9000–8000 years BP.
Already in the Holocene, in the period late boreal – Atlantic early subboreal
(8000–4000 years BP), the so-called Holocene maximum emerged, when the average surface temperatures were about 2 ◦ C higher compared with contemporary
ones [9 , Fig. 50].
According to some researchers between 900 and 1300 AD, Medieval Warm
Period (MWP) (or Medieval Climate Optimum) manifested, ending with moderate conditions in the 15th century. This MWP was followed by a period with
significantly colder climates called the Little Ice Age, which was expressed most
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T. Nikolov, N. Petrov
Fig. 6. Solar-output model from 14 000 years BP to 2000 years AP compared with sea-level
deviations [33 ] and selected events (after Perry and Hsu [31 ], Fig. 2; courtesy Ch. Perry). Indexes [31 ]: point D – Older Drias; point E – Younger Dryas; point A (about 9000 BP) –
start of sea-level rising. It is at this point in time that the solar-output model is date calibrated
(point A, Fig. 1). The “Global Chill” after Perry and Hsu centred near 8200 years BP (point H)
on the solar output model is reflected in a small dip in the otherwise steady rise in the Ters
sea-level curve. The point O (about 7600 BP) coincides with rapid warming and the flooding
of the Black Sea basin. Nearly 1300 years later between 6500 and 6000 years BP (point P)
later, there is evidence for two “Sahara Aridity” cold periods, one near 7000 years BP (point I)
and another at 5500 years BP (point J). The “4000 BP Event” that in fact prevailed from
4400 to 3800 BP (point K) [34 ] may have been the coldest period since the Younger Dryas cold
period. The next warm period ushered in the Bronze Age, which began about 3800 years BP
(point R); this probably was the most favourable climatic period of the Holocene and is also
referred to as the Holocene Maximum [34 ]. The “Centuries of Darkness” from 3250 to 2750 years
BP (point L) included the downfall of the great empires of the Bronze Age [34 ]. The Bronze
Age came to an end with the “Centuries of Darkness” chill, but warming returned during the
“Greco-Roman Age” from 2750 to 2060 years BP [34 ] (point S). Another little ice age occurred
during the period from 2060 to 1400 years BP [60 before Christ (B.C.) to anno Domini (A.D.)
600] (point M) called the “Migration of Nations” [34 ]. The next warm period was known as the
Medieval Optimum [31, 34 ] (point T), which was just beginning near 1400 years BP and lasted
until the Little Ice Age began about 700 years later. The most recent of the climatic cooling
periods was experienced during 720 to 140 years BP (A.D. 1280–1860) (point N) when the
climate worldwide was probably the coldest since the continental glaciers melted 10 000 years
ago and is referred to as the “Little Ice Age”. Currently, the Earth is enjoying the latest warm
period (point U), which has been underway for almost two centuries [31 ]
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
1471
strongly between 1450–1850, when average global temperature was about 1–1.5 ◦ C
lower than today. It is assumed that LIA is the coldest episode in the Holocene.
We share the idea that the last cycle of Global warming began in the early
Holocene – 10 ka BP and this is seen in [31 , Fig. 2]. This causes a gradual
increase in global sea level. Against this global trend observed small glacial cycles. Modern global warming started at the beginning of the Industrial era (circa
1850–1860 AD) under the strong impact of the industrial activities of mankind,
which contributes to an increase in GHE and especially CO2 in the atmosphere.
“However, geophysical, archaeological, and historical evidence is consistent with
warming and cooling periods during the Holocene as indicated by the solar-output
model” of Perry and Hsu [31 ].
The Earth’s climate has always experienced cyclic fluctuations. The causes
of these cycles are known – mainly astronomical and orbital effects on which the
influence of the overall economic activity of man strengthened in recent centuries
of modern era and especially carbon dioxide’s impact on climate. “The Intergovernmental Panel on Climate Change” focuses on the latter fact. It should be
emphasized that in the long history of the Earth, climate change occurred under
the influence of several factors; there was always a priority role of a major factor
but one factor has never been decisive for climate reality.
Negative impact of man on nature and in particularly on the Earth’s climate
system in modern times is real, comparable to the most powerful factors of the
geological history of the Earth. But the immediate impact on climate change
from the beginning of the Holocene and especially after the start of the Industrial
Revolution (1850) is not only the increase of greenhouse gases, and particularly
CO2 emissions but also the overall impact of man on the environment: massive
deforestation, expansion of desert areas and destruction of habitats; disturbance
in ecosystems and dangerously reducing of biodiversity; pollution of land, air,
ocean and round-Earth space; intensive exploitation of natural resources and
others. Therefore it is important to stress more strongly the responsibility of
man for balance of nature and for the balance nature-society.
The supporters of the idea of global warming on our planet are not few.
On the one hand, there is ample evidence for such a claim: the largest reports
for the amount of carbon emissions in the history of the planet; increase (even
a little) of the average temperatures and sea levels. On the other hand, from
geological perspective, we should enter the next era of global cooling. This could
be related to subsequent deep minimum of solar activity, which perhaps starts,
and also changes of the orbital parameters. But there is a third party, which
may cause unpredictable rapid global climate change on Earth: the eruption of
super volcanoes, strong change in Earth’s magnetic field, strike of the Earth with
a large asteroid or comet, strong radiation flow from the exploding stars close to
us and others.
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As the saying goes: Nothing is new under the Sun. Climate changes through
time. Today, with the aspiration to reduce the emissions of greenhouse gases by
80%, and CO2 in particular, trillions of dollars are spent in the world . . . while
just a small part has been allocated to education, education that will encompass
all strata of society, and above all politicians. At present the knowledge about
the Earth, its structure, intensive dynamics and complex and long history of
development is at a low level. The climate system represents a part of planet
Earth, determined to a great extent by its dynamics, and for this reason we have
to acquire knowledge about Earth as an integrated system, the planet as a whole,
in order to achieve a deeper understanding of climate dynamics. Without such
knowledge the joint efforts for reasonable actions against the trend towards global
warming will not achieve substantial progress.
In 2012, Nikolov [35 ] proposed the hypothesis of the influence of hot spots
in lithosphere on some meteorological phenomena and climatic variations. This
hypothesis is based on the fact that hot spots are a huge source of heat, coming
from the core-mantle zone, which obviously affects the thermal dynamics of the
oceans. Moreover, the spatial manifestation of ENSO (El Niño and La Niña),
NAO (Azores and Iceland), as well as some other smaller oscillations suggest the
idea of significant relationship and impact of the internal terrestrial dynamics
and hotspots on certain weather phenomena with sequences for climate change.
Obviously, there is a link between some hotspots with definite weather events –
for example, there is a striking coincidence of Icelandic atmospheric minimum
with Icelandic hotspot (Iceland hot spot), and Azores atmospheric maximum
with Azores hot spot. Little attention has been paid on this fact except the
remarkable article written by Mann et al. [32 ], but today we have to admit that
the hot spots in the equatorial Pacific must be among the factors stimulating the
events of El Niño. Obviously the dynamics of atmospheric processes and climate
variability for shorter intervals play a specific role in the so called Energetically
Active Zones of the oceans (EAZO) (f. e. Sargasso Sea). These energy active
zones in the ocean are a kind of analogue of the hot spots in the lithosphere
plates that are situated over updrafts of molten mantle material [33 ]. It is quite
likely that in some regions (Sargasso Sea) methane hydrates influence the thermal
dynamics of the ocean water [9 ].
Conclusion. The climate system of the Earth is very complex and is characterized by chaotic dynamics. It has been formed and is under the constant
influence of several key factors:
(1) variations in solar radiation caused by dynamic processes of the Sun;
(2) changes in the orbital parameters of the Earth in its movement around
the Sun;
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
1473
(3) changes in the intensity of galactic cosmic rays alter the Earth’s cloudiness;
(4) geophysical and geological (tectonic) processes that generate the internal
structure of the Earth, the structure and movement of lithospheric plates,
formation of mountain systems, the opening and closing of oceans and
formation of the main geomorphological features of the planet;
(5) human activity that has a strong growing impact since the early Holocene.
These factors can be divided into three groups: external (astronomical and
orbital), internal (earth – geophysical, geological and geographical) and anthropogenic. Climate changes are caused by the combined effect of these factors,
among which the orbital effects are of paramount importance.
The role of the Sun as the primary energy source for the Earth and driver
of global climate change is particularly important. But we also pointed many
controversial issues about the exact physical processes that cause climate change.
One major problem is connected with the clarification of the relationship between
solar variability and solar forcing, and also the insufficient reliable statistical data
for determining of more accurate physical models, leading to inability to predict
the long term weather events [2, 4–6 ].
We live in a dynamic climatic environment with very short time intervals
of changes from few tens of years to a few hundreds of years. This dynamics
is favourable for us but our prosperity cannot last forever. And we, ourselves,
are one of the most disturbing factors in this respect. The development of our
civilization is now a determining factor in the acceleration of global climate processes on Earth in one direction or another. Adaptation to climate change is
necessary as well as reconstruction of the contemporary global picture with view
to balanced (sustainable) development as coevolution of nature and society. We
cannot stop climate change, but we can reduce the negative human impact on
the environment.
Acknowledgements. The authors thank G. Miloshev and P. Velinov for
their thoughtful and stimulating reviews. Special thanks to I. Zagorchev for
his comments and valuable suggestions to the manuscript. A great thanks to
Charles Perry for his permission to use as illustration a figure from his article
with K. Hsu [31 ].
REFERENCES
[1 ] Broecker W. S. Science, 189, 1975, 460–463.
[2 ] De Boer P. L., D. G. Smith. Spec. Publs Int. Ass. Sediment, 19, 1994, 1–14.
[3 ] Nikolov T. J. of BAS, 112, 1999, No 2, 11–24.
1474
T. Nikolov, N. Petrov
[4 ] Beer J., W. Mende, R. Stellmacher. Quaternary Science Reviews, 19, 2000,
403–415.
[5 ] Bard E., M. Frank. Earth and Planetary Science Letter, 248, 2006, 1–14.
[6 ] Gray L. J., J. Beer, J. D. Haigh et al. Rev. Geophys., 48, 2010, 1–53.
[7 ] Thaddeus P., G. A. Chanan. Nature, 314, 1985, 73–75.
[8 ] Bahcall J. N., S. Bahcall. Nature, 316, 1985, 706–708.
[9 ] Nikolov T. Global Changes of Climates in Earth’s History. Prof. Marin Drinov
Academic Publishing House, Sofia, 2011 (in Bulgarian with English summary).
[10 ] Monin A. S., J. A. Shishkov. Climate History. Gidrometeoizdat, Leningrad, 1979,
407 pp (in Russian).
[11 ] Kopp G., J. L. Lean. Geophysical Research Letters, 38, 2011, No 1, ??–??.
[12 ] Schwarzschield M. Structure and evolution of the stars. Princeton Univ. Press,
Princeton, 1958, 296 pp.
[13 ] Ruddiman W. F. Earth’s Climate. Past and Future. Freeman and Co., N.Y., 2001,
465 pp.
[14 ] Aller L. H. Atoms, Stars and Nebule. Ed. Cambr. (Mass.). Harvard Univ. Press,
1971, 351 pp.
[15 ] Scotese C. R. Paleomap Project – Climate History, 2008, http://www.scotese.
com/climate.htm.
[16 ] Imbrie J., K. P. Imbrie. Ice Ages: Solving the Mystery. Harvard Univ. Press,
1986, 224 pp.
[17 ] Mustel’ E. R. Sun and Earth’s Athmosphere. GITTL, Moscow, 1957, 103 pp (in
Russian).
[18 ] Hale G., F. Ellerman, S. Nicholson, A. H. Joy. Astrophysical J., 49, 1919,
153.
[19 ] Fritts H. C. Tree Rings and Climate. Academic Press, London, 1976, 576 pp.
[20 ] Usoskin I. G. Living Rev. Solar Phys., 10, 2013, 1–94.
[21 ] Budyko M. I. Climate in paste and future. Gidrometeoizdat, Leningrad, 1980,
350 pp (in Russian).
[22 ] Berger A. L. Rev. Geophys., 26, 1988, 624–657.
[23 ] Berger A. L. Astronomy and Astrophysics, 51, 1976, No 1, 127–135.
[24 ] Schwartzacher W., A. G. Fischer. In: Cyclic and Event Stratification (eds
G. Einsele, A. Seilacher). Springer Verlag, Berlin, 1982, 72–95.
[25 ] Milankovič M. Canon of Insolation and Ice Age Problem. Zavod za udžbenike i
nastavna sredstva, Belgrade (first published in German by Serbian Royal Academy,
1941), 1998.
[26 ] Svensmark H. Astronomy & Geophysics, 48, 2007, No 1, 1.18–1.24.
[27 ] Shaviv N. J., J. Veizer. GSA Today, July 2003, 4–10.
[28 ] Kump L. R. Nature, 419, 2002, 188–190.
[29 ] Metz B., O. R. Davidson, P. R. Bosch, R. Dave, L. A. Meyer (eds). The
Intergovernmental Panel on Climate Change. Climate Change 2007. Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment
Report of the IPCC. Cambridge University Press, 2007.
[30 ] Crowley T. J. Consecances, 2, 1996, No 1.
[31 ] Perry Ch. A., K. J. Hsu. PNAS, 97, 2000, No 23, 12433–12438.
[32 ] Mann M. E., M. A. Cane, S. E. Zebiak, A. Clement. American Meteorological
Society, 18, 2004, 447–456.
Compt. rend. Acad. bulg. Sci., 67, No 11, 2014
1475
[33 ] Ters M. Climate History, Periodicity, and Predictability (eds M. R. Rampino, J. E.
Sanders, W. S. Newman, L. K. Konigsson). Van Nostrand Reinhold, New York, 1987,
204–336.
[34 ] Hsu K. J. Climate and Peoples: A Theory of History. Orell Fussli, Zurich, 2000.
[35 ] Nikolov T. Comp. rend. Acad. bulg. Sci., 65, 2012, No 6, 839–846, http:
//newserver.stil.bas.bg/proceedings/PDF1/C_06-16s.pdf.
National Institute of Geophysics
Geodesy and Geography
Bulgarian Academy of Sciences
1113 Sofia, Bulgaria
e-mail: [email protected]
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∗
Institute of Astronomy and NAO
Bulgarian Academy of Sciences
72, Tsarigradsko Shosse Blvd
1784 Sofia, Bulgaria
e-mail: [email protected]
T. Nikolov, N. Petrov