MAIN FACTORS INFLUENCING CLIMATE CHANGE: A REVIEW PDF

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Доклади на Българската академия на науките Comptes rendus de l’Acad´ emie 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 E...

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MAIN FACTORS INFLUENCING CLIMATE CHANGE: A REVIEW T. Nikolov, Todor Nikolov

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Доклади на Българската академия на науките Comptes rendus de l’Acad´emie 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 1455

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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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%. 1458

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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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Fig. 2. Solar disk images from SOHO (NASA/ESA) spacecraft, operated by Extreme Ultraviolet Imaging Telescope (in the light of 304 and 171˚ A 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...