FACULTY OF MATHEMATICS AND PHYSICS

DEPARTMENT OF PHYSICS

THE DAY AFTER TOMORROW

The Physical Basics of Greenhouse Effect and Climate Change

Aleks Žagar Advisor: prof. dr. Jože Rakovec

Ljubljana, 12th March 2008

Abstract

Global climate change has become one of the most contested science-based public policy issues in recent years, but the debate suffers from lack of scientific breath and data. In this seminar some basic aspects of grenhouse effect on energy balance in Earth system and the influence of various factors on Earth`s climate change are presented. In order to show, how greenhouse gases and other changes affect Earth`s climate, IPCC (Intergovernmental Panel on Climate Change) conclusions are described. In the end data that supports this theory and results of computer models is provided.

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Table Of Contents 1. Introduction.......................................................................................................3 2. Physical background..........................................................................................4 2.1. Radiation..................................................................................................................5 2.2. Energy balance........................................................................................................6 2.2.1. Thermal equilibrium for the Earth (without the atmosphere).............................6 2.2.2. Thermal equilibrium for the Earth (with the atmosphere – greenhouse model).......7 3. Greenhouse gases..............................................................................................8 3.1. Molecular responses to radiation..............................................................................9 3.2. CO2 response..........................................................................................................10 3.3. H20 response...........................................................................................................10 3.4. Absorption bands....................................................................................................11 3.5. Anthropogenic greenhouse effect...........................................................................12 3.5.1. CO2..................................................................................................................................12 3.5.2. CH4..................................................................................................................................13 3.5.3. N2O..................................................................................................................................13 3.5.4. CFC.................................................................................................................................13 4. Solar variations................................................................................................14 5. Orbital variations (Milakovich cycles)............................................................16 5.2. Precession...............................................................................................................16 5.1. Eccentricity.............................................................................................................16 5.3. Obliquity.................................................................................................................17 6. Plate tectonics theory.......................................................................................18 6.1. Configuration of the continents..........................................................................................18 6.2. Elevation of the continents.................................................................................................18 7. Reconstruction of climate changes..................................................................19 8. Current data and future predictions.................................................................20 9. Conclusion.......................................................................................................21 References...........................................................................................................21 Literature.............................................................................................................21

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1. Introduction Changes in Earth`s climate started long before the "Industrial Revolution" and the invention of the internal combustion engine. Earth's climate varied between conditions that support largescale continental glaciation and extremly tropical conditions with lack of permanent ice caps at the poles.

Figure 1: The graph shows estimated temperature (blue curve), CO2 (green curve) and windblown glacial dust (red curve) from Vostok, Antarctica ice core as reported by Petit et al., 1999. The graph reads backward in time, with today being on the far left. Zero on the y axis is relative to today's average temperature. Higher dust levels are believed to be caused by cold, dry periods. Note the short warm periods between long periods of highly variable and cold climate, with the length of glacial cycles averages 100,000 years. Also, note how comparatively stable the past 10,000 years have been as compared to the past 400,000 years. [1] Separating whether climate is changing from what causes climate to change is a first step in restoring scientific integrity to the debate on climate change and global warming in the past century. Climate is changing all of the time, but unlike weather, its rate of change cannot be quantified over short time. Climate does not only influence development of life on Earth, but also influences the development of some of Earth`s mineral deposits and energy resources. Understanding and explaining the past and present climate evolution and forecasting the future climatic changes in order to establish efficient global policies on environmental protection and aviod the future possible disasters are strong stimuli for the development of new advanced theories of evolution of Earth`s climate. Computer models are used and confirm the measured data of the past century and predict further raise of temperatures. [2] In short conclusion Earth`s climate is determined by the mass and composition of the atmosphere and energy brought to, contained in, and transfered from the atmosphere. Geologically short term (less than 120,000 year [3]) temperatures are believed to be driven by orbital factors. The arrangements of land masses on the Earth's surface are believed to reinforce these orbital forcing effects. Continental drift obviously affects the thermohaline circulation, which transfers heat between the equatorial regions and the poles and also affects the extent of polar ice coverage. The presence of snow and ice is also a well understood positive feedback mechanism for climate. Among many different climate driving and forcing factors energy output of the sun and greenhouse gases is believed to have the greatest influence on Earth`s climate and life on Earth. Basic physical concepts of some factors and the effects will be presented further on in the seminar.

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2. Physical background Weather is the day-to-day state of the atmosphere, and is a chaotic non-linear dynamical system. On the other hand, climate, the average state of weather, is fairly stable and should be predictable. Climate includes the average temperature, amount of precipitation, days of sunlight, and other variables that might be measured at any given site. However, there are also changes within the Earth's environment that can affect the climate. Even small changes could be reinforced or have multiple feedback mechanisms. The average temperature on Earth today exceeds the temperature of the previous century for more than 0.6 °C. Due to global warming sea leves has rise from 15 to 20 cm in the same period. Polar ice is melting. Concentrations of atmospheric CO2, one of the greenhouse gases responsible for scalding temperatures on Venus and at least 33 °C of normal warming here on Earth (caused mostly by water vapour), are on the rise and the higest in the past 750000 years. [2] The greenhouse effect is the process in which the emission of infrared radiation by the atmosphere warms the Earth's surface. The name comes from an incorrect analogy with the warming of air inside a greenhouse compared to the air outside the greenhouse. The greenhouse effect was discovered by Joseph Fourier in 1824. [2] It's estimated that the Earth's surface would be about 255 K (-18 °C) with atmosphere and clouds but without the greenhouse effect and that the "natural" greenhouse effect raises the Earth's temperature by aproximately 33 °C. A schematic figure of Earth`s energy balance and greenhouse effect is shown below in figure 2.

Figure 2: Details of Earth's energy balance. Numbers are in watts per square meter of Earth's surface, and some may be uncertain by as much as 20%. The greenhouse effect is associated with the absorption and reradiation of energy by atmospheric greenhouse gases and particles, resulting in a downward flux of infrared radiation from the atmosphere to the surface (back radiation) and therefore in a higher surface temperature. Note that the total rate at which energy leaves Earth (107 W/m2 of reflected sunlight plus 235 W/m2 of infrared [long-wave] radiation) is equal to the 342 W/m2 of incident sunlight. Thus Earth is in approximate energy balance in this analysis. [4]

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Energy incoming in the atmosphere causes temperature fluctuations on small scales. Temperature changes depend on the quantity of absorbed and emited heat, thus such changes can be defined if energy flux, absorbtion, density etc. are known. These changes can be caused by dynamic process on Earth, external forcings including variations in sunlight intensity, and more recently by human activities. Mainly causes for temperature changes in the atmosphere are divided to:

- solar radiation (IR, visible and UV) - surface radiation (longwave IR radiation of Earth`s surface) - conduction - energy transfers - turbulence and convection - phase changes of water (latent heat transport) - expansion and compression of air masses

Still, the main and most efficient process in heating up the atmosphere is solar radiation, other processes basicly just redistribute and transfer energy from one place to another. 2.1. Radiation Earth`s climate is fundamentally controled by the way solar radiation interacts with Earth`s surface and atmosphere. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. Thus by Wien`s law λmaxT = 2.897 ×10–3 m K, (1)

the peak and about half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum as seen in figure 3 below. More than 90% of total energy lies between 0.25 µm and 2.5 µm. [5] The amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays is called solar constant: j0 = 1367 W/m2. [6] (2)

The solar constant fluctuates by maximum couple of percent on daily and monthly basis and by 0.1% in 11 years cycle, due to the earth's varying distance from the sun and changes in the sun`s magnetic field (Figure 11). Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.74 ×1017 W, plus or minus ~ 2 %. [6] The Earth receives a total amount of radiation determined by its cross section, πr2, but as the planet rotates this energy is distributed across the entire surface area, 4πr2. Hence, the average incoming solar radiation (solar irradiance), taking into account the half of the planet not receiving any solar radiation at all, is one fourth the solar constant or ~ 342 W/m². At any given location and time, the amount received at the surface depends on the state of the atmosphere (density and composition of different gases) and the latitude. [5] Earth reflects about 30% (mostly due to clouds, snow and ice, with albedo α ~ 0.5) of the solar irradiance and absorbs the remaining 70%, warming the land, atmosphere and oceans and reradiating it in infrared. The decrease in solar radiation and absorbtion at the atmosphere is shown in figure 4 and 8. The visible spectrum of solar radiation mostly heats the surface, whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not the surface. The infrared photons

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emitted by the surface are mostly absorbed in the atmosphere by greenhouse gases and clouds and do not escape directly to space. Earth emits longer infrared waves, it`s surface and atmosphere are heated between 210 K and 310 K, therefore from equation (1) the electromagnetic spectrum of terrestrial radiation is between 0.75 µm and 24 µm, whereas the peak of solar radiation is between 0.4 µm and 0.75 µm as seen in Figure 3. Figure 4 (at right) compares the emission from the Earth with the amount of solar energy that reaches the Earth.

Figure 3 (left) and 4 (right): Comparison of the emission spectra of the sun and the Earth. Notice the difference in the amount of energy emitted by the sun and the Earth (left). The peak of the sun's energy emission is in the wavelengths of visible light, although it emits other types of energy (like ultraviolet and infrared) also. The Figure 4 at right compares the emission from the Earth with the amount of solar energy that reaches the Earth. The amount of energy from the Sun that actually reaches the Earth's surface is smaller realy, and has some very definite spectral features due to absorption and scattering that occur at specific wavelengths in the atmosphere. In particular, most of the UV radiation is absorbed in the ozone layer. Notice that the emission from the Earth is larger than the energy received from the sun, for wavelengths longer than about 5µm. This slightly fuzzy cross-over point is used to define two principal wavelength ranges, shortwave (SW) radiation and longwave (LW) radiation. [7] 2.2. Energy balance Knowing how the radiation energy is transported some basic calculations can be made. Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. Thus, energy balance can be defined if Earth`s surface, atmosphere and oceans are taken for collective system. The planet is approximately in equilibrium, long-term speaking. 2.2.1. Thermal equilibrium for the Earth (without the atmosphere) In a simple global energy balance model, the only variable is the emission temperature of the Earth. Considering long-term equilibrium and neglecting some basic facts (like atmopshere effects and energy accumulation), effective temperature of the Earth can be calculated.

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The sun behaves approximately like a black body, with radius rs, at a temperature of Ts=5800 K. The radiative flux (radiation per unit area) at the sun's surface is therefore given by Stefan`s law: 4

Tj σ= , (3)

where σ is the Stefan-Boltzmann Constant, σ = 5.6704 x10-8 Wm2K4. Thus, at the Earth's distance from the sun, res, this flux is reduced by the factor (rs/res)

2. The intercepted radiation warms up the Earth to Te and solar radiation absorbed by the Earth, with radius re, is balanced with terrestrial flux radiated to space:

( ) 024

2

2242 )1(14 jrT

r

rrTr eS

es

e

eee πασπασπε −=−= , (4)

4 0

4

)1(

σε

α jTe

−= . (5)

This leads to a solution Te = 255 K (-18 °C) if assuming α = 0.3, ε = 1. Te is about 33 K cooler than the globaly averaged observed temperature, which is about 288 K. [2] Notice that the effective temperature depends only on albedo and the distance from the sun. 2.2.2. Thermal equilibrium for the Earth (with the atmosphere – greenhouse model) Atmosphere is rather opaque to infrared, so terrestrial radiation cannot be tought as being radiated into space directly from the surface. Much of the radiation from the surface si absorbed by H2O and CO2 before passing through the atmosphere. On average the absorbtion occurs in lower parts of the atmosphere and emission to space will emanate from some level in the atmosphere (typically around 5 km), so the region above that level is mostly transparent to infrared. This region of the atmosphere, rather than the surface, must be at the emission temperature. This radiation from the atmosphere is basicly directed upward and downward, resulting in the counterradiation warming the surface, besides solar irradiation. The Earth`s surface, due to greenhouse effect, is therfore warmer than Te, calculated if the atmosphere were not present. Let`s take a look at descirbed 3 layer model: surface (Tsurf) – atmosphere (Te) – space. [8] Since the atmosphere is thin let`s simplfy things by considering a planar geometry. The atmosphere is represented by a single layer of temperature, Te. Another important assumption is, that the atmosphere is completely transparent to shortwave solar radiation. On the other hand, the atmosphere has absorptivity, ε, so a fraction of upwelling infrared radiation from the surface is absorbed within the atmosphere. In totall, the upper layers of the atmosphere and space must be in equlibrium:

440 )1(4

)1(esurf TT

jσσε

α+−=

−. (6)

Net flux at surface in equlibrium must also be zero:

440

4

)1(surfe TT

jσσ

α=+

−. (7)

Combining both equations (6) and (7), Tsurf is obtained:

esurf TT4/1)/2( ε= . (8)

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In the limit, if →ε ∞, atmosphere acts as transparent and the temperatures are Tsurf = Te, which is similar to previous calculated case. In the limit, if →ε 1, atmosphere acts as completely opaque and absorbs all infrared radiation. Applying atmosphere to calculations of the energy balance model, assuming ε = 1, therefore increases surface temperature by factor 1.19. If in equation (5) ε = 0.7 is put instead of 1, surface temperature Tsurf = 279 K (6 °C) is obtained, which by equation (8) gives Te = 215 K (-58 °C) for the atmosphere (Te = 234 K (-39 °C), if assumed the atmosphere has radiative flux 4

eTεσ ). This is closer to the actual mean

surface temperature of 288 K, thus the greenhouse model is a satisfactory aproximation. This analyse shows that the atmosphere partially transmits terrestrial radiation. The actual energy flow balance is more complex, as the atmosphere itself. Theoretically, if the planet's surface were cooled by radiation alone, then the greenhouse induced surface temperature would be much warmer, about 350 K (77 °C). [5] Atmospheric motion, convective towers carrying latent and sensible heat upwards and large scale circulation carrying it both upwards and polewards, circumvent much of the greenhouse effect and significantly increase the escape of energy to space, leaving Earth's surface cooler than a static atmosphere would do.

3. Greenhouse gases Atmosphere contains roughly 78% N2, 21% O2, 0.93% Ar, 0.038% CO2, trace amounts of other gases and a variable amount of water vapor. Tabel 1 shows the dry atmospheric composition by volume (ppm - parts per million by volume).

gas formula volume [ppm (%)]

nitrogen N2 780840 ppm (78 %) oxygen O2 209460 ppm (20.946 %) argon Ar 9340 ppm (0.9340 %) carbon dioxide CO2 383 ppm (0.0383 %) neon Ne 18.18 ppm (0.001818 %) helium He 5.24 ppm (0.000524 %) methan CH4 1.74 ppm (0.000174 %) hydrogen H2 0.55 ppm (0.000055 %) nitrous oxide N2O 0.30 ppm (0.00003%) ozon O3 0.07 ppm (0.000007 %) water vapor H2O varies ~ 1% to 4%

Table 1: Tabel 1 shows the average dry atmospheric composition to the level 30 km above ground, where 99 % of the mass is gathered. [9] Minor components of air not listed. For comparison water vapor is added below. Note that the composition figures above are by volume-fraction (parts per million by volume), which for ideal gases is equal to mole-fraction. (IPCC) [10], [11] Water vapor, CO2, CH4, N2O, O3 and a few other gases (CFC) are greenhouse gases. They all are molecules composed of more than two component atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The major components of the atmosphere are two-atom molecules too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect. The main reason infact is the simmetry.

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3.1. Molecular responses to radiation Atoms and molecules can absorb electromagnetic radiation, but only at certain energies (wavelengths). The diagram in figure 5 illustrates the relationships between different energy levels within a molecule. Certain energies in the visible and UV regions of the spectrum can cause electrons to be excited into higher energy orbitals. Very energetic photons (UV to X-ray region of the spectrum) may cause ionization and dissociation of molecules. Photons in the infrared region of the spectrum have much less energy than photons in the visible or UV regions of the electromagnetic spectrum. They can excite vibrations in molecules. There are many possible vibrational levels within each electronic state. The higher energy near-infrared (0.8 – 1.4 µm) radioation can excite overtone or harmonic vibrations, which explain the absorbtion bands in higher wavelenghts of CO2 and H2O spectrum in figure 8. The mid-infrared (1.4 - 30 µm) radiation may excite the fundamental vibrations and associated rotational-vibrational transitions. Microwave radiation and the far-infrared (1000–30 µm) is even less energetic. It cannot excite electrons in molecules, nor can it excite vibrations. It can only cause molecules to rotate.

Figure 5: The diagram illustrates absorbtion of radiation by molecules. The three groups of lines correspond to different electronic configurations. The lowest energy, most stable electron configuration, is the ground state electron configuration, where excited vibrations in molecules occur. Certain energies in the visible and UV regions of the spectrum can cause electrons to be excited into higher energy orbitals. Some of the possible absorption transitions are indicated by the vertical arrows. Transitions between the vibrational levels are indicated by the vertical arrows on the left side of the diagram. [12] Eventually, the vibrating molecule will emit the radiation, with lower energy, again, and it will likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface. The number of vibrational modes in a molecule is 3N-5 for linear molecules and 3N-6 for nonlinear molecules, where N is the number of atoms. The vibrational frequency depends on the stiffness of the chemical bond and on the mass of the atoms. Stiff double bonds, i.e. in CO2, vibrate faster than loose single bonds and heavy atoms will vibrate slower than light atoms. The energy of a vibrating molecule En is given by a vibrational quantum number n, Planck's constant h, and the vibration frequency υ as

En= (n+1/2)hυ. (8)

hυ is the fundamental quantum of energy and equals the energy spacing between each energy level. At mean surface temperature, calculated in equation 8, most of the molecules will be in their lowest stable vibrational energy state, E0. Absorption of light with the appropriate

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energy, which equals the energy gap between two energy levels, hυ = E1-E0, allows the molecule to become excited into the next higher vibrational energy state E1. Such absorption of infrared light can only occur, if the dipole moment of the molecule is different in the two vibrational states. Vibrational transitions that obey this spectroscopic selection rule are said to be infrared active and will intensely absorb. For example O2 and N2 are symetric diatomic molecules and do not have strongly defined dipole moment, therefore they do not absorb in IR, eventhough the rotational and vibrational transitions are possible. Further more, in a symmetric polyatomic molecule like carbon dioxide only the asymmetric vibrations that create a dipole will be infrared-active. 3.2. CO2 response Carbon dioxide is a linear molecule, therfore has 3 x 3 - 5 = 4 vibrations. These vibrational modes, shown in figure 6, are responsible for the greenhouse effect in which heat radiated from the earth is absorbed by CO2 molecules in the atmosphere. The asymmetric stretch is infrared active because there is a change in the molecular dipole moment during this vibration. Infrared radiation at 4.26 µm excites this particular vibration. The symmetric stretch is not infrared active, and so this vibration is not observed in the infrared spectrum of CO2. The two equal-energy bending vibrations in CO2 are identical but perpendicular to each other. Infrared radiation of 15 µm excites these vibrations. 3.3. H20 response Water vapor is the main absorber of the sunlight, it is responsible for about 70% of all atmospheric absorption. It contributes significantly to the greenhouse effect, but also operates a negative feedback effect, due to cloud formation reflecting the sunlight and attenuate global warming. Worth noticing is the relatively short residence time for water in the atmosphere (around 10 days), compared to the residence time for perturbations to CO2 (decades to centuries) or CH4 (a decade). [13] The water molecule has a large permanent dipole and is asymetric, so all three normal modes would be infrared-active. In the gas state, the vibrations involve combinations of symmetric stretch (v1), asymmetric stretch (v3) and bending (v2) of the covalent bonds, shown in figure 7. It has a very small moment of inertia on rotation which gives rise to rich combined vibrational-rotational spectral bands in the vapor containing tens of thousands to millions of absorption lines, mostly for λ > 12 µm. Figure 6 (left) and 7 (right): Vibrational modes of CO2 and H2O. The arrows indicate the directions of motion. Strech vibrations of CO2 represent the stretching of the chemical bonds, one in a symmetric fashion, in which both bonds lengthen and contract in-phase, and the other in an asymmetric fashion, in which one bond shortens while the other lengthens. Similar is valid for H2O modes. [12], [14]

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3.4. Absorption bands Figure 8 gives the amount of energy absorbed by greenhouse gases in various wavelength regions, from ultraviolet radiation on the left, to visible light in the middle, to infrared radiation on the right. The CFCs are not plotted here but will be mentioned separately. For each gas is given a plot of the absorptance of the gas, ranging from 0 to 1, for each wavelength.

Figure 8: Plot shows the degree of absorbtion of light by the atmosphere across the spectrum. At the top black body emission solar and terrestrial spectrum is shown. In the middle the fraction of absorbed radiation in the atmosphere in total and absorptivity of various gases and due to Rayleigh scattering at the bottom. Note that the scale is schematic and not normalised. [15] Notice high absorptivity in the plot of oxygen and ozone in the ultraviolet region but essentially zero in the visible and infrared regions, except for isolated peaks. Other gases have much different absorption properties. Methane, for example, has a couple of very small wavelength regions in which it absorbs strongly and these occur at about 3.5 µm and 8 µm, which are in the infrared region. Nitrous oxide having peaks at about 5 µm and 8 µm, absorbs in fairly narrow wavelength ranges. CO2 has a more complex absorption spectrum with isolated peaks at about 2.6 µm and 4 µm and a strong absorption of infrared radiation at about

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15 µm, due to bending vibrations. As seen carbon dioxide is a strong absorber of infrared radiation. The plot for water vapor shows an even more complex absorption spectrum, with numerous broad peaks in the infrared region between 0.8 and 10 µm, due to mostly vibrations. At about 12 µm rotational transitions start to absorb most of the spectrum. Notice that CO2 peaks and H2O pekas overlap in regions around 2 µm, 3 µm and partially around 15 µm. These regions are mostly saturated by H2O absorbtion, therefore increasing concentration of CO2 manly affects absorbtion around 4 and 15 µm. Discussion of the relative importance of different greenhouse gases is confused by the overlap between the spectral lines due to different gases, widened by pressure broadening. As a result, the absorption due to one gas cannot be thought of as independent of the presence of other gases. One convenient approach is to remove the chosen constituent, leaving all other absorbers and the temperatures untouched and monitoring the infrared radiation escaping to space as it has been done in GISS-GCM Model simulation. [13] Results are given in table 2.

removed gas reduction in GH [%]

H2O 36 CO2 9 O3 3 H2O + clouds 66

Table 2: The table shows the instantaneous change in longwave absorption when each component or combination of components is removed using the radiation code from the GISS GCM. Because of the overlaps (i.e. mentioned overlaps of H2O and CO2), the combined changes are larger than the changes due to each individual component. The overlaps complicate things, but it's clear that water vapour is the single most important absorber (between 36% and 66% of the greenhouse effect), and together with clouds makes up between 66% and 85%. CO2 alone makes up between 9 and 26%, while the O3 and the other minor GHG absorbers consist of up to 7 and 8% of the effect. [13] The total spectrum of all atmospheric gases is given in the middle of the plot. This shows a series of gaps between 0.3 and 0.8 µm, which allows solar radiation, without UV to reach the Earth's surface. Terrestrial infrared radiation has a maximum near 10 µm in so called atmospheric window between 8 µm and 13 µm, where less greenhouse gases are active (almost none except for an oxygen spike). As seen, changing the amount of a gas like H2O, may not increase the absorbtion of the atmosphere much, because nearlly all the radiation in these bands is being absorbed anyway. However increasing the low concentracion of gases absorbing in the atmospheric window, such as freons (CFC) and ozon (O3), would increase the absorbed amount of radiation and caused warming.

3.5. Anthropogenic greenhouse effect Human activities contribute to climate change by causing changes in Earth`s albedo and Earth`s atmosphere in the amounts of greenhause gases, aerosols and cloudiness. 3.5.1. CO2

As a result of human activities the level of CO2 in the atmosphere has increased by 30 % in the course of the last two centuries. Measurements of CO2 amounts from Mauna Loa observatory show that CO2 has increased from about 313 ppm in 1960 to about 375 ppm in 2005. This is considerably higher than at any time during the last 750000 years, the period for which data has been extracted from ice cores. In Europe, it is estimated that between 70 % and 90 % of CO2 emissions result from the combustion of fossil fuels and about 10 % are

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linked to agriculture and deforestation. Future CO2 levels are expected to rise due to ongoing burning of fossil fuels, with the rate of the rise depending on uncertain economic, sociological, technological and natural developments. The IPCC report gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100, as seen further on. [2] 3.5.2. CH4

CH4 is present in the atmosphere in very low quantities. Until 1850 its concentration was 0,8 ppm, since the beginning of the industrial age, its concentration in the atmosphere has not stopped increasing, 1 ppm in 1900, 1,1 ppm in 1950 and 1,8 ppm in 2007. Agricultural activities are the cause of 57 % of the discharges of CH4 into the atmosphere. Nevertheless, CH4 is easily oxidised and its residence time is about a decade. [2] 3.5.3. N2O

Agricultural activities, particularly the use of nitrogen fertilisers, are also the cause of a rise in the concentration of N2O. N2O is present naturally in the atmosphere in minure quantities, but its concentration has increased by 15 % since the start of the industrial age and its residence time in the atmosphere is relatively long (120 years). [2] 3.5.4. CFC

The CFC are synthesised gases, which have been banned since 1990. They were used in aerosols (shaving creams, deodorants, etc.) and as refrigerating and propellant gas. The increase in their concentration during the last century was 4 %. Given that their power of infrared radiation absorption is very high, due to absorbing in atmosphee window, the CFC’s contribution is significant. [2]

The addition of this anthropogenic greenhouse effect to the natural greenhouse effect has direct consequences for the Earth’s temperature and climate. Graphs in figure 9 illustrate the described increase of anthropogenic greenhouse gases in past years.

Figure 9: shows global trends in mentioned anthropogenic greenhouse gases for the past 20 years. The natural greenhouse gases account about 97 % of the direct climate forcing by 1750. Note the anual cycle in CO2 and CH4, due to seasional changes in temperature, vegetation, etc., refered as carbon and methane cycle. [12 (2005)]

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Figure 10 illustrates relative fraction of anthropogenic greenhouse gases by sectors.

Figure 10: This figure shows the relative fraction of anthropogenic greenhouse gases coming from different categories of sources (Emission Database for Global Atmospheric Research). The top panel shows the sum over all man made greenhouse gases, weighted by their global warming potential over the next 100 years. This consists of 72% CO2, 18% CH4, 9% N2O and 1% other gases. Lower panels show the comparable information for each of these three primary greenhouse gases, with the same coloring of sectors as used in the top chart. Segments with less than 1% fraction are not labeled. [15] 4. Solar variations Solar variations are changes in the solar irradience. It is observed to vary in phase with the solar cycle, with yearly averages going from 1365.5 W/m2 at solar minimum, up to 1366.6 W/m2 at solar maximum. The min-to-max average variation (black line), at the 0.1% level, is too small to affect Earth's climate directly, but the solar constant fluctuates by ~ 1 % during a year, as seen in figure 11. Figure 11: The extended PMOD composite of the solar irradiance as daily values, plotted in different colors for the different originating experiments (radiometers on different space platforms since November 1975: HF on Nimbus7, ACRIM I on SMM, ACRIM II on UARS, VIRGO on SOHO, and ACRIM III on ACRIM-Sat.). The differences between the minima values is also indicated, together with amplitudes of the three cycles. [16]

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When solar activity is high, the solar wind deflects the cosmic rays that produce the cosmogenic isotopes 14C and 10Be. [12] Therefore, high solar activity result in decreased amounts of cosmogenic isotopes reaching the Earth's surface. Historical variations in solar activity can be reconstructed from 14C and 10Be isotopes found in tree rings and ice cores respectively. Solar variability can also be estimated through records of sunspot counts. There are still uncertainties and dissagrement in debate of solar variability having affect on climate in the last few decades. Modeling studies reported in the IPCC Third Assessment Report (TAR) found that volcanic and solar forcing may account for half of the temperature variations prior to 1950, but the net effect of such natural forcing has been roughly neutral since then. The IPCC Fourth Assessment Report (AR4) gives a best estimate for radioactive forcing from changes in solar activity of 0.12 W/m2. This is less than half of the estimate given in the TAR, thus the combined effects of all human activity are estimated to be an order of magnitude greater. Variations in solar output are too small to have contributed appreciably to global warming since 1970 and that there is no evidence of a net increase during this period. [2] Some researchers (i.e. Stott et al. 2003) believe that the effect of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of recent greenhouse warming. Others (i.e. Marsh and Svensmark 2000) have proposed that feedback from clouds or other processes enhance the direct effect of solar variation, which, if true, would also suggest that the effect of solar variability was being underestimated. The present level of solar activity is historically high. Solanki et al. in 2004 [17] suggest that solar activity for the last 60 to 70 years may be at its highest level in 8000 years. Muscheler et al. disagree, suggesting that other comparably high levels of activity have occurred several times in the last few thousand years. Solanki concluded based on their analysis that there is a 92 % probability that solar activity will decrease over the next 50 years. [17] Additionally, in 2005 researchers at Duke University have found that 10–30% of the warming over the last two decades may be due to increased solar output, as shown in figure 12. Note that the temperature curves do not follow the sollar irradience data after 1970.

Figure 12: Shows total solar irradience and terrestrial temperature vs. time for solar radiation reconstruction with an increase in the 11-year averaged irradiance between 1700 and 1980 by Krivova and Solanki in 2003. The solid curves prior to 1985 represent irradiance reconstructions (thick curve: cycle-length based, thin: cycle-amplitude based). From 1985 onwards they represent total irradiance meassurements (solid: Lean et al. 1998, dot-dashed: Willson et al. 1997). The dashed curves represent global (thick) and northern heimisphere (thin) temperatures. All curves have been smothed by an 11-year running mean. After the epoc marked by vertical dotted line the average period has been successively reduced. [17]

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Besides solar variations another external forcing factor function on climate system are variations in the Earth's orbit around the sun.

5. Orbital variations (Milakovich cycles)

In the 1930s, the idea that astronomical factors explained ice ages was defined by a Serbian mathematician named Milutin Milankovich. The eccentricity, axial tilt and precession of the Earth's orbit vary. Changes in Earth's orbit around the sun, also known as the Milankovich cycles, partially explain the glaciation periods and interglacial periods. Although the precise cause of glacial cycles is still under debate, the idea of periodic fluctuations in irradiance due to orbital changes has been accepted as a major forcing function of climatic change. Milankovitch cycles consist of variations in three components: eccentricity, obliquity and precession.

Figure 13: Illustrated Earth`s orbit around the sun. The eccentricity, axial tilt and precession of the Earth's orbit vary. Notice the difference between perihelion and aphelion distance, around 3 %. Also illustrated is the present position of solstice and equinox. [12]

5.1. Eccentricity Eccentricity, due to gravitational disturbances of other planets, varies between more or less elliptical on a cycle of about 110 000 years (actually composed of periods of 96 000 years and 410 000 years). These variations are important because they change distance from the sun. This is the only one of the cycles that affects the actual amount of radiation reaching the Earth. Variability in irradiance as a function of eccentricity is less than 1 %. When the Earth's orbit is more elliptical the amount of energy received would be vary much more between seasons. 5.2. Precession Earth is not perfectly round, instead, it has a slight bulge at the equator due to tidal forces ot the moon and the sun. This causes a slight wobble in the Earth's spin, which causes the positions of the equinoxes and solstices to vary around Earth's orbit. Earth`s axis oscillates in

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a period of about 22 000 years (which is actually composed of periods of 19 000 and 24 000 years). Precession is climatically significant and causes large seasonal variation in temperature, like harsh winters and hot summers. 5.3. Obliquity Obliquity is a measure of the tilt of the Earth's axis from the normal to the plane of its orbit. This inclination oscillates in a range of 22o and 24.5o, over a period of 41 000 years. The difference in tilt affects the latitudinal distribution of solar irradiation, which has global climatic consequences. Again, like precession above, axial tilt could cause warmer winters and cooler summers. At present axial tilt is in the middle of its range at 23.5°. Figure 14 illustrates all of the above mentioned orbital variations.

Figure 14: This figure shows the variations in Earth's orbit, the resulting changes in solar energy flux at high latitude, and the observed glacial cycles. According to Milankovitch theory the precession of the equinoxes, variations in the tilt of the Earth's axis (obliquity) and changes in the eccentricity of the Earth's orbit fit the observed 100 000 years cycle in ice ages. These changes in Earth's orbit are the predictable consequence of interactions between the Earth, the moon, the sun and the other planets. The orbital data shown are from Quinn et al., 1991. Principal frequencies for each of the three kinds of variations are labeled. The solar forcing curve is derived from July 1st sunlight at 65 °N latitude according to Jonathan Levine's insolation calculator. The glacial data is from Lisiecki and Raymo, 2005, and gray bars indicate interglacial periods. [5] Although eccentricity, tilt and precession only produce minor changes in the Earth's total insolation, as seen above they greatly affect its local distribution and seasonal cycle. Anyway, the relatively small changes in insolation do not by themselves appear to be capable of causing continental glaciation. Either they act as a trigger mechanism or are magnified by other processes to produce glaciation or warming. Let`s just mentione some of the processes.

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6. Plate tectonics theory

The timing of ice ages through geologic history is partlly controlled by the position of the continental plates. The theory of plate tectonics describes the global scale dynamics of the lithospheric plates as a result of motion in the earth's asthenosphere. These processes significantly influence world climate. Plate tectonics affects the climate system through three major mechanisms, altering the distribution of continental land masses, changing continental elevations and affecting variability in atmospheric concentration of carbon dioxide. Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as today. Reason for that is the presence of snow and ice being a positive feedback mechanism for climate. 6.1. Configuration of the continents

The distribution of the continents has changed dramatically over geologic time. For example, just 165 million years ago there was only one continent, Pangea and one ocean, Panthalassa. Antarctica hasn't always been centered at the south pole and extensive coal beds indicate a much more temperate climate in the past. In assessing the effects of the distribution of continents on global climate, five major factors must be addressed:

• differences in surface albedo:

Lower latitudes receive a greater amount of incoming solar radiation, than the poleward latitudes. Thus, the amount of ocean (α ~ 7%) versus land surface area (α ~ 20%) at low latitudes has a greater affect on the amount of solar energy absorbed or reflected by the earth.

• land area at high latitudes:

Solar irradiation decreases as latitude increases, snow and ice accumulate more readily on high latitude terrain. This accumulation of snow and ice in turn has a positive feedback effect, for as albedo increases over the white reflective surface (α ~ 65-80 %), even more snow and ice are able to accumulate.

• the transfer of latent heat:

Evaporation is greater over oceanic regions than over land. Greater oceanic surface area at low latitudes thus results in greater evaporative heat loss.

• restrictions on ocean currents:

Oceanic circulation is a primary mechanism by which heat is redistributed from equatorial to polar latitudes. Continental barriers to oceanic heat transport restrict the transfer of heat toward the poles and can influence snow and ice cover.

• the thermal inertia of continents and oceans:

While land masses respond quickly to changes in solar input, oceans have a high thermal heat capacity. This is another way in which the distribution and relative abundance of land and ocean areas can affect climate. 6.2. Elevation of the continents

Collisions between lithospheric plates change the topography of the land. High elevations such as mountain ranges and plateaus can dramatically affect climate by altering patterns of wind circulation, temperature and precipitation.

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7. Reconstruction of climate changes There has been many climate changes through Earth`s geologic history, as figure 15 illustrates. Perhaps one of the most mentioned climate changes these days is the Little Ice Age. It refers to time between the 13th to 19th centuries during which many parts of the globe recorded global cooling. Glaciers expanded, crops failed, rivers such as the Thames in London froze over and colonies in Greenland died out due to the increasingly harsh climate. The Little Ice Age is an exciting event to study because it occurred recently enough that many diverse sources of data exist and there is enough climate information, including dates of harvests, fishing catch records and paintings of advancing mountain glaciers. Geological data are also available from polar and high altitude ice cores, tree rings, deep-sea sediments and corals. For climate changes further back in time a primary source of data is recovered also from the calcareous shells of microscopic marine organisms. When these organisms precipitate their shells they preserve a record of the oxygen isotopic composition of the ocean water in which they grew. The oxygen isotopic composition of the ocean reflects a combination of processes, including seawater temperature and the volume of seawater stored on land as glacial ice. During evaporation, water containing the lighter isotope of oxygen (16O) is preferentially evaporated, so precipitation is enriched in 16O. During glacial intervals, more 16O is locked up as ice on land and the oceans become enriched in 18O. Thus the oxygen isotopic composition of calcareous organisms recovered from marine sediment cores serves as a proxy record of seawater chemistry, ocean temperature, and glacial ice volume. The actual cause of the Little Ice Age is still under debate. However, it does correspond in time to the Maunder minimum, a period of decreased solar activity. Estimates of the magnitude of cooling during the Little Ice Age are from 0.5 °C to 1.5 °C, while models of the Maunder minimum for irradiation reduction of 0.25 % only give a 0.45 °C temperature drop. Therefore, if the estimates of temperature for the Little Ice Age are assumed correct, the Maunder minimum was not acting alone as a forcing function. Other proposed contributing forcing functions include changes in the ocean's heat transport circulation patterns.

Figure 15: Illustrates a combination of reconstructions of climate changes through Earth`s geological history. Graphs at 21th century illustrates different scenarious predicted by simulations (GCM IPCC). [18]

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8. Current data and future predictions Current global wide data shows increase in global temperature of 0.74 ± 0.18 °C. [2] Computer models fit the data and predict the observed rise in temperature. Figure 16 demonstrates temperature trends from 1906 to 2005 over the globe. Computer simulations results for the same period considering natural forcing only as shown don`t fit the observed data in the last decades. Future predictions by simulations vary, highly depending on intial conditions, but have one thing in common, they all predict the furher rising of temperatures. The measure of the temperature response to increased greenhouse gas concentrations and other anthropogenic and natural climate forcing is climate sensitivity. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The current estimates sensitivity is in the range of 1.5 to 4.5 °C. [2] The IPCC predicts global temperature change of 1.4-5.8 °C due to global warming from 1990-2100. [2] Much of this uncertainty results from different climate models, though additional uncertainty comes from different emissions scenarios and initial conditions.

Figure 16: Figure shows temperature changes relative to the corresponding average for 1901-1950 from decade to decade from 1906 to 2005 over the Earth`s continents, as well as the entire globe, global land area and the global ocean (lower graphs). The black line indicates observed temperature change, while blue indicates simulations that include natural factors only. Dashed black lines indicate decades and continental regions for which there are substantially fewer observations. [2] As shown below, climate models routinely predict that land will warm more rapidly than ocean, due to it's lower specific heat.

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9. Conclusion Clearly, the climate system is quite complex. The parameters involved in climatic change interact in a intricate system of positive and negative feedbacks that make it difficult to predict the specific results of an individual perturbation. Parameters usually change simultaneously and the climate system is continuously responding to each push and pull. Because of this complexity there may be significant lag times between a parameter's change and the climate system's response. So climatic changes and their forcing factors are not easily linked. In spite of this uncertainty, being able to understand and predict climate change is critical in a world where the human population already approaches five and a half billion people. We still do not understand many parts of the climate system, yet changes, both natural and anthropogenic, are occurring. In which extent which of the two factors prevails let the reader estimates himself. References: [1] http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html [2] The Intergovernmental Panel on Climate Change, IPCC, AR 4, WG 1 [3] http://www.scotese.com/earth.htm Paleomar Project [4] Kiehl, J. T. and Trenberth, K. E., 1997 Bull. Amer. Meteor. Soc., 78, 197-208. http://www.cgd.ucar.edu/cas/abstracts/files/kevin1997_1.html [5] http://wikipedia.org/ [6] http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant [7] http://mynasadata.larc.nasa.gov/Radiation_Explanation.html [8] http://www.shodor.org/master/environmental/general/energy/application.html [9] M. Joseph, Meteorology : The atmosphere and the science of weather (Prentice Hall, 1997). [10] http://www.grida.no/climate/ipcc_tar/wg1/221.htm#tab61 [11] http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html [12] http://physicsworld.com/ [13] http://www.realclimate.org/index.php?p=142 GISS-GCM Model simulation [14] D. Eisenberg and W. Kauzmann, The structure and properties of water (Oxford Universty, London, 1969) [15] http://www.globalwarmingart.com [16] http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant [17] N.A. Krivova and S.K. Solanki, Solar total and spectral irradiance: Modelling and a possible impact on climate (ESA SP, 2004), Solar Variability of Possible Relevance for Planetary Climates (Sp. Sci. Rev., 2006) [18] www.bom.gov.au/info/GreenhouseEffectAndClimateChange.pdf Greenhouse effect and climate change

Other literature:

Tomaž Vrhovec, Jože Rakovec, Osnove meteorologije (DMFA, 2000). Drake, F., Global warming: The science of climate change (Oxford University, London, 2000). Houghton, John T., Global warming: The complete briefing (Cambridge, 2004). Holton, James R., An introduction to dynamic meteorology (San Diego, Academic Press, 1992). Svensmark, H., The chilling stars : A new theory of climate change (Thriplow, Icon Books, 2007).