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CLIMATE CHANGE Mariam Elizbarashvili Global Energy Balance Milankovich theory of climate change The Climate THE WEATHER • What is the weather, what is the climate? •What is the weather like today? Weather is the specific condition of the atmosphere at a particular place and time and is measured in terms of things such as wind, temperature, humidity, atmospheric pressure, cloudiness, and precipitation. Weather describes the short-term state of the atmosphere. Climate is the average pattern of weather for a particular region, usually taken over a 30-year time period. Climatic elements can include precipitation, temperature, humidity, wind velocity, fog, frost and other measures. The Sun is the only significant source of energy for Earth’s Atmosphere. Millions of Other stars radiate energy, but they are too far away to affect Erath. Energy is also released from inside Earth, primarily as radioactive decay of minerals, although not enough to influence the atmosphere significantly. Thus, The Sun supplies essentially all of the energy that drives most of the atmospheric processes. SUN PROPERTIES: NOTES: 1. Energy source is nuclear fusion (hydrogen, mainly) 2. Luminosity increased about 30% over earth lifetime 3. Projected lifetime: ~ 11 billion years 4. Sun is about middle age Global climate is determined by the radiation balance of the planet. As solar energy reaches the Earth’s surface, a fraction of it is absorbed and the Earth’s surface warms up. The remaining fraction is reflected immediately off the surface back into the atmosphere and space. The surface of the Earth (land and water) that has been warmed by the radiation then emits energy back in the form of heat into the atmosphere and toward space. Since the Earth’s surface temperature is much lower than that of the Sun, it emits radiation at longer wavelengths and with energy levels much lower than that from the Sun, in this case at infrared (heat) wavelengths (not at visible wavelengths like the Sun). Earth's atmosphere is made up of gases, and these gases allow some solar radiation to reach the planet but also absorb some of the heat radiating from the planet, trapping it and radiating it back downward to the surface. This cycle is called the greenhouse effect, because it is similar to the warming process inside a glass-walled greenhouse. Earth's atmosphere traps enough heat to keep the entire planet warm; without it, the average temperature of the Earth's surface would be much colder. What is the average temperature on the Earth? If there was no greenhouse effect, what would be the avarege Temperature? If the Earth and the atmosphere did not emit radiation but only absorbed radiation, the Earth and the atmosphere would continue to get hotter and hotter until it would be uninhabitable. If more radiation were emitted than absorbed, over time the Earth would get colder and colder. Neither of these happens because the Earth is roughly in energy balance. At a particular time and place, the energy emitted by the Earth might not balance the energy absorbed by the Sun, but when averaged over the Earth’s entire surface for a long time period, the input and output of energy are nearly in balance. QUESTION 1: CALCULATE THE EMISSION TEMPERATURE OF VENUS. the emission temperature is the temperature that the planet needs to maintain energy balance. ENERGY BALANCE OF THE EARTH We will use the principle of planetary energy balance: assuming that i) no energy is stored by the earth, and that ii) the earth does not do work to its surroundings then by the first law of thermodynamics, the net energy going into the earth must be zero. It means that Incoming solar radiation = outgoing planetary radiation Incoming solar Outgoing longwave radiative energy FLUX, FLUX DENSITY, SOLAR CONSTANT The Sun puts out a nearly constant flux of energy that we call the luminosity Lo 3.9×1026 W Assume radiative flux is spherically uniform . flux density (Sd) power per unit area. We assume that the flux density is uniform over sphere, and write the flux density at any distance d from the sun as Sd. Since space is effectively a vacuum and Energy is conserved, the amount of energy passing outward through any sphere with the sun at its center should be equal to the luminosity, or total energy flux from the sun. So: Lo = flux density x area of sphere Sd × 4 π d2 d Solar constant (So) flux density at distance d Lo / 4 π d2 =1367 W/m2 The mean earth-sun distance d 1.5 x 1011 m So Flux density Sd is inversely proportional to the square to distance to the sun. We assume that the Stefan-Boltzmann Law can be used E = σ T4 σ 5.67x10-8 Wm-2K-4 NOTES: 1. E has units of power per unit area (W/m2) 2. Temperature has to be in Kelvin in calculations EMISSION TEMPERATURE LET’S APPLY PLANETARY ENERGY BALANCE TO THE EARTH: the emission temperature is the temperature that the earth needs to maintain energy balance. Incoming solar radiation = So(1-αp) π rp2 where rp is the radius of the earth albedo (αp) fraction of solar radiation reflected Outgoing planetary radiation (radiation emitted) x (area of planet) σT44 π rp2 Solar radiation is essentially a parallel and uniform beam for a planetary body in the solar system, because the planets all have diameters that are small compared to their distance from the sun. the amount of energy incident on a planet is equal to the solar constant times the area that the planet sweeps out of the beam of parallel energy flux. IF YOU EQUATE THE INCOMING RADIATION TO OUTGOING RADIATION, YOU CAN SOLVE FOR THE TEMPERATURE TO GET EQUATION Te 4 ( So / 4)(1 p ) This is the emission temperature which is the temperature that the earth needs to maintain energy balance. 1. It depends on the solar constant and albedo of earth. 2. Emission temperiture for earth ~ 255K (18 C) QUESTION 1: CALCULATE THE EMISSION TEMPERATURE OF VENUS. Te 4 ( S o / 4)1(1 p ) σ 5.67x10-8 Wm-2K-4 Solar constant (So) flux density at distance d Lo / 4 π d2 The emission temperature is much less than the observed global mean surface temperature of 288 K = 15 C. To understand the difference we need to consider the greenhouse effect. GREENHOUSE EFFECT Let’s now add a simplified ‘slab’ atmosphere to our planet with the properties that the atmosphere: 1. Lets sunlight through without absorbing or reflecting it 2. Lets this “stab” absorbs all terrestrial radiation 3. Has the same temperature everywhere We can then work out the energy balance for each of the layers in this atmosphere-earth system - the top of the atmosphere (TOA), in the atmosphere, and on the surface. Let Ts be the surface temperature, and TA the atmospheric temperature, then I. Top of Atmosphere (TOA) balance: II. Atmospheric balance: III. Surface balance: Ts = 21/4Te Since Te = 255K (earth’s emission temperature), it follows that Ts = 303K or 30 degrees C! The surface temperature is increased because the atmosphere does not inhibit the flow of solar energy to the surface, but augments the solar heating of the surface with its own downward emissin of longwave radiation, which in this case is equal to the solar heating. There are three fundamental ways the Earth’s radiation balance can change, thereby causing a climate change: (1) changing the incoming solar radiation (e.g., by changes in the Earth’s orbit or in the Sun itself), (2) changing the fraction of solar radiation that is reflected (this fraction is called the albedo – it can be changed, for example, by changes in cloud cover, small particles called aerosols or land cover), and (3) altering the longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations). Milankovich Theory of climate change Characteristics of earth’s orbit around the Sun The earth’s orbit is slightly elliptical. Eccentricity is a measure of how far earth’s orbit is from being circular. Define eccentricity (e) by raphelion= (1 +e) rmean ; e ~ 0.0167 today Mean distance: 1.496 x 1011m Max (aphelion): 1.521 x 1011m Min (perihelion): 1.471 x 1011m Obliquity (tilt) is the angle between the rotation axis of the planet and the plane of the orbit around the sun Currently 23.45 degrees; varies between 22.1and 24.5 degrees over a period of ~41,000 years. This change affects the amount of solar radiation received by the higher latitudes. More tilt results in more solar radiation being received at higher latitudes during the summer, while less tilt results in less solar radiation being received at higher latitudes during the summer. Earth axis “wobbles” like a spinning top, and so over time it points in different directions relative to the stars in a 25 800 26 000 year cycle called precession. Precession alters the timing of the seasons relative to Earth’s position in its orbit around the Sun. Hipparchos (190 BC – 120 BC), was a Greek astronomer, geographer, and mathematician of the Hellenistic period. He is considered the founder of trigonometry but is most famous for his incidental discovery of precession of the equinoxes. Question 2: Over the last 2000 years on what angle “rottated” the Earth axis as a result of precession? It is well known from astronomical calculations that periodic changes in parameters of the orbit of the Earth around the Sun modify the seasonal and latitudinal distribution of incoming solar radiation at the top of the atmosphere (‘insolation’). These cycles are known as Milankovich cycles, after Milutin Milankovich. As these long-term cycles “overlap” there are periods of time when significantly less radiation reaches Erath surface (especially in the high latitudes), and periods of time when there are greater or smaller seasonal contrasts. For examle, during periods when there are smalller “seasonality” – in other words, when there are smaller contrasts between winter and summer – more snow can accumulate in high latitudes due to greater snow fall from winters and less melting will take place due to lower summer temperature. THANK YOU FOR ATTENTION! YOU CAN USE THE SAME EQUATION TO GET THE EMISSION TEMPERATURE FOR OTHER PLANETS, BUT YOU’LL NEED TO CHANGE THE SOLAR CONSTANT TO THE SOLAR FLUX APPROPRIATE FOR THAT PLANET, AS WELL AS THE ALBEDO. Te 4 ( S o / 4)1(1 p ) σ 5.67x10-8 Wm-2K-4 Solar constant (So) flux density at distance d Lo / 4 π d2 THE DIFFERENCE BETWEEN THE ACTUAL SURFACE TEMP WITH THE EMISSION TEMP REFLECTS THE INFLUENCE OF DIFFERENT PLANETARY ALBEDO AND GREENHOUSE EFFECTS. Venus (actual temp ~730K) Earth’s Te ~255K Earth (actual temp ~288K) Emission temp Te (K) Distance from Sun (x106 km)