ESE / GE 148a: Introduction to Climate. Organizational Details - I

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1 ESE / GE 148a: Introduction to Climate Organizational Details - I TA: Tim Merlis and Zhihong Tan Tim is a graduate student in Environmental Science and Engineering and is currently working on climate dynamics in the atmosphere with Prof. Tapio Schneider Zhihong is also and ESE graduate student and will take over from Tim later in the quarter. Tim will schedule office hour(s) after the next class 1

2 Organizational Details - II Course WebPage: Textbooks: 1) Atmosphere, Ocean, and Climate Dynamics, Marshall and Plumb, Elsevier, 008 ) Principles of Planetary Climate, Pierrehumbert, Cambridge University Press, 0?? (copies to be provided). P/F daily homework (aka Ticket) 5%; problem set (approximately every other week) 0%; an oral presentation 0%; final exam 35% Oral presentations: Each student gets a particular month, e.g., January 009, from which to choose two published articles (journals such as Science and Nature, Climate Dyn., J. Climate, Geophys. Res. Lett., J. Atmos. Sci., J Phys. Oceanog., J. Geophys. Res., etc.). The student gives a 10-minute talk on each article. The goal is to instruct the class, and the class is responsible for the material presented. ESE 148b winter term w/ Simona Bordoni details how energy, water, etc. are transported in the Earth Atmosphere/Oceans. ESE 148c spring Alex Sessions describes the cycling of elements by the plants, bugs, and animals. Daily P/F homework To help motivate the day s lecture, a short problem is to be worked and handed in at the beginning of class. These are listed on the class web page. The problems are not meant to be difficult, if the reading has been done. If you find yourself spending a lot of time on a problem, you are probably not thinking about it correctly! Here is the problem for Wed (as detailed on the web page): Various global warming scenarios for the next century suggest that Earth's surface temperature might increase by 3 degrees. Here we estimate what change in external forcing would be required to produce a similar change (all else staying constant - a poor assumption as we will see later). Calculate what change in the mean distance between sun and Earth would change Earth's emission temperature (T e ) by 3 degrees.

3 T/F? 1. CO is the most important greenhouse gas in Earth s atmosphere.. Surface temperature has warmed about 1 C in the last 100 years. 3. Ice once covered the entire Earth. 4. Hurricanes will become more intense if the Earth warms as is predicted. 5. Plants grow faster under elevated CO. 6. The Kyoto agreement required developed countries to reduce their greenhouse gas emissions to 1990 levels. 7. Fossil fuel supplies will run low soon, producing rapidly increasing prices and thereby limiting our emissions of CO. CLIMATE Climate is the synthesis of weather in a particular region. The climate variables of most interest: *Temperature - Figure 1 *Precipitation Wind Pressure Cloudiness Humidity => Climate is the expectation for a given month or season. Distinguish from synoptic scale weather. Climate is what we expect, weather is what we get Generally we are interested in surface climate, though recent observations provide measure of climate at other altitudes, e.g. stratospheric climate determined by ozone and temperature observations; recent mid-tropospheric satellite temperature measurements allow tests of warming theory. 3

4 Figure 1. winter summer temperature difference. Ruddiman, 001. Temperature <T> surface = 88K (15 C). In lowest 10 km, the lapse rate, Γ, averages: The lapse rate varies with season, altitude, and latitude (See Figure 1., 1.3, 1.4). As we will discuss at length, the decrease of temperature with altitude in the troposphere is critical for the maintenance of the equitable climate. 4

5 Cavity Radiation - Stefan-Boltzman Law. Blackbody Radiation. The radiation field within a closed cavity in thermodynamic equilibrium has a value uniquely related to the temperature of the cavity wall, regardless of the material of which the cavity is made. [This radiant intensity is called the blackbody radiation, since it corresponds to the emission from of a surface with unit emissivity (later)]. Intuition: consider two blocks of different material each with an internal cavity but at the same temperature; they are placed together such that the cavities connect thru a small hole. The radiation passing in each direction must be the same total intensity (and also of the same color) The intensity of cavity radiation (and therefore blackbody emission) follows the Stefan-Boltzmann Law: E BB = σ T 4 ; σ = 5.67 x 10-8 W m - K -4 Emission Temperature of the Sun The solar spectrum is similar to that of a blackbody with temperature of 5800K. Using the Stefan-Boltzmann formula, we estimate the flux density of the photosphere is 6.4 x 10 7 W m -. T e (photosphere) = [(E/m )/ σ] 1/4 = 5800 K E/m = 6.4 x 10 7 W/m Total luminosity d=1.4 x 10 9 m Surface area Sun = πd ; d=1.4 x 10 9 m Solar luminosity, S 0 = 3.9 x 10 6 W Solar Flux at Earth 1 Astronomical Unit = kilometers S p = 3.9 x 10 6 W / 4πr = 1367 W/m 5

6 Emissivity For an arbitrary body at equilibrium with an measured emission of E R we define the emissivity, ε, as: ε = E R / E BB = E R / (σ T 4 ) The emissivity of an object is wavelength dependent and related to the reflectivity as (Kirchhoff s Law): ε = 1 - R Metals such as tungsten have emissivities ~ 0.5 in the near IR; liquid water has an emissivity near 1 at all wavelengths - which as we will see later is critical for Earth's climate]. Emission Temperature of Planets The emission temperature of a planet, T e, is the temperature with which it needs to emit in order to achieve energy balance (assuming the average temperature is not decreasing c.f. Jovian planets). We equate the absorbed solar energy with the energy emitted by a blackbody: Solar radiation absorbed = planetary radiation emitted Absorbed Solar Radiation = S p π r p (1-α p ) α p is the planetary reflectivity or albedo. For Earth, α p is ~ 0.3. For Venus, α p is ~ 0.7 and so solar energy input per unit area is less despite being at 0.7 AU. Emitted radiation = σ T 4 4 π r p The 4 π r p accounts for the fact that emission occurs over the entire area of the sphere. Equating the absorbed and emitted radiation: S p π r p (1-α p ) = σ T e 4 4 π r p T e = [(S p / 4)( 1-α p )/ σ] 1/4 6

7 Today: Emission Temperature of Earth T e = [(1367 W m - / 4) (1 0.3) / ( W m - K -4 )] ¼ = 55 K = -18 C (O o F) Obviously this is not the emission temperature of the surface! As we will see in the next few lectures, the greenhouse effect warms the surface significantly above T e (fortunately). The problem is even more severe early in the solar evolution: 4 Billion years ago: T e = [(1000 W m - / 4) (1 0.3(?)) / ( W m - K -4 )] ¼ = 35 K = -38 C (-36 o F) Contrast with Jupiter: Jupiter is km from the sun. Its albedo is S Jupiter = 3.9 x 10 6 W / (4 π d Jupiter ) = 50 W m - T e = [(50 W m - / 4) (1 0.73) / ( W m - K -4 )] ¼ = 88 K Actual emission temperature of Jupiter is ~ 134 K. Most of the energy emitted by Jupiter today (> 80% - note T 4 dependence) is associated with cooling from gravitational accretion. 7