GEOG 401: Week 2 Earth s Energy Balance

Transcription

1 GEOG 401: Week 2 Earth s Energy Balance Dr. John Abatzoglou Spring 2011 Lecture Goals (1) Understand the global balance of energy and associated fluxes of energy and energy types (2) Qualita@ve and quan@ta@ve understanding of the greenhouse effect (3) Ability to es@mate radia@ve Energy Balance for various planets (including EarthJ ) Sun: The UlFmate Source Source of energy for life, primary energy input to climate system 1. Solar input plays a first order effect on global temperature!!! 2. Solar input changes (solar cycles), but generally constant 1

2 Factors that determine planetary surface temperature Distance from the Sun (primary energy source) Energy permeates outward in 3- d space following area of sphere = 4 πd 2 Reflec@vity of the planetary body Albedo How the planet absorbs and retains energy Greenhouse Effect The Electromagne@c Spectrum Radia@on energy transmized as a wave Short waves, vibrate very fast, HIGH ENERGY Long waves, vibrate slower, LOWER ENERGY Basic RadiaFon Laws Wavelengths and intensity of radia@on are related to temperature of emi\ng body. 1. Wien s Law: warmer objects emit a higher propor@on of their energy at short wavelengths than cold objects. 2. Stefan- Boltzman Law: Warmer objects emit more intensely than cold objects. 2

3 Wien s Law emission wavelength and temperature λ max = a / T Where: λ max is wavelength of peak emized radia@on (in µm) a = 2898 µmk, constant T emizer temperature (in K) Recall that K = T C Sun s temperature is 6000K What s its wavelength? Earth s average temperature is around 280K Stefan - Boltzmann Law Energy emized is dependent on its temperature: I = σ T 4 Where: I = Black body energy radia@on σ = (Constant) 5.67x10-8 W/m 2 /K 4 T = temperature in Kelvin Example: Sun surface is 6000K, so I = 7 x 10 7 W/m 2 Solar vs. Terrestrial Radia@on Radia@on emized from the sun and emized from the Earth are in discrete wavelength bands 3

4 Solar energy reaching the Earth. 1. How much does the sun emit? - Use Stefan- Boltzman Law 2. How much radia@on is incident on a planar surface at the top of the Earth s atmosphere? - Use Inverse distance squared law 3. What is the emission temperature of Earth? Balance Thermal Out Solar In Energy In 1367 W/m 2 * Surface area = πr 2 Energy Out I = σ T 4 * Surface area = 4πr 2 Albedo Defined: The frac@on of light reflected by an object or a surface, ranges from 0 to 1 White objects have high albedos (~1) Black objects have low albedos (~0) Increasing albedo 4

5 Constant Gases (inert, passive) Nitrogen(N 2 ): 78% Oxygen (O 2 ): 21% Variable Gases (radia@ve) Water Vapor (H 2 O) Carbon Dioxide (CO 2 ) Methane (CH4) Ozone (O 3 ) Atmospheric Composi@on These are Greenhouse Gases and absorb long wavelength energy 5

6 Each molecule can absorb and energy at different wavelengths Two atom molecules absorb negligible longwave energy E.g., Nitrogen (N 2 ) and Oxygen (O 2 ) Three or more atom molecules absorb much more longwave energy E.g., Carbon Dioxide (CO 2 ), water vapor (H 2 O), methane (CH 4 ) Tri- atomic and greater molecules have rota@onal/vibra@onal modes excited by IR radia@on Proper@es of Greenhouse Gases (1) Their rela@ve ability to absorb thermal radia@on emized from the Earth s surface (2) Their quan@ty (3) Their longevity in the atmosphere (4) Their rela@ve natural and man- made sources and sinks Greenhouses Gases: Water Vapor Due to shear numbers, water vapor accounts for vast majority of current greenhouse effect Natural Sources: the hydrologic cycle Anthropogenic Sources: Warmer air can hold more water vapor (climate feedback) Current Concentra@on: 0-4% Atmospheric Life@me: measure of how long a molecule remains in atmosphere, on average. so its NOT well mixed in the atmosphere 6

7 Greenhouses Gases: Carbon Dioxide The Notorious CO2 Natural Sources: ocean, volcanoes. Anthropogenic Sources: Incomplete of fossil fuels, biomass burning Atmospheric years so it is well mixed in the atmosphere~ implies global coverage Greenhouses Gases: Methane Carbon containing molecule that is 25X more potent on a molecule- molecule basis than CO2 Natural Sources: peat bogs, soil Anthropogenic Sources: Industrial and auto emissions, agriculture (belch!) Current Concentra@on: 2 ppm Atmospheric Life@me: 10 years so its well mixed in the atmosphere The Greenhouse Effect Greenhouse effect: the process of absorbing and trapping infrared radia@on emized from the planet s surface by the atmosphere. The greenhouse effect makes a planet s surface temperature warmer than it would otherwise be. The stronger the greenhouse effect, the warmer the surface (other factors being equal). Important Point: For an atmosphere to provide a greenhouse effect it must dis@nguish between shortwave and longwave radia@on On Earth, longwave radia@on (emized by the Earth s surface) is preferen@ally absorbed by greenhouse gases 7

8 Strength of the Greenhouse Yes, related to # of GHG, more GHG = stronger effect For a strong Greenhouse effect the amount of radia@on exi@ng the Earth- Atmosphere system should be significantly less than that exi@ng the Earth s surface Compare TOA vs. Surface Bigger Difference == More heat trapping Normalized blackbody emission spectra % radia@on abs in troposphere % radia@on abs in upper- atm Absorp@on by the atmosphere Radia@on emized Shortwave (sun) Longwave (Earth) Radia@on transmized through atmosphere AbsorpFon Profiles Atmospheric Windows Keeping Cool 8

9 Incoming Solar RadiaFon of shortwave by atmospheric gas molecules is fairly weak (ex. O3) As a result, the atmosphere is transparent to shortwave radia@on Sun s energy is either ScaZered (reflected away) or Absorbed ScaZering happens by bouncing off Par@cles in the atmosphere Earth s surface Outgoing Longwave RadiaFon Absorp@on of longwave radia@on by atmospheric gas molecules is fairly strong!!! Certain gases in the atmosphere capture longer wavelengths emized from Earth s surface The atmosphere is opaque to longwave radia@on (10% escapes directly to space) This acts to trap more heat within the Earth- Atmosphere system, inhibi@ng cooling. Note that the SAME AMOUNT OF RADIATION STILL IS EXPELLED BACK TO SPACE REGARDLESS OF THE GREENHOUSE EFEECT Energy Balance Atmosphere transparent to solar, opaque to infrared radia@on Energy balance at two levels, top of atmosphere and surface - TOA, same as emission temperature - Surface T A T S 9

10 1/18/12 Leaky Greenhouse Earth s Energy Balance Earth s Atmosphere Transparent to Solar RadiaFon Some is scazered by clouds, aerosols Absorp@on of UV in ozone shields Earth from harmful radia@on Opaque to Thermal RadiaFon Most is absorbed by atmosphere Atmosphere re- radiates energy back to surface Less than 10% of longwave energy emized from surface actually escapes to space, most is absorbed by atmosphere. 33 C warming by our Natural Greenhouse Effect = Reason for Life on our Planet 10

11 Forcing Defined: Change in net (both solar and thermal) for the Earth s climate system compared to a control state. Used to assess how perturba@ons may be driving climate change Control state: Earth s climate system prior to the I.R. Experiment: Earth s climate system today, or into the future with both natural and anthropogenic changes applied. Consider the following perturba@on: oceans turn milky white A decrease in net radia6on is termed a nega6ve radia6ve forcing and would cool the planet Sudden Removal of all Greenhouse Gases OUTPUT Thermal (sfc) INPUT Solar No GHG gases means that the only input is solar Imbalance: Now outgoing energy away exceeds incoming energy. Thus, average surface temperature starts to decrease. Sudden Removal of all Greenhouse Gases As surface cools, emission of radia@on decreases un@l balance is restored. At this point, cooling stops Average surface temperature = -18 C 11

12 1/18/12 Energy Budget The Enhanced Greenhouse Effect Theory +4W/m2 Forcing (a) Current top of atmosphere (TOA) balance (b) Increased GHG inhibits cooling, so thermal out to space (IMBALANCE) Warming since input>output (c) atmospheric column must warm to achieve balance, thus in the level of emission increasing in Last Words on the Greenhouse Effect Atmosphere radiates energy back to Earth s surface The strength of the greenhouse effect can be thought of as the difference in the amount of radia@on leaving the Earth s surface from that leaving the top of the atmosphere (Thermal SFC - Thermal TOA) Intensity of radia@on related to temperature of emi\ng object GHG higher in the atmosphere have a stronger effect Increased # GHG = stronger effect More heat trapped in Earth- Atmosphere system = stronger effect 1- layer model is a simplifica@on of atmosphere, reality is that GHG exist through atmosphere and radia@ve exchange occurs at mul@ple levels 12