1 MET 112 Global Climate Change MET 112 Global Climate Change - Lecture 2 The Earth’s Energy Balance Dr. Craig Clements San José State University Outline  Energy  Solar and terrestrial radiation  Absorption and scattering  Global energy balance

2 MET 112 Global Climate Change Review: Why is CO 2 So Important?  Carbon Dioxide is a greenhouse gas.  Greenhouse gases are those gases that cause the greenhouse effect.  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).  Consider the blanket analogy

3 MET 112 Global Climate Change Earth’s Energy Balance  Energy entering top of atmosphere  Energy entering the Earth’s surface = Energy leaving top of atmosphere = Energy leaving Earth’s surface Conservation of Energy

4 MET 112 Global Climate Change There are three modes of energy transmission in the atmosphere.  Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion.  Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy.  Radiation: the propagation of electromagnetic waves through space. Energy transmission

Conduction

Convection

Electromagnetic radiation  Radiation is the transfer of energy by rapid oscillations of electromagnetic fields.  The most important general characteristic is its wavelength ( ), ______________________________.  Radiation travels through space at the speed of light (3 x 10 8 m s -1 ). Defined as the crest-to-crest distance

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9 Radiation  What emits radiation? –All objects with a temperature greater than 0°K emit some type of radiation (energy)  Examples:  Radiation laws: –Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law) –Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

10 MET 112 Global Climate Change Review questions  Considering the previous discussion –Which object would emit more (intensity) radiation: Earth or Sun? –If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength? –What wavelength radiation are you emitting right now? Sun Earth infrared

Solar Radiation (Sunlight)  Sunlight is primarily made up of the following: –Visible Light (44%) –Infrared Radiation (48%) –Ultraviolet Radiation (7%) Unit: 1  m = m

12 MET 112 Global Climate Change Terrestrial or Longwave Radiation  Planets mainly emit infrared radiation  Radiation emitted by planets occurs mainly at wavelengths _____ than those contained in solar radiation Solar Radiation (“Shortwave”) Terrestrial Radiation (“Longwave”) longer

13 MET 112 Global Climate Change Solar vs. Terrestrial Radiation  The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation ;  Thus …

14 MET 112 Global Climate Change Energy from the Sun  Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get?  Sun’s energy is either –Scattered (reflected away) or –Absorbed  Scattering happens by bouncing off –Particles in the atmosphere –Earth’s surface  Absorption happens when certain gases absorb the energy –The reality is the only certain gases absorb certain wavelengths.

15 MET 112 Global Climate Change Absorption of radiation  Absorption of shortwave radiation by atmospheric gas molecules is fairly weak; –most absorption of shortwave radiation occurs at the Earth’s surface.  Most gases do not interact strongly with longwave radiation, however –Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

17 MET 112 Global Climate Change Incoming solar radiation  Each ‘beam’ of incoming sunlight can be either: –Reflected back to space:  Clouds  Atmosphere  Surface –Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface. Albedo

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19 MET 112 Global Climate Change Recap  ______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation  __________________ radiation comes from the Earth and is composed of infrared radiation  Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength.

20 MET 112 Global Climate Change Recap  ______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation  __________________ radiation comes from the Earth and is composed of infrared radiation  Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength. Shortwave or solar Longwave, terrestrial or infrared

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25 MET 112 Global Climate Change Review - sensors that measure radiation  A _________________ measures solar radiation.  A__________________ measures infrared radiation (terrestrial) that comes from the Earth. Pyranometer Pyrgeometer

Longwave radiation is emitted from surface. Some surface radiation escapes to space Most outgoing longwave is absorbed in atmosphere (by greenhouse gases) Greenhouse gases emit longwave upward and downward Some atmospheric radiation escapes to space Some atmospheric radiation is absorbed at the surface

29 MET 112 Global Climate Change Greenhouse Effect Sequence of steps: 1.Solar radiation absorbed by earth’s surface. 2.Earth gives off infrared radiation. 3.Greenhouse gases absorb some of the Earth’s infrared radiation. 4.Greenhouse gases (water and CO 2 ) give off infrared radiation in all directions. 5.Earth absorbs downward directed infrared radiation Result: warmer surface temperature

30 MET 112 Global Climate Change Energy Balance  Assume that the Earth’s surface is in thermodynamic equilibrium:  Thermodynamic Equilibrium: –The flow of energy away the surface equals the flow of energy toward the surface Surface Average surface temperature = 15°C

31 MET 112 Global Climate Change Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface. Sudden Removal of all Greenhouse Gases

32 MET 112 Global Climate Change Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface. Thus, average surface temperature starts to decrease. Sudden Removal of all Greenhouse Gases

33 MET 112 Global Climate Change As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops Sudden Removal of all Greenhouse Gases

34 MET 112 Global Climate Change As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops and equilibrium is restored. Average surface temperature = -18°C Result: A Very Cold Planet!

35 MET 112 Global Climate Change Earth’s Greenhouse Effect  Without the greenhouse effect, the surface temperature of Earth would be –Way Cold (-18°C)  Greenhouse gases play an important role in shaping climate. –More GHGs – warmer climate –Less GHGs – cooler climate

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37 MET 112 Global Climate Change Homework 1-1  Start with the following diagram and assume the earth’s surface temperature is 15C and that the atmosphere has greenhouse gases.  Imagine that the concentrations of greenhouse gases were to increase by 50%. 1. Draw two more diagrams illustrating (with arrows) how the energy balance would change with the increase in greenhouse gases and explain why. 2. How would the average surface temperature change? Surface Average surface temperature = 15°C

38 MET 112 Global Climate Change 1.What percentage of the sun’s radiation is a)absorbed by the Earth’s surface? b)absorbed by the atmosphere c)reflected out to space? 2.What percentage of the energy received by the earth’s surface comes directly from greenhouse gas emissions? 3.If the sun’s radiation was to increase by 10%, how would the following energy units change (increase, decrease or stay the same) a)Energy gained by the Earth’s surface. b)Energy lost by the Earth’s surface. c)Energy emitted by greenhouse gases. d)Energy lost to space. Homework 1-2