The Greenhouse Effect CLIM 101 // Fall 2012 George Mason University 13 Sep 2012

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 The Greenhouse Effect Solar radiation passes through the atmosphere and warms the Earth’s surface The Earth emits thermal radiation (also called infrared radiation) back to space, part of which is absorbed by the molecules of “greenhouse gasses” (water vapor, H 2 O; carbon dioxide, CO 2 ; some other micro gases) in the atmosphere and warms the atmosphere This warming effect of the greenhouse gases is called the “Greenhouse Effect” (Global Warming: Houghton)

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 The Greenhouse Effect 1827: Fourier pointed out the similarity between the blanketing effect of greenhouse gases in the atmosphere and what happens in a real greenhouse, hence the name: Greenhouse Effect (Not a good analogy) 1860: Tyndall measured the actual absorption of infrared radiation by CO 2 and H 2 O 1896: Arrhenius calculated the effects of increasing greenhouse gases in the atmosphere; estimated the magnitude of global warming due to doubling of CO 2 (Global Warming: Houghton)

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 The Greenhouse Effect Natural Greenhouse Effect: Due to greenhouse gases present for natural reasons; these gases (viz. CO 2 ) were in the atmosphere (except CFCs) long before human beings came on the scene. Enhanced Greenhouse Effect: The additional greenhouse effect caused by the additional greenhouse gases in the atmosphere due to human activities (fossil fuel burning; deforestation, etc.) If there were no greenhouse gases (hence no greenhouse effect) the Earth’s temperature would be -18  C (not +15  C as it is at present) The Greenhouse Effect is real; without it, the Earth would be uninhabitable. But it is possible to have too much of a good thing!

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012

Energy Equilibrium

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Earth’s Energy Balance Solar Radiation S = 1380 Wm -2 (plane, parallel) In equilibrium, INCOMING ENERGY = OUTGOING ENERGY (1 -  ) S  r 2 = E (4  r 2 ) E = 1/4 (1 -  ) S Measured albedo (  ) = 0.31 Measured planetary E = 237 Wm -2 Implied T E = 255 K (-18°C; 0°F) Planetary Emission Measured surface E s = 390 Wm -2 Measured T s = 288°K (+ 15°C; 59°F) WHAT’S GOING ON?

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Factors Determining Earth’s Average Temperature All “bodies” radiate energy. The hotter the body, the more the emitted radiation. Earth’s equilibrium climate results from a balance between the incoming solar radiation and the outgoing terrestrial radiation. The equilibrium temperature and the mean climate depend on the greenhouse gases in the atmosphere. Earth is radiating energy (and cooling) all the time (24 hours). We do not notice cooling during the day because the sun is heating more than the Earth is cooling. Nights are colder because there is no incoming solar radiation. The temperature goes down at night, because the Earth continues to radiate energy and cool.

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 MARS EARTH VENUS A Tale of Three Planets

CLIM 101 // Global Warming, Weather, Climate and Society // Fall AU F = 592 W / m AU F = 1380 W / m AU F = 2639 W / m 2

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 MARS 1.56 AU from Sun F = 592 W/m 2 Albedo = 17% T eff = 216 K Effective Temperature A Tale of Three Planets EARTH 1.00 AU from Sun F = 1367 W/m 2 Albedo = 30% T eff = 255 K VENUS 0.72 AU from Sun F = 2639 W/m 2 Albedo = 78% T eff = 225 K

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 A Tale of Three Planets Mars: −63 o C Earth: 15 o C (59 o F) Venus: 464 o C MARS 1.56 AU from Sun F = 592 W/m 2 Albedo = 17% CO 2 = ~0% T sfc = 210 K EARTH 1.00 AU from Sun F = 1367 W/m 2 Albedo = 30% CO 2 = ~0.03% T sfc = 288 K VENUS 0.72 AU from Sun F = 2639 W/m 2 Albedo = 78% CO 2 = ~96% T sfc = 737 K Surface Temperature

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 T sfc T Eff T sfc – T Eff Venus Earth Mars – 6 Why is T s different from T Eff ? Why is T s – T Eff so different for different planets? Answer: The greenhouse effect Temperature ( o C)

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 FAQ 1.3, Figure 1

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 If the atmosphere had no GHG, clouds, or aerosols, infrared radiation would pass through it without being absorbed. Note: size of arrows coming from the surface and going to space are equal.

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 But if we suddenly cover the Earth with an atmosphere, most of the radiation emitted by the Earth’s surface gets absorbed, and only a small fraction passes through. Note: atmosphere absorbs radiation coming from Earth’s surface; upper arrows much smaller than lower arrows.

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 The atmosphere emits the radiation it absorbs. Most of it is emitted downward, back towards the Earth’s surface. Note: lower arrows go towards the Earth means atmosphere emits radiation to the Earth, and also to the Space (upper arrows).

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 The downward flux of infrared radiation from the atmosphere makes the Earth’s surface warm up, and as it gets warmer it emits more radiation Note: lower arrows are bigger to denote that Earth’s surface has gotten warmer and hence emitting more radiation.

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Eventually the system (Earth + atmosphere) comes into equilibrium. The Earth’s surface has been warmed by the addition of an atmosphere. This is the essence of the greenhouse effect. Note: Greenhouse effect is real; without it, Earth would be uninhabitable.

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 A Tale of Three Planets Mars: −63 o C Earth: 15 o C (59 o F) Venus: 464 o C MARS 1.56 AU from Sun F = 592 W/m 2 Albedo = 17% CO 2 = ~0% T sfc = 210 K EARTH 1.00 AU from Sun F = 1367 W/m 2 Albedo = 30% CO 2 = ~0.03% T sfc = 288 K VENUS 0.72 AU from Sun F = 2639 W/m 2 Albedo = 78% CO 2 = ~96% T sfc = 737 K Surface Temperature

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 If there were no greenhouse gases the Earth’s temperature would be about 0  F (not the 59  F we observe now) The greenhouse effect is real; without it, the Earth would be uninhabitable. Feedbacks amplify the warming by greenhouse gases. The Greenhouse Effect The Greenhouse Effect (The Cause of Global Warming)

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Radiation – visible and invisible Waves (or particles) that transmit energy in a specific direction It’s the only way of transmitting energy through a vacuum Matter emits and absorbs radiation The higher the (absolute) temperature of a body the more radiation it emits (S-B) the shorter the wavelength of the emitted radiation (WDL)

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Electromagnetic Spectrum 255 K

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012

Thermal infrared imaging

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 CH 4 O 2 & O 3 CO 2 H2OH2O N2ON2O methane nitrous oxide oxygen ozone carbon dioxide water vapor

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Warming 1. Greenhouse gases (CO 2, CH 4, N 2 O) CO 2 : Carbon Dioxide : Emission from fossil fuel CH 4 : Methane : Agriculture N 2 O: Nitrous Oxide 2. Land use change Cooling 1.Aerosols Man made/Natural Volcanoes ※ Rate of increase of GHG is largest in 10,000 years (Net) Global Warming

CLIM 101 // Global Warming, Weather, Climate and Society // Fall 2012 Summary The “greenhouse effect”, which results from the absorption and emission of energy by gases in Earth’s atmosphere, helps Earth’s surface maintain a temperature that is within a range suitable for water in all 3 phases and supporting life The gases that produce the greenhouse effect (“greenhouse gases”, GHG) selectively allow solar radiation to pass through and absorb/emit radiation from the Earth The balance of incoming and outgoing energy produces an equilibrium temperature for the planet that would be very different without GHG