Astro 202Prof. Jim Bell ([email protected])Spring 2008Lecture 12: The Greenhouse EffectBut first...PDFs for Lectures 7-12 are postedPaper 3 is due nowPaper 4 is now posted onlinePaper 4 is due at beginning of class on Mar. 13No extensions over Spring Break!A review of the end of Lecture 11...3An example of some human climate "help"...The Antarctic Ozone Hole19791998High O3Low O3Why is Ozone important?How did this happen?Ozone Layer• Creation of ozone blocks high energy UV radiation from getting to Earth's surface• Makes life possible!5The Ozone Layer• Destruction of ozone allows more UV to get to the surface:• Health risks!Cl and other O3destroying phases last hundreds of years in the atmosphere!Another example...Atmospheric carbon dioxidemeasured from Mauna Loa, HIWhy is CO2 important?http://en.wikipedia.org/wiki/Image:CO2-Mauna-Loa.pngprevious natural “record”The Earth's Surface TemperatureA simple problem with a confusing answer!!What is the "equilibrium" temperature of Earth?– (What comes in must go out...)–Incoming: Sunlight @ 1368 W/m2 = S0 (solar constant)•Earth "intercepts" an amount = !RE2 • S0• Some fraction, A, is reflected back into space; A = “Albedo” ! 0.3• So the fraction (1-A) is absorbed by the ground and heats it up•Therefore the heat input to Earth is (1-A) • !RE2 • S0 W/m2–Outgoing: An object at temperature T emits "T4 W/m2 • This is the Stefan-Boltzmann Law in Physics•Earth's entire surface emits 4!RE2 • "T4 W/m2–So to balance, (1-A) • !RE2 • S0 = 4!RE2 • "T4 !Solution: T = 255 K = -17°C = 0°F = DAMNED COLD!!So WHY AREN'T EARTH'S OCEANS FROZEN?Answer: The Greenhouse Effect!Some of the infrared (heat) radiation that would normally escape to space is absorbed by certain gases in the Earth's atmosphere!This is the greenhouse effect, responsible for warming the Earth by more than 30°C above its equilibrium temperature!Water vapor (H2O) and carbon dioxide (CO2) are Earth's main greenhouse gases!Without the greenhouse effect, we would not be here!GreenhouseEffectMitchell (1989)Mars #T ~ 5°CEarth #T ~ 35°CVenus #T ~ 500°C!!11Mitchell (1989)12Mitchell (1989)WeakStrongWeakStrongMitchell (1989)• Water Vapor is our major greenhouse gas!• CO2 is in second place, but catching up...• Think about the feedbacks...Mitchell (1989)Radiative Balance:“What comes in, must go out”= 100Feedbacks and Interactions...!The climate is a complex system!How will it react to increasing CO2, H2O?– Complex feedbacks: Clouds, Snow/Ice, Albedo, ...– Uncertainties: “Prehistoric” CO2 levels, role of clouds, “thermal inertia” of the oceans, global temp. record!Active scientific research area...!Political hot-button topic!!More in Lecture 15...16Cosmic Influences on the Biosphere!Impacts have occurred on Earth!These catastrophic events appear to have had serious consequences for life on Earth!– Severe climate changes: "impact winter"– Mass extinctions– Probably influenced the origin of life itself!!Much more detail in next part of the course17Computer Simulation~65 million years ago, a 10-km class asteroid or comet crashed into the Earth, raising a huge cloud of dust that lowered global temperatures for years and probably led to the extinction of the dinosaurs!Taking A Global View of the Earth...19Discussed in Pale Blue Dot, Chapter 5(Top): Global view of Earth in visible light (left), infrared light (middle), and within a water vapor absorption band (right)(Right): Global infrared view of Earth from the Galileo spacecraft during Earth gravity assist flyby:Blue: OxygenWhite: CloudsRed: ChlorophyllViews of Earth as seen from a low-resolution orbiting satellite platform (tens of km resolution)Views of Earth as seen from a higher-resolution orbiting satellite platform (a few km resolution)No obvious signs of life at these scales in the pictures, but there are signs of life detectable in other ways...Spectroscopy of the Earth...• Galileo flyby “experiment” describedby Sagan in PBD...• We will come back to this when discussing extrasolar planets...22Views of Earth as seen from a very high resolution orbiting satellite platform (a few tens of meters resolution)Views of Earth as seen from an ultra high resolution orbiting satellite or an airborne platform (a few meters resolution)We start to see regular, geometric patterns (cities, farms, other structures) at these scales...James Lovelock and the “Gaia” hypothesis...Discover article posted on the course web site...Taking A Global View of the Earth...Summary!The Greenhouse Effect– Some of a planet’s outgoing thermal infrared radiation can be trapped by its atmosphere, further warming the surface!Major greenhouse gases: H2O, CO2, CH4!Life on Earth depends on this!!Earth’s CO2 is increasing dramatically– By itself, this should make the planet warmer...– But the climate system is complex!Next Time... Venus, Earth, & MarsReading: PBD 13, NSS