Astro 2037 Spring 07, Lecture 16 Greenhouse Effect Causes Temperature Increase on both Earth and Venus Greenhouse Effect: the trapping of infrared radiation by a planet’s atmosphere (mainly by CO2 and H2O absorption and re-radiation) Water, Carbon Dioxide, Methane and Other Molecules Have Strong Absorption Bands in the Infrared for Absorbing Infrared Photons 20 m 10 m The greenhouse effect on Earth is SELF – REGULATING ! Carbonate – Silicate Cycle ( Self- regulating mechanism ) Temperature Increases More Water Vapor (moisture) in the atmosphere Less CO2 removed Increase in Rainfall and runoff because of reduced weathering Increased weathering More CO2 removed from Decrease in the atmosphere amount of rainfall Less Water Vapor Temperature drops due to in the atmosphere reduced greenhouse effect Comparisons of Actual Temperatures and “No Greenhouse” Temperatures for Venus, Earth, and Mars Runaway Greenhouse Effect Causes Venus to be Extremely Hot (Inhabitable!) • Proximity of Venus to the Sun and large amounts of CO2 causes the global temperature to be higher than Earth. Water is vaporized and absorbs additional solar radiation to further increase its temperature. Eventually, all of water becomes vaporized. This phenomenon is called a Runaway Greenhouse effect • Water vapor rises high enough to be broken apart (photo-dissociated) by high energy ultraviolet radiation from the sun The hydrogen atoms would escape from the planet’s gravitation field since they have so little mass, while the heavier oxygen atoms would remain behind. Illustration of how to turn the Earth into Venus by placing it at at the Venus’s distance, the runaway greenhouse effect causes the oceans to evaporate completely On Earth, CO2 content in the air is regulated by the Carbon Dioxide Cycle (the carbonate-silicate cycle, or the inorganic carbon cycle) to avoid runaway Greenhouse effect • The cycle is about 400,000 years • The CO2 cycle acts as a thermostat • If the earth warms up slightly, carbonate minerals form in the oceans at a more rapid rate and takes away more CO2 Limestone rocks The Boundary of the Sun’s Habitable Zone Today Global Temperature Increase may Cause a Runaway Greenhouse Effect!