1. Global Warming: the Basics CSCI 1210 Fall 2003

2. Dimensions of the Problem Climate science Biological science Technology design Technology policy Global governance

3. The Greenhouse Effect Earth absorbs visible light from the Sun Reflects visible and infrared light Atmosphere is opaque to infrared

4. Modeling Heat Transfer Radiation – direct transfer of heat through space Conduction – as in a metal object Convection – heat carried by mixing fluid

5. Energy-Balance model Energy input from the sun… minus what is reflected into space… … must be emitted as infrared radiation This determines Earth temperature

6. Albedo – a key climate variable Albedo – the fraction of reflected light High albedo for a bright shiny surface Low albedo for a dark surface Albedo effects climate… … but is also affected by climate! (Clouds and snow) Sounds like a feedback loop!

7. Setting up the E-B model Newton’s Law of cooling: Radiated energy varies as the fourth power of absolute temperature Area of the Earth that is directly radiated by the sun = 4  R 2 (area of a circle seen from the sun) Solar Constant = 1370 watts per square meter

8. …And the final answer is: 1370 * 4  R 2 (1-  ) =  T 4 Energy in (left side)… Equals energy out (right side)! Earth temperature T must adjust to balance!

9. What the E-B model tells us Changes in the solar constant would cause Earth temperature to rise and fall We can observe actual changes in the solar constant… … but, these are too small to account for observed global warming

10. One-dimensional model Divide Earth into latitude zones Zones near equator get more heat Zones near poles have higher albedo (icy) Heat flows between zones

11. Hysteresis in climate models Sometimes when we run the 1-D model: Temperature drops from random effects This increases the zones of snow and ice…. … which increases Earth’s albedo… … so Earth absorbs less energy… … and gets stuck in a frozen state! This ‘stuck’ phenonenon is hysteresis

12. The Hysteresis effect Ice Ages could be an example: Earth gets stuck in a cold state Suddenly flips to a warmer state Nobody knows for sure!

13. The effect of clouds A warmer Earth will have more water vapor and more clouds Some clouds cool the Earth (sunscreen effect) Some clouds warm the Earth (blanket effect)

14. Clouds and Feedback Sunshade effect creates negative feedback Blanket effect creates positive feedback Which feedback effect is stronger?

15. Radiative-Convective Models Visible light directly warms Earth surface Each layer absorbs infrared light Each layer is in energy balance Lower layers are warmer

16. RC Models and clouds On a sunny summer day: Lower layers of atmosphere get warmer If the temperature difference gets big enough, air starts to mix (convection) Cumulus clouds begin to form… Maybe we’ll have a thunderstorm!

17. The Big Enchilada: GCM Most sophisticated models Earth broken into 3-D grid cells Run simulation in time steps (really 4-D!) Limited by supercomputer power! Image source: Global Warming: what can we hope to know? By Starley Thompson, Livermore Labs