Some resources (more websites later) Intergovernmental Panel Climate Change 2001: The Scientific Basis at http://www.ipcc.ch/pub/reports.htm John Houghton Global Warming - the complete briefing Cambridge UP, 2nd edition 1997 Spencer Weart The discovery of Global Warming Harvard UP, 2003 James Hansen (NASA) Defusing the Global Warming Time Bomb Scientific American, March 2004, p.68
What s in the atmosphere? Changing! What s missing? H 2 O highly variable but very important
Some Properties Nitrogen N 2 inert Oxygen O 2 important for incoming radiation Noble gases Ar, Ne, He, Kr inert Hydrogen H absent: escapes (too light) Ozone O 3 special in stratosphere (see later) Carbon Dioxide CO 2 important for outgoing radiation Methane CH 4 same as CO 2 Water H 2 O important for in and outgoing radiation
Radiation Issues R = s T 4 Spectrum different at each T Electromagnetic waves Speed c = 3 x 10 8 m/s (speed of light) Frequency: # of times and Ø per second In vacuum c fixed: c = wavelength x frequency = l x f Higher frequency fi more energy http://chemistry.beloit.edu/ BlueLight/waves/
Types of Electromagnetic Radiation Infrared 10-3 m > l > 0.75 mm Visible 0.75 mm > l > 0.4 mm Ultraviolet 0.4 mm > l > 12 x 10-9 m 60 Hz fi 60 s -1 fi l ~ 5,000 km (Power Stations)
Blackbody Radiation Note: Different types of scales Top and bottom of the atmosphere! Role of different gases!
Outgoing Radiation Infrared role of CO 2 N 2 O nitrous oxide CH 4
Heat Transport Less dense warm air More dense cold air fl Dominates < 10 km Temperature decreases by about 6 C per km Some windows transparent others absorbed by CO 2, H 2 O, Can also emit; but at lower T! (see previous slide)
Blanket effect Greenhouse gases Cold Warm Earth s surface
Radiation Budget Albedo Components of the radiation (in watts per square meter) which on average enter and leave the Earth s atmosphere and make up its radiation budget. Problem is imbalance due to extra greenhouse gases
Natural Greenhouse Effect vs Enhanced H 2 O CO 2 temperature of the oceans evaporation at surface not directly influenced by human activity (but indirectly) different since industrial revolution more than 30% increase contributes to global warming (enhanced greenhouse effect)
CO 2 2x? Feedbacks: positive fi add to warming negative fi reduce warming Best estimate ~ 2.5 C (not 1.2 C)
Examples Higher T fi more H 2 O fi more greenhousing Less ice cover fi less albedo fi more gh effect And so on Note: calculated Greenhouse effect for Mars & Venus coincide with the experimental observations
CO 2 and the carbon cycle Reservoirs Oceans ~ 40,000 gigatonnes (Gt) Land biota ~ 2,000 Gt Fossil fuels ~ 5,000-10,000 Gt Atmosphere ~ 780 Gt (in 2000): net addition ~3.3Gt/yr Processes Photosynthesis Respiration & Decomposition Physical & chemical processes Deforestation Fossil-fuel use Emitted CO 2 distributed among reservoirs Unlike other greenhouse gases, it is not destroyed
Picture
Keeling curve Steady concentration for thousands of years before industrial revolution (ice core measurements) within about 10 parts per million of a mean value of about 280 parts per million by volume (ppmv) Mid fifties Keeling started to make really accurate measurements fi now captured by the so-called Keeling curve Adding 1.5 ppm each year!
Other greenhouse gases (infrared absorbers) Gas Concentration Growth Rate Greenhouse Warming (ppm) (%/yr) Potential (relative to CO 2 ) CO 2 ~380 0.3 1 CH 4 ~1.75 1.1 11 N 2 O ~0.32 0.25 270 CFCs flat for now CFC-11 2.4 x 10-4 3400 CFC-12 4.15 x 10-4 7100 HCFC-22 1.00 x 10-4 1600
Radiative Forcing Radiation imbalance (example: doubling of CO 2 discussed earlier) Expressed in W/m 2 Distinguish Anthopogenic forcing of the climate Natural forcing of the climate Slide from IPCC (Intergovernmental Panel on Climate Change) 2001
Climates from the past From ice core data (see Weart book)
Changes in Earth s orbit relevant for ice ages Hansen article
Calculating the climate / weather Numerical models of climate/weather based on Fundamental mathematical equations describing the physics and dynamics of the movements and processes taking place in the atmosphere, the oceans, the ice and on land with input from empirical information (starting values)
Model Equations OK Limitations: sensitivity to initial conditions fi weather not accurate enough fi average weather (climate) OK
Feedbacks Water vapor warmer atmosphere more evaporation fi positive feedback Cloud-radiation reflect part of incoming solar radiation blankets to thermal radiation Which dominates depends on T of cloud Low clouds High clouds cool heat
More feedbacks Ocean-circulation oceans are very important Source of H 2 O in the atmosphere Absorb heat (warm more slowly than atmosphere) Circulation distributes heat (necessary for regional effects) Imbalance takes ~100 yrs to disappear Ice-albedo Positive feedback All must be included in: Global Climate Models Circulation
Illustration
Grids
Initialization