Greenhouse Gases Layer model assumes atmosphere acts as a blackbody in the IR, absorbing and emitting in all frequencies of light. In reality, gases absorb very selectively. Difference has to do with the effect of molecular vibrations on the electromagnetic field.
Difference in gases Because gases absorb IR selectivelyBecause gases absorb IR selectively Some radiation bands are completely absorbed.Some radiation bands are completely absorbed. Others are ‘atmospheric windows’Others are ‘atmospheric windows’ This leads to higher greenhouse forcing per molecule for some gases.This leads to higher greenhouse forcing per molecule for some gases.
Need a Modification to the Layer Model By looking more closely at gases in atm Concentration – # of molecules within a volume. –Fewer molecules per volume as gas expands. Proportions (%) – easier –O 2 21%, N 2 78%, CO 2 0.038% –CO 2 380 parts per million (ppm) Mixing Ratio – CO 2 380 parts per million (ppm) –CO 2 is rising at a rate of 1.5ppm/yr
4/12/2015 Earth – Range of primary wavelengths being emitted. Sun – Range of visible wavelengths 1. Nitrous Oxide (N2O) 2. Methane (CH4) 3. Ozone (O3) 4. Water Vapor (H2O) 5. Carbon Dioxide (CO2)
Gases, vibration,light Most mass of an atom is in its nucleus. Electrons - orbitals around the nucleus. Nuclei of 2 different atoms repel each other (+ charges). But some electrons of different atoms can fit together in their orbitals. Atoms are held together (like a spring) not too close, not too far. –Too close they repel each other, too far and there is less energy gain from sharing the electrons.
Gases, vibration,light Chemical bond - physical process responsible for the attractive interactions between atoms and molecules, gives stability to chemical compounds.
Gases, vibration,light Gases are the simplest molecules. –Vibrate only in a particular frequency. Vibrations of the major gases in the atm (O 2, N 2 ) are invisible to the electromagnetic field –Transparent to VIS and IR
Gases, vibration,light O2 & N2: These are symmetrical molecules - Invisible because electric field doesn’t change. –only 2 atoms (that are the same) –Not infrared active –So, not gh gases In general, symmetrical molecules with only 2 atoms are not gh gases.
Gases, vibration,light Gas molecules with more then 2 atoms have more than one chemical bond. Water has three normal modes of vibration All 3 are IR active Positivecharge negativecharge 3.0 um 6.0 um
Water - main absorber 13 million tons of water in the atmosphere Water content the atm varies (0 to 4%) ~70% of atmospheric absorption of IR Contributes significantly to greenhouse effect –Ensures a warm habitable planet But also, negative feedback effect, clouds reflect sunlight. http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water
Gases, vibration,light CO 2 - is in a straight line with C in the middle It is a symmetric molecule - both O molecules pull equally. Most climatically important IR Active
How gh gases interact with IR Earth‘s IR (from clouds, atm, and ground)
How gh gases interact with IR Gases are not good blackbodies! Very choosy about what wavelengths they absorb and emit. Blackbody spectra for Earth - T ranges from hot summer day to cold upper atm (model generated data) CH 4 – 1300 cycle/cm (8 m) CO 2 effects outgoing IR more because there’s more IR energy at 700 cycle/cm (14 m) Jagged curve – IR light measured by a spectrophotometer looking down 7.0 m 50 m 10 m 14 m
Optically thick CO 2 bend frequency – absorption in upper atm where T is lower. atm. window Comparison of IR 14 um 11 um
Carbon Dioxide Band Saturation Atm w/no CO 2 Atm w/10 ppm CO 2 Atm w/100 ppm CO 2 Atm w/1000 ppm CO 2 Figure 4.5
Band Saturation - Figure 4.5 Current CO 2 is 380 ppm Figure goes from 0 to 1000 ppm A. atm transparent to IR at 700 (no CO 2 ) B. Add 10 ppm has noticeable change. C. and D. inc. of 100 and 1000 more subtle effect.
That’s what figure 4.6 is showing You start to get less bang for your buck as you get to higher concentrations.
Methane & CO 2 The band saturation for CO 2 means that it is a less potent gh gas than methane. Even though; Methane absorbs in the tail –CO2 is under the curve –Methane has a much lower concentration in the atm. –Methane absorption band not saturated. A molecule of CH 4 is 20x More powerful then a molecule of CO 2
Figure 4.8 - Demo of gh effect A - No CO 2 – –Earth in energy balance – –Ground T 270 K B - Add 1000 ppm – –Dec. outgoing energy flux – –See CO 2 trough C - Ground and atm respond by warming up 8.5 deg K – –New output spectrum has risen – –System rebalances
Exercise #2 Upward IR Heat Flux 289.602W/m 2 IR Heat Loss (Background) 289.29W/m 2 Ground Temperature 299.7K I out
Exercise #2 Upward IR Heat Flux 320.28 W/m 2 IR Heat Loss (Background) 289.29W/m 2 Ground Temperature 299.7K I out 400 ppm Carbon Dioxide
Take Home Points 1.Gases absorb/emit IR energy if they vibrate at the frequency of the IR energy, and if its vibration effects the electric field. –O2 and N2 are not gh gases –All molecules of 3 atoms or more are IR active. 2.A gh gas has a strongest impact on the radiative balance of Earth if it interacts with energy in the middle of the earth energy spectrum. 3.Band saturation: a gh gas at high concentrations will be less effective molecule by molecule than a dilute gas. 4.Converting between wavelength, frequency and wave number - www.cactus2000.de/uk/unit/masswav.shtml
–Keeps Earth ~33 deg C warmer than without –Past century, T inc. by ~0.5 deg C –After 1997 Kyoto Protocol world has finally taken steps in reducing emissions. GH Gases
Kyoto Treaty (1997) Commits industrialized nations to reduce gh gases by ~5.2% below 1990 levels over next decade. Agreement needed to be ratified by countries responsible 55% of world's carbon emissions. Dealt severe blow in 2001 when Pres. George W. Bush announced United States would never sign it.
Reasons for not ratifying Treaty were expressed by U.S. Senate and Clinton Administration in 1997, and Bush Administration in 2001. In 1997, U.S. Senate voted 95-0 that Treaty would not enter Senate floor for ratification. Said ratification would result in harm to American economy and workforce. Loose restrictions imposed on India and China would render treaty ineffective in the long run for goal of lowering total carbon emissions.
–CO2 colorless, odorless, most prominent gh gas –Recycled through atm by photosynthesis –Every year humans add over 30 billion tons of carbon dioxide to atm.
–Mauna Loa (Hawaii) - dataset started by Keeling, 1958. –316 ppm in 1959 to 360 ppm in 1996
Ice core samples - also show inc. in carbon dioxide levels. Ice core samples - also show inc. in carbon dioxide levels.
–Inc. from developing countries are 3x times developed countries. –Middle East 35%, Africa 12%, Eastern Europe 75% from 1990-1995.
Methane Colorless, odorless, flammable. Formed under anaerobic conditions –when plants decay –bacteria - wetlands –bacteria - found in cows, sheep, goats, buffalo, termites, and camels. –bacteria - found in cows, sheep, goats, buffalo, termites, and camels. Since 1750, methane has doubled, could double again by 2050. Since 1750, methane has doubled, could double again by 2050.
Methane Each year humans add 350-500 million tons of methane to atm livestock, coal mining, drilling for oil and natural gas, rice cultivation, and landfills.
Rice cultivation - doubled in the past 45 years. –feeds 1/3 of World's pop. –feeds 1/3 of World's pop. –methanogenesis - bacteria in warm, waterlogged soil releases methane. 50 -100 million tons/yr. possibly biggest anthropogenic source. Solutions –many rice varieties can be grown under much drier conditions –improved varieties of rice, higher yield per acre.
The average cow belches ~250 liters of methane/day.
Nitrous oxide Colorless Released naturally from oceans and by bacteria in soils. Released naturally from oceans and by bacteria in soils. Risen by more than 15% since 1750. Risen by more than 15% since 1750.
Nitrogen based fertilizer use has doubled in past 15 years. Nitrogen based fertilizer use has doubled in past 15 years. –provide nutrients for crops –but, breakdown in soil, nitrous oxide released into atm –but, breakdown in soil, nitrous oxide released into atm –Automobiles - released at lower rate than carbon dioxide, more carbon in gasoline than nitrogen. –Other sources, animal manure, sewage treatment –Also produced naturally from a wide variety of biological sources in soil and water microbial action in wet tropical forests.
Fluorocarbons Fluorocarbons General term for any synthetic organic compounds with fluorine and carbon. General term for any synthetic organic compounds with fluorine and carbon. –easily converted from liquid to gas. –easily converted from liquid to gas. –used in aerosol cans, refrigerators, air conditioners. –used in aerosol cans, refrigerators, air conditioners. 1970s - CFCs linked to ozone layer. 1970s - CFCs linked to ozone layer. –use decreased, banned from production in U. S. Sub for CFCs are hydrofluorocarbons (HFCs). –doesn’t harm ozone, but is a gh gas.
Gas molecules with 3 or more atoms are greenhouse gases Water vapor (H 2 O), ozone (O 3 ), carbon dioxide (CO 2 ), and methane (CH 4 ). Also, trace quantities of chloro-fluoro-carbons (CFCs) can have a disproportionately large effect.
The 3 most powerful greenhouse gases are; –Water vapor 36-70% –Carbon dioxide 9 - 26% –Methane 4 – 9% Misc trace gases: –Ozone, N20, CFCs (HFCs) N 2 and O 2 major atmospheric constituents not greenhouse gases. –don’t absorb or emit IR radiation –no change in these molecules when they vibrate.
4/12/2015 Earth – Range of primary wavelengths being emitted. Sun – Range of visible wavelengths Main absorbers in atm: 1. Nitrous Oxide (N2O) 2. Methane (CH4) 3. Ozone (O3) 4. Water Vapor (H2O) 5. Carbon Dioxide (CO2)