Radiation Balance of the Earth-Atmosphere System In Balance: Energy flow in = Energy flow out PowerPoint 97 To download: Shift LeftClick Please respect copyright on this material

Reaction Topic u What would be the impacts on life in Wisconsin if the climate were 10 o F warmer year round?

Key Points u Climate depends on radiation from sun –Sun higher above horizon in summer => more radiation per unit area of earth surface –Atmosphere mostly transparent to solar –Surface heating reduced by clouds & snow u Because CO2 acts like a blanket, surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level) u Increased atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Ocean heat storage in ocean takes decades to come to new balance u For given CO2 low clouds cool, high clouds warm

The Seasons u Equinox u Summer Madison

The effect of distance from the sun u Assumed mathematics –Sphere of radius r F Surface area = 4  r 2 –Disc of radius r F One sided area =  r 2 u Intensity = Power received by object divided by the area of object = Energy flow / unit surface area

Radiation from the Sun r R S Earth Sphere Sun u Intensity over Earth = 1368  r 2 / (4  r 2 ) = 342 W m -2 (averages time of day & latitude) u Intensity direct from sun = S / (4  R 2 ) = 1368 W m -2 (the inverse square law)

How Strong is the Sun at Mars? u S = total power emitted by Sun – output of nuclear furnace deep inside u R = distance of Earth from Sun u Distance of Mars from Sun = 1.4 R u Direct Intensity = S / {4  (1.4 R) 2 } = 1368 / (1.4) 2 W m -2 = 698 W m -2 u Sunlight is half as strong on Mars

Electromagnetic Spectrum Visible Near Infrared Infrared 10,000 Microwave X-rays Ultraviolet.0001 Wavelength (microns)

Opacity of Atmosphere (no clouds) u Horel & Geisler Chapter 3 u Fraction of radiation absorbed by entire atmosphere (mostly in the troposphere) From Sun From Earth 0 % 50 % 100 % Wavelength (micrometers)

Key Points u Climate depends on radiation from sun u Because CO2 acts as a blanket, surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level) u Increased atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Because of heat storage in the ocean, the system to takes decades to come to new balance u For given CO2 low clouds cool, high clouds warm

Solar Radiation Budget 100% 342 W/m % 105 W/m % 237 W/m 2 Reflected Incoming Absorbed u Earth as a whole

The Effective Radiating Level ERL u ERL: The lowest level in the atmosphere from which infra red radiation is able, on average, to escape upwards to outer space without being reabsorbed u As concentrations of absorbing gases such as CO 2 are increased, the ERL rises, decreasing the total mass of air above and keeping the opacity of that air constant. u The intensity of the radiation emitted to space increases with the temperature at the ERL

Temperature and Altitude u Adapted from H & W Figure Stratosphere Troposphere 5% mass 20% mass 75% mass Tropopause 10 km 20 km 30 km 40 km 200 K250 K300 K Convection & clouds Little Mass to Absorb Radiation Slow Overturning (years)

Key Points u Climate depends on radiation from sun u Surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed) u Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Ocean heat storage in ocean takes decades to come to new balance u For given CO2 concentration, low clouds cool, high clouds warm

Absorbent Atmosphere Temperature ERL Infrared Solar u ERL =Effective Radiating Level Solar in = IR out H2O, CO2,... Tropopause

Key Points u Climate depends on radiation from sun u Surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed) u Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Ocean heat storage in ocean takes decades to come to new balance u For given CO2 concentration, low clouds cool, high clouds warm

Greenhouse Warming: a simple model u Hold absorption of incoming solar radiation radiation fixed u Infrared radiation leaves earth for space from upper troposphere (ERL). Amount increases with temperature at ERL (immediate). Height of ERL is such that total CO2 above it is constant. u Additional carbon dioxide mixes rapidly in troposphere (weeks) –ERL rises to where temperature is lower, less outgoing radiation., u Earth surface+ troposphere warms till outgoing radiation from ERL balances incoming (years to centuries)

Effects of Enhanced CO2 initial change in radiation Temperature ERL Infrared Solar u ERL =Effective Radiating Level Solar in = IR out H2O, CO2,... Tropopause

Effects of Enhanced CO2 after rebalance Temperature ERL Infrared Solar u ERL =Effective Radiating Level Solar in = IR out H2O, CO2,... Tropopause

Key Points u Climate depends on radiation from sun u Surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed) u Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Because of heat storage in the ocean, it takes decades to come to new balance u For given CO2 concentration, low clouds cool, high clouds warm

How long to achieve balance? u Land surface & atmosphere ~ 1 month u Ocean surface layers ~ decades u Deep ocean ~ millenia

Key Points u Climate depends on radiation from sun u Surface and lower atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed) u Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate u Heat storage in ocean takes decades to come to new balance u For given CO2 concentration, low clouds cool, high clouds warm

Effects of Low Clouds u reflect sunlight (to space as well as to your eyes) u reduce absorbed solar radiation u little effect on outgoing infrared u tend to cool the earth

Effects of High Clouds u reflect little sunlight (hard to see) u are much colder than earth surface u absorb and re-emit outgoing infrared like a greenhouse gas u tend to warm the earth

Earth from Space u NASA u Which wins?

Sources of Information u Horel & Geisler Chapter 2