Radiation Balance of the Earth-Atmosphere System

In Balance:

Energy flow in = Energy flow out

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What would be the impacts on life in Wisconsin if the climate were 10oF warmer year round?

Key Points 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 Because CO2 acts like a blanket, surface and lower

atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level)

Increased atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Ocean heat storage in ocean takes decades to come to new balance

For given CO2 low clouds cool , high clouds warm

The Seasons

Equinox Summer

Madison

The effect of distance from the sun

Assumed mathematics– Sphere of radius r

Surface area = 4 r2

– Disc of radius r One sided area = r2

Intensity = Power received by object divided by the area of object = Energy flow / unit surface area

Radiation from the Sun

rR

S

Earth

Sphere

Sun

Intensity over Earth = 1368 r2 / (4 r2) = 342 W m-2

(averages time of day & latitude)

Intensity direct from sun = S / (4 R2) = 1368 W m-2

(the inverse square law)

How Strong is the Sun at Mars?

S = total power emitted by Sun – output of nuclear furnace deep inside

R = distance of Earth from Sun Distance of Mars from Sun = 1.4 R Direct Intensity = S / {4 (1.4 R)2}

= 1368 / (1.4)2 W m-2 = 698 W m-2

Sunlight is half as strong on Mars

Electromagnetic Spectrum

.1 .5 1 5 10

Vis

ible

Nea

r In

frar

ed

Infr

ared

10,000

Mic

row

ave

X-r

ays

Ultr

avio

let

.0001

Wavelength (microns)

Opacity of Atmosphere (no clouds)

Horel & Geisler Chapter 3

Fraction of radiation absorbed by entire atmosphere (mostly in the troposphere)

From Sun From Earth

0 %

50 %

100 %

20151050.70.50.30.10.07

Wavelength (micrometers)

Key Points Climate depends on radiation from sun Because CO2 acts as a blanket, surface and lower

atmosphere cool by infrared radiation to space from upper troposphere (ERL= effective radiating level)

Increased atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Because of heat storage in the ocean, the system to takes decades to come to new balance

For given CO2 low clouds cool , high clouds warm

Solar Radiation Budget

100% 342 W/m2

30.7% 105 W/m2

69.3% 237 W/m2

Reflected

Incoming

Absorbed

Earth as a whole

The Effective Radiating Level

ERL 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

As concentrations of absorbing gases such as CO2 are increased, the ERL rises, decreasing the total mass of air above and keeping the opacity of that air constant.

The intensity of the radiation emitted to space increases with the temperature at the ERL

Temperature and Altitude

Adapted from H & W Figure

Stratosphere

Troposphere

5% mass

20% mass

75% mass

Tropopause10 km

20 km

30 km

40 km

200 K 250 K 300 K

Convection & clouds

Little Mass to Absorb RadiationSlow Overturning (years)

Key Points Climate depends on radiation from sun Surface and lower atmosphere cool by infrared

radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed)

Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Ocean heat storage in ocean takes decades to come to new balance

For given CO2 concentration, low clouds cool , high clouds warm

Absorbent Atmosphere

Temperature

ERL

InfraredSolar

ERL =Effective Radiating Level

Solar in = IR out

H2O, CO2, ...

Tropopause

Key Points Climate depends on radiation from sun Surface and lower atmosphere cool by infrared

radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed)

Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Ocean heat storage in ocean takes decades to come to new balance

For given CO2 concentration, low clouds cool , high clouds warm

Greenhouse Warming: a simple model

Hold absorption of incoming solar radiation radiation fixed

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.

Additional carbon dioxide mixes rapidly in troposphere (weeks) – ERL rises to where temperature is lower, less

outgoing radiation., Earth surface+ troposphere warms till outgoing

radiation from ERL balances incoming (years to centuries)

Effects of Enhanced CO2initial change in radiation

Temperature

ERL

InfraredSolar

ERL =Effective Radiating Level

Solar in = IR out

H2O, CO2, ...

Tropopause

Effects of Enhanced CO2after rebalance

Temperature

ERL

InfraredSolar

ERL =Effective Radiating Level

Solar in = IR out

H2O, CO2, ...

Tropopause

Key Points Climate depends on radiation from sun Surface and lower atmosphere cool by infrared

radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed)

Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Because of heat storage in the ocean, it takes decades to come to new balance

For given CO2 concentration, low clouds cool , high clouds warm

How long to achieve balance?

Land surface & atmosphere ~ 1 month

Ocean surface layers ~ decades

Deep ocean ~ millenia

Key Points Climate depends on radiation from sun Surface and lower atmosphere cool by infrared

radiation to space from upper troposphere (ERL= effective radiating level, such that total CO2 above is fixed)

Increased concentration of atmospheric CO2 raises ERL, reducing outgoing IR, until troposphere has warmed to compensate

Heat storage in ocean takes decades to come to new balance

For given CO2 concentration, low clouds cool , high clouds warm

Effects of Low Clouds

reflect sunlight (to space as well as to your eyes)

reduce absorbed solar radiation little effect on outgoing infrared tend to cool the earth

Effects of High Clouds

reflect little sunlight (hard to see) are much colder than earth surface absorb and re-emit outgoing infrared like

a greenhouse gas tend to warm the earth

Earth from

Space

NASA

Which wins?

Sources of Information

Horel & Geisler Chapter 2