The Greenhouse Effect

Solar Radiation, Earth's Atmosphere, and the Greenhouse Effect.

Nili HarnikDEES, Lamont-Doherty Earth Observatory

nili@ldeo.columbia.edu

Temperature• All matter is made up of atoms, which are in constant

jiggling motion. • Temperature is a measure of the energy which is stored

in these motions – heat energy. • The larger the temperature, the larger the motion of

atoms and molecules. • The temperature scale which defines the amount of this

motion is the Kelvin (absolute temperature). Absolute zero is the state with the least possible energy (almost no motion).

• The Celsius and Fahrenheit temperature scales are defined based on water boiling and freezing points.

Temperature Scales

Always use K (Kelvins) in Radiation calculations

0 K = -273 0C0 0C = 273 K1 K = 1 0CT(0C)=[ T(0F)-32]*5/9

Three Aspects of Radiationinteracting with matter

Transparency - the radiation is unchanged when passing through the medium.

Reflection – the radiation changes direction when it hits the reflecting medium but otherwise remains the same.

Absorption – some or all the radiation energy is transferred to the medium. This energy may take a variety of forms, but in general the temperature of the absorbing medium will rise. In a state of equilibrium, this energy will be re-emitted, but the wavelength of the emitted radiation will be determined by the absorbing/emitting medium.

Absorption of radiation by matter may result in:

An increase in the vibrational energy of the molecules of the absorbing medium if it is a solid (temperature rises).Increased molecular velocities, if the absorbing medium is a gas.A chemical change, if the radiant energy is of sufficiently high energy (high frequency) to break chemical bonds or change the energy level of electrons.

Radiation absorption by a gasEnergetic radiation (UV) can break apart the

molecules.

Infra-red radiation: can excite rotational and vibrational modes, depending on the nature of the molecule. Asymmetric molecules, with an electric dipole (H2O), will absorb more efficiently.

Each mode of energy absorption occurs at a specific narrow band of the solar spectrum.

Composition of the Atmosphere

The Earth's atmosphere contains many trace (or minor) components

Interaction of solar radiation with the atmosphere:

• Reflection: about 20% is reflected by clouds and 5% by the surface, and about 5% is scattered (reflected in many directions) by the atmosphere.

• Absorption: The most energetic radiation is absorbed high up in the atmosphere, by diatomic oxygen (and nitrogen), resulting in chemical changes – breakup of diatomic oxygen to form ozone, and breakup of ozone to form diatomic oxygen again.

• Transmission: most of the solar radiation, which is in the visible range, is transmitted through the atmosphere. This radiation is absorbed by the Earth.

•The major atmospheric components, Nitrogen and Oxygen, absorb little or not radiation. Oxygen absorbs only the most energetic solar radiation (UV). •Water vapor absorbs effectively in large sections of the IR wavelength range. CO2 absorbes in the IR. One of the CO2

absorption bands overlaps a gap in the water vapor band (referred to as “window”). By closing the window, CO2 increases the absorptive power of the atmosphere. Hence its importance in the climate system.

Greenhouse Effect:Absorption by trace gases

Greenhouse Effect:Absorption by trace gases

Absorption by trace gases influences the atmosphere’s temperature

O2 + ultraviolet light = O + O

O + O2 = O3

O3 + UV light = O2 + O

Effective Temperature

Greenhouse Effect

How big is the greenhouse effect on our planet ?Lets do a simple calculation.

Lets assume the atmosphere is entirely transparent to the incoming solar flux,and completely absorbs the radiation emitted by the planet…

X X

Absorption of IR radiation in the atmosphere:

• The IR radiation emitted by the Earth gradually gets absorbed as it propagates upward.

• The altitude at which all the radiation gets absorbed is referred to as a unit optical depth.

• The radiation absorbed by the atmosphere is remitted back in all directions.

• To simplify matters, we approximate the continuous atmosphere by a thin absorbing sheet, which absorbs all the IR it receives from below, and reemits half of it upward and half downward.

• The reemitted radiation can get reabsorbed again before it leaves the atmosphere (an optical depth of 2).

• This would correspond to two absorbing sheets stacked one on top of the other, each absorbing all the radiation it recieves and remitting it equally in both directions.

Effect of Atmospheric Absorption: 1 sheetw/o absorption w. absorption

ground

α

(1-α)

atmosphere

S04

σΤg4

absorption of solar emission of IR

S04

S04

ground

α

atmosphere

σΤg4IR

absorbinglayer

H=σΤa4

H(1-α)

S04

S04

S04

= S04(1-α)σΤa

4

σΤg4 = S0

4(1-α) + σΤa4

σΤg4 = S0

4(1-α)

Effect of Atmospheric Absorption: 1 sheet

w/o absorption w. absorption

= S04(1-α)σΤa

4σΤg4 = S0

4(1-α)

Τg = S04σ(1-α)

4

= 239 Wm-2

σΤg4 = S0

4(1-α) + σΤa4

σΤg4 = S0

2(1-α)

Τg =S02σ(1-α)

4

The Greenhouse

Effect

W/O absorption: Tg=255 K or -18°C W. absorption Tg=303.5 K or +30.5°C

Radiative Transfer in the Atmosphere Reality is much more complex:

Some solar energy is absorbed in the atmosphere.IR absorption happens continually through the troposphere but is not perfect. The surface looses heat not only through radiation, but also through convection (the transfer of heat through turbulent fluid motion), which moves warm and moist air masses from the surface up causing surface sensible and latent heat loss (more on that at later lectures). All this yields a surface temperature of Tg =(390/σ)1/4=288 K=15°C

greenhouse effect=390-235=155 Wm-2

Annual mean latitudinal distribution of incoming shortwave and outgoing longwave radiation

Rad

iatio

n W

m-2

The total longwave and shortwave radiations are equal in the global average.

There is an excess of incoming radiation in the tropical and subtropical regions and a deficit thereof in the middle and high latitudes.

Atmospheric motions transfer heat from the equator to the poles.

Summary• The greenhouse effect results from atmospheric absorption of IR (longwave) radiation

emitted from the Earth’s surface. • IR absorption occurs in broad spectral bands related to molecular motion and energy

levels. • The major constituent of the atmosphere, Nitrogen (78.01% by volume in dry air), is

transparent to both short- and longwave radiation. Oxygen (20.95% by volume) absorbs mainly shortwave (UV) radiation.

• It is the minor atmospheric constituents, mainly water vapor (about 0.33% of total atmospheric mass and 0.48% by volume) and CO2 (0.035% by volume) that efficiently absorb the longwave radiation from the Earth’s surface.

• Longwave absorption by the atmosphere strongly reduces the efficiency of Earth’s cooling to space, forcing its surface to “overheat” in order to balance the incoming solar radiation. The end result is a higher surface temperature than the “emission”temperature of 255 K).

• At present conditions, the earth “has to” emit 350 Wm-2 to allow 235 Wm-2 to escape to space and balance the 235 Wm-2 received from the sun. This results in an average surface temperature of 288 K or 15°C