Earth’s radiation budget

We have learned about the greenhouse effect. Now, with the greenhouse effect in mind, lets dive a little deeper and see what is happening in the atmosphere.

The Structure of the Atmosphere

Temperature varia3ons define the atmosphere�s four principal layers: the troposphere, stratosphere, mesosphere, and exosphere. The ozone layer is within the stratosphere.

This picture represents the temperature profile of the atmosphere. As you will recall

from Week 2’s lecture, most gas molecules are within the lower atmosphere,

specifically, the troposphere. Within the troposphere, temperatures decrease with

increases in altitude. This is because incoming solar energy is absorbed by the Earth, a

blackbody, and back radiation from the Earth heats up the atmosphere (Gas molecules

in the atmosphere can be blackbodies, however, they are not as effective at absorbing

and maintaining heat as the solid Earth). Thus, temperature vertically decreases as

you move away from the heat source (the Earth).

Interestingly, there is an inversion within the stratosphere. Temperature increases

when you go higher in altitude. This makes stratosphere a very stable environment.

Warmer, therefore less dense, air sits on top of colder, therefore denser, air. With this,

the atmosphere in the stratosphere is very difficult to disturbed (thus stratified). This is

mainly because of the ozone layer, a layer that contains a relatively high concentration

of ozone, that exists within the stratosphere. As you learned in

�Geograph110_6_Greenhouse Effect III�, ozone in the stratosphere absorbs mainly incoming shortwave energy. This is good for us because the ozone layer blocks ultra

violet light and other shortwave energy that are harmful to living organisms. This

absorbed incoming energy keeps the stratosphere warmer with increases in altitude.

The Atmosphere Screens Earth from Harmful Solar Radiation

Shorter wavelength gamma and X-ray

radiation and large amounts of infrared

radiation are completely absorbed by the atmosphere.

The ozone layer absorbs the most

harmful ultraviolet wavelengths. Only radio waves, visible

light, and some ultraviolet radiation

reach Earth�s surface relatively unimpeded.

Portion of the electromagnetic spectrum

A schematic image showing incoming and outgoing radiation and their interference within the atmosphere. Purple bars represent incoming shortwave energy and red bars are outgoing longwave energy. As you can see, the majority of shorter wavelength energy is absorbed within the atmosphere and never reaches the surface. Also, part of the outgoing back radiation is absorbed in the atmosphere due to greenhouse gases.

Solar Radiation

Infrared radiation: Wavelengths longer than 780 nm. Quickly absorbed and converted to heat in the upper few meters of a body of

water.

Ultraviolet radiation: (< 380 nm) Forms only a small fraction of total radiation.

Usually rapidly scattered and absorbed, except in the clearest body of water.

Visible spectrum :(400-700nm) � Penetrates deeper into the sea. ParJcularly important for

animals with vision.

� Approx. the same wavelength as used by plants for

photosynthesis, so oLen called

photosyntheJcally acJve

radiaJon (PAR).

Absorption Spectra of

Greenhouse Gases

The shapes of the blackbody

spectra of Earth and the sun

Percentage of radiation

absorbed through the atmosphere

Here is a re-cap of what we leaned:

• The radiation emitted by a blackbody has a characteristic wavelength distribution that depends on the body’s absolute temperature (the Earth’s

blackbody radiation = infrared wavelength).

• “Percentage of radiation absorbed through the atmosphere”, absorption of 100 % means that no radiation penetrates the atmosphere. CO2, O3, N2O,

CH4, H2O are the media that absorb associated wavelength energy – and we

now know that these media are called greenhouse gases!

Outgoing spectrum of the Earth with an atmosphere

This figure shows the Blackbody spectrum for objects with temperatures ranging from 300 K (surface temperature) on a hot summer day, down to 220 K, about the coldest it gets in the atmosphere, up near the troposphere at about 10-km altitude. The jagged-looking curve (denoted as “Atmosphere”) is a model-generated spectrum of infrared light escaping to space from the top of the atmosphere. This is jagged-looking, because CO2, water vapor, ozone, and methane absorb specific wavelengths of outgoing energy emitted from the ground.

MODTRAN MODEL Please visit the following interactive Modtran Model site, developed by David Archer

of Chicago University to explore the Earth’s outgoing spectrum.

hGp://climatemodels.uchicago.edu/modtran/

So what would the Earth’s surface temperature look like from space if

the Earth had no atmosphere?

Outgoing spectrum of the Earth With an atmosphere

270 K

Without an atmosphere, more energy will be radiated due to an absence of the greenhouse effect. In fact, the outgoing spectrum will look like a blackbody spectrum at 270 K (= -3 C�, 26.6 F), between the 260 K and 280 K spectra shown in this figure.

Greenhouse Effect What is the hottest planet in the solar system?

H2, He, O2

CO2, N2, H2O, SO2

The ho&est planet in the solar system?

Mercury – closest to the Sun – is not the ho&est. Very thin atmosphere composed of H2, He, O2 – no greenhouse gases. The temperature is 426C during the day, -173C in the shadow.

The atmosphere of Venus is very thick. It is composed of CO2 gas (96%), with some nitrogen (3%) and a very small amount of water vapor (0.003%). Venus also has a thick layer of sulfuric acid clouds. Although Venus is much further away from the Sun, due its thick atmosphere made up of greenhouse gases, Venus remains the same temperature no ma&er where you go on the plant; at the North Pole, day or night: 461C

Mercury is hot, but Venus is ho&er (greenhouse effect)!

A greenhouse gas is classified as any gas that:

A. traps visible rays and thereby promotes global warming.

B. traps gamma rays and thereby reduces global warming.

C. traps infrared rays and thereby promotes global warming.

D. traps infrared rays and thereby reduces global warming.

Answer is C!

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