The solar spectrum compared to a black body

Sun ~6000K

Sun radiates a lot more energy that the Earth!

Earth ~290K

Blackbody radiation curves typical for the Sun and Earth

Normalized blackbody radiation curves for Earth and Sun

• Divide each radiation curve by its maximum value to normalize curves:

• Very little overlap of the normalized radiation curves

How steady is the Sun’s output?

• Measurements of solar radiation from space, rockets, and balloons

• Note on short timescales, some large fluctuations are possible.

IPCC, 2001C. FRÖHLICH

Solar variability: the sunspot cycle

Reconstructions of solar variability over time

IPCC, 2001Note the scientific trend too…

TSI = total solar irradiance

How do we get these temperatures?-Infrared temperatures from Aqua satellite, April 2003.

Reflectivity (albedo) of Solar (shortwave) radiation

Global average ~30%Albedo increases with latitudeOceans are quite dark (low reflectivity)

smsc.cnes.fr/IcPARASOL

September, 2005

Emissivity of infrared radiation at the surface

Emissivity/absorptivity is close to 1. This implies a good approximation to black body in the infrared

cimss.ssec.wisc.edu/iremis/

Emissivity, is a measure of how well blackbody radiation is obeyed:F=T4

Summary (important)-

At visible wavelengths, the Earth reflects about 30% of the incident radiation.

At infrared wavelengths, most natural materials absorb almost Everything (~95 to 98%), so the Earth behaves quite closely as atrue blackbody.

-Go to calculation of black body temperature

Radiation and physical objectsAny physical material (solid, liquid, gas) interacts with electromagnetic waves (radiation) in one of four different ways.

TRANSMISSION: waves pass through the material

ABSORPTION: some of the waves are absorbed (& heat)

REFLECTION: some of the waves are reflected in the direction they came from.

EMISSION: Every object (above absolute zero) emits radiation because it possesses thermal energy

Less important:-

SCATTERING: waves are deflected (hence blue sky…)

Radiation and physical objects

How a material interacts with radiation (transmission, absorption,emission, reflection) depends on what it is made of.

For example: what’s the difference between the yellow light in these 3 pictures?

A key fact for Earth’s climate is that gases in the atmosphere absorb radiation.

• Molecules absorb radiation at particular wavelengths, depending on amount of energy required to cause vibration or rotation of atomic bond.

• Two essential things for the greenhouse effect:

– The Earth’s atmosphere is mostly transparent to visible radiation (why not totally)

– The Earth’s atmosphere is mostly opaque to infrared radiation.

The composition of the Earth’s atmosphere matters...

(Plus other trace components, e.g. methane, CFCs, ozone)

• Bi-atomic molecules (O2, N2) can only absorb high energy photons, meaning ultraviolet wavelengths and shorter.

• Tri-atomic molecules (H2O, CO2) can absorb lower energy photons, with wavelengths in the infrared

CH4

N20

O2,03

CO2

H20

Atmospheric absoption by atmospheric constituents

Peixoto and Oort, 1992

solar &terrestrialemissions as a function of wavelength

100%-0%-

Key things from previous slide:-

- Atmosphere mostly transparent to solar radiation (except in uv)

- Atmosphere mostly opaque to terrestrial radiation (infrared)

- Water vapor is the most important greenhouse gas (by far)

- Carbon dioxide is a problem because of a ‘window’ in H2O absorption spectrum.

This physics is very, very well known

Atmospheric absorption

• Shortwave (i.e. solar) radiation measured from the top of atmosphere and from the ground.

Peixoto and Oort, 1992

The (clear) atmosphere is not totally transparent to solar radiation:

- back scatter by dust, aerosols

- absorption by constituent gases

- amount varies as a function of wavelength

IPCC, 2007

Energy pathways in the atmosphere

This is wrong – why?

Greenhouse effect summary

• CO2 and H20 (and some other gasses) effectively absorb radiation at the same wavelengths that the Earth

emits at.

• Some of that radiation is then re-emitted back towards the ground keeping the surface warmer than it would otherwise be.

Essential to remember: - CO2 , H20 in the atmosphere absorbs and re-emits infrared

radiation - It does NOT (not, not, not) reflect radiation