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The solar spectrum compared to a black body. Blackbody radiation curves typical for the Sun and Earth. Sun ~6000K. Earth ~290K. Sun radiates a lot more energy that the Earth!. Normalized blackbody radiation curves for Earth and Sun. Divide each radiation curve by its maximum value - PowerPoint PPT Presentation
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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