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, 2001 C. FRÖHLICH

Solar variability: the sunspot cycle

Reconstructions of solar variability over time IPCC, 2001 Note 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 latitude Oceans 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=  T 4

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 a true blackbody. -Go to calculation of black body temperature

Radiation and physical objects Any 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 (O 2, N 2 ) can only absorb high energy photons, meaning ultraviolet wavelengths and shorter. Tri-atomic molecules (H 2 O, CO 2 ) can absorb lower energy photons, with wavelengths in the infrared

CH 4 N 2 0 O 2,0 3 CO 2 H 2 0 Atmospheric absoption by atmospheric constituents Peixoto and Oort, 1992 solar & terrestrial emissions 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 H 2 O 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 CO 2 and H 2 0 (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: - CO 2, H20 in the atmosphere absorbs and re-emits infrared radiation - It does NOT (not, not, not) reflect radiation