EXPLAIN 2. Earth’s Energy Balance. Energy Balance Let the rate of energy flow from the Sun to the...
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Transcript of EXPLAIN 2. Earth’s Energy Balance. Energy Balance Let the rate of energy flow from the Sun to the...
Energy Balance
• Let the rate of energy flow from the Sun to the Earth be called Fin
• Let the rate of energy flow from the Earth to outer space be called Fout
Energy In = Energy Out
Energy Balance
• Assume both Sun and Earth are blackbodies, so F = s T4
• Fin = absorbed sunlight x daylight area
• Fout = s Tearth4 x total area
Fin = Fout
Energy Balance
Fin = Fout
Fin = Solar brightness x (1 – albedo) x (area of Earth’s shadow)
Fout = s (Tearth)4 x (area of Earth’s surface)
• 30% reflected by clouds, air, dust, and surface• 19% absorbed by the atmosphere (mostly
clouds)• 51% absorbed at the surface
Solar Radiation
Both are almost perfect blackbodies!
The hot sun radiates at shorter (visible) wavelengths that carry more energy
Energy absorbed by the cooler earth is then re-radiated at longer (thermal infrared) wavelengths
Earth & Sun Emission
SUN
EARTH255 K
Bare Rock Model• If sunlight were
the only source of energy, the surface of the Earth would be way too cold for life!
• 255 °K = -18 °C = -1 °F
Surface
Sunlig
ht
Fsurface
Fin = Fout
Two-Layer Model• Now imagine
a layer of glass above the solid rock
• Glass is transparent to sunlight
• Glass is opaque to “Earthlight”(thermal IR)
Glass
Surface
Sunlig
ht
Fglass
Fglass
Fsurface
Seen from Above
Both models: • Absorb the same sunshine• Reflect the same amount of sunlight back out• Emit the same amount of Earthlight
Glass
Surface
Sunlig
ht
Fglass
Fglass
Fsurface
Sunlig
ht
Fsurface
boundary
Summary• Layer models are not meant to be
accurate or predictive, just to help us understand how the world works
• Energy balance of “bare rock” model is way too cold to support life!
• Adding a layer of glass makes the surface nice and toasty
• Where did the heat come from?
Bathtub Analogy• If faucet runs faster than
drain, level rises• And vice versa• Drain runs faster when
water is deep• Adding glass to layer
model acts like a clog in the drain
• Water rises until drainage = inflow again
Faucet ~ SunDrain ~ Thermal emissionWater level ~ temperature
Gases• Gases are made of
moving molecules separated by empty space
• Kinetic energy of molecular motion is proportional to temperature
• Gases don’t behave as blackbodies or even graybodies!
• Let’s take a look!99% of Earth’s
atmosphere is N2 & O2
Waves and Photons
Experiments show both kinds of behavior:
– Oscillating fields with troughs and crests (waves)
– Individual packets of energy (particles)
Long Waves = small photons
Short Waves = BIG PHOTONS
Electromagnetic radiation behaves as both waves and particles!
Energy is “Quantized”• When radiation
interacts with atoms and molecules, only certain “jumps” in energy are possible
• Electrons orbit at specific energy levels above an atomic nucleus
• Absorption of a photon of just the right energy can make them “jump up” to the next level
• Emission of a photon occurs when an electron “falls” down to a level below
UV & Visible Light• Big jumps take
lots of energy, small jumps take less
• Big drops emit energetic photons (short wavelengths)
• Small drops emit less energetic photons (longer wavelengths)
Most electron transitions in gases absorb/emit visible or UV light
Molecules and IR light• Molecules are groups of atoms
that share electrons (chemical bonds)
• Molecular transitions involve changes in vibration, rotation, bending, and stretching of chemical bonds
• Photons can interact with molecules to change states
• Transitions involve specific amounts of energy, so only certain wavelengths are active
Molecular transitions typically absorb & emit in thermal infrared
Dancing Molecules and Heat Rays!
• Nearly all of the air is made of oxygen (O2) and nitrogen (N2) in which two atoms of the same element share electrons
• Infrared (heat) energy radiated up from the surface can be absorbed by these molecules, but not very well
N N
O O
Diatomic molecules can vibrate back and forth like balls on a spring, but the ends are identical
No electric dipole!
Dancing Molecules and Heat Rays!
• Carbon dioxide (CO2) and water vapor (H2O) are different!
• They have many more ways to vibrate and rotate, so they are very good at absorbing and emitting infrared (heat) radiation Molecules that have
many ways to wiggle are called “Greenhouse” molecules
O OC
H H
O
Atmospheric Absorption
• Visible radiation passes almost freely through Earth's atmosphere
• Earth's emitted thermal energy either fits through a narrow “window” or is absorbed by greenhouse gases and reradiated
• Complete absorption from 5-8 mm (H2O) and > 14 mm (CO2)
• Little absorption between about 8 m and 11 mm (“window”)
Earth-Atmosphere Energy Balance
Earth's surface absorbs the 51 units of shortwave and 96 more of longwave energy units from atmospheric gases and clouds.These 147 units gained by earth are due to shortwave and longwave greenhouse gas absorption and emittance. Earth's surface loses these 147 units through convection, evaporation, and radiation.
9 PM surface temperature = 25 °C = 77 °F
Summer Nightin Colorado
6 AM surface temperature = -50 °C = -58 °F
radiationemittedby soil
390 W m-2 4 inches= 10 cm
6 AM surface temperature = 15 °C = 50 °F
Summer Nightin Colorado
radiationemittedby soil
390 W m-2 4 inches= 10 cm
radiationemittedby air
340 W m-2
Common Sense• Doubling CO2
would add 4 watts to every square meter of the surface of the Earth, 24/7
• Doing that would make the surface warmer
• This was known before light bulbs were invented!
4 Watts
1 m
1 m
John Tyndall, January 1863
Common Myth #1“Scientists are worried about climate change
because it’s been warming up recently”
WRONG! We’re worried
because we know that when we add heat to things, they warm up