earth’s energy budget earth has 2 heat engines: – internal – external internal heat engine –...
TRANSCRIPT
Earth’s Energy Budget• Earth has 2 heat engines:
– Internal– External
• Internal Heat Engine– Energy that drives plate tectonics– Source = radioactive decay– ~1/10000th the energy provided by the sun
Earth’s Energy Budget
• Earth’s External Heat Engine– Energy provided by the sun– Average global surface temperature = 15°C
• Balance between* Incoming solar radiation (electromagnetic
spectrum, mostly visible light)* Outgoing terrestrial radiation (infrared radiation)
Solar radiation• Stephan-Boltzman Law (Energy emitted), E = T4
= Stephan-Boltzman constant = 5.67 x 10-8 Wm-2K-4
• P is proportional to the area under the Planck function curve• Psun = 6.3 x 107 W/m2 (every second)• Only a small fraction of sun’s energy is received by the Earth every
second: 1368 W/m2, as measured by satellites at top of atmosphere
• Wien’s Law - Peak wavelength emitted by a body peak (in microns) = 2877/TK
• For Sun: 5780K = temp of photosphere peak sun = 2877/5780
= 497 nm = 4.97 x 10-7 m
Peak wavelength of the sun is around 500 nm (497)
Earth
• Incoming radiation is short wave.• Outgoing radiation is long wave.
• If the Earth is a blackbody at steady state– Incoming = outgoing (remember our assumption)
– Powerin = Powerout
Geometry & assumptions
From: http://www.windows2universe.org/earth/climate/sun_radiation_at_earth.html accessed 3.26.12
Earth- Incoming Radiation• Solar radiation intersects Earth as a disk (r2)
• Powerin = Powerin from sun (S) – Reflected Solar power= r2 S - r2 SWhere:r = radius of Earth (6360 km)S = solar constant (1368 W/m2) = albedo (earth’s reflectivity) (~30%)
= r2 S (1-
Earth- Outgoing Radiation
• Earth radiates as a sphere with area 4r2
• Stephan-Boltzmann equations defines outgoing energy based on radiating temperature
• Powerout = 4r2 Te4
(units (m2)(Wm-2K-4)(K4) = WTotal energy emitted by the Earth
Earth’s Radiation Budget
• If the earth were a black body the in = out
• Set incoming = outgoingr2 S (1- 4r2 Te
4
Simplify:S/4 (1- Te
4
Solve for Te
Te = 255K (-18 °C)
What if it were different?
Earth as a Black Body• Earth’s actual surface temperature
Ts = 288K (15°C)peak (m) = 2877/288 = 10 m (IR)
Ts - Te = 288 – 255 = 33 difference
• Interactions within atmosphere alter radiation budget • Earth is not a perfect black body, some of the outgoing
radiation is reflected & re-radiated• Greenhouse Effect
Greenhouse Gases
H2O = 1-3%
CO2 = .035%
CH4
N2O
O3
CFC’s
HighConc.
Trace
Naturally occurring
Anthropogenic
Sources referenced
• Ruddiman, W. “Earth’s Climate: Past and Future”. Online: http://bcs.whfreeman.com/ruddiman/
• Archer & Rahmstorf. “The Climate Crisis”. 2010. • Martin, E. Energy budget Powerpoint. Accessed
online 3.23.12.• Climate and Earth’s Energy Budget. NASA. Online:
http://earthobservatory.nasa.gov/Features/EnergyBalance