Earth’s radiative balance
or, how I learned to stop worrying and love the greenhouse effect
If the Earth had no atmosphere:
Stefan-Boltzmann constant
(5.67x10-8)
Solar constant (1370 W/m2)
T4 = S/4(1 — A)
Albedo (reflectivity)
Absorbed and
re-radiated as heat
reflected
T = 255 K
-18 ˚C
0 ˚F
Actual surface
288 - 255 = 33 ˚ of ‘normal’ greenhouse warming
No atmosphere
Infra-red radiation
Sunlight
Air
Global Temperatures for the last 150 years
Temperature anomaly (˚C)
(5-year running average; as of ~2005)
0.4
Al Gore invents
the internet
0
World War II
Civil war
-0.4
Model T
1860
1900
1940
1980
Climate change in the age of Disco
After Jones and Wigley, 1980
“ . . . a global deterioration of climate, by order of magnitude larger than any hitherto experienced by
civilized mankind, is a very real possibility and indeed may be due very soon. ”
The last ~10 years: another plateau resembling ~1950
Currently favored explanation of the plateaus is that
these are periods when accumulated heat gets
preferentially stored in ocean instead of atmosphere…
Warming has a strong geographic bias
(2000-2009 avg. vs. 1950-1980 mean)
±2.5 ˚C scale
A long-suspected driving force — rise in CO2
Mauna Loa 1958-2004 AD
Global average 1700-2000 AD
The CO2 Greenhouse Effect
What should physical principles lead me to expect of global warming?
33 ˚ of warming = 20˚ from H2Ov + 10 ˚ from CO2 + 3 ˚ from other things
CO2 has risen 10’s of %, so T should
have risen by a degree or more
Complications
• Solar ‘constant’ is a variable
• Water-vapor feedback
• Albedo
Arrhenius, 1896
Doubling CO2 = 5-6 ˚C warming
• Ice
• Clouds
• Aerosols and dust
• Ground cover
Cylon invasion happens here
Sunspots and early 20th century temperatures
Reid, 1999; by way of www.oar.noaa.gov
So what factor is to blame for 20th century T rise?
Wrong question — lots of things are to ‘blame’ but some
are more important than others, and at different times