Earth’s radiative balance or, how I learned to stop worrying and love the greenhouse effect

If the Earth had no atmosphere:

T4 = S/4(1 — A)

T = 255 K

-18 ˚C

0 ˚F

Albedo (reflectivity)

Solar constant (1370 W/m2) Stefan-Boltzmann constant

(5.67x10-8)

reflected

Absorbed and

re-radiated as heat

288 - 255 = 33 ˚ of ‘normal’ greenhouse warming

Actual surface

No atmosphere

Sunlight Infra-red radiation

Air

0

-0.4

0.4

1900 1940 1980 1860

Te

mp

era

ture

an

om

aly

(˚C

)

Global Temperatures for the last 150 years (5-year running average; as of ~2005)

Civil war

Model T

World War II

Al Gore invents

the internet

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.”

Climate change in the age of Disco

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

Global average 1700-2000 AD

Mauna Loa 1958-2004 AD

The CO2 Greenhouse Effect What should physical principles lead me to expect of global warming?

Complications

• Solar ‘constant’ is a variable

• Water-vapor feedback

• Albedo

• Ice

• Clouds

• Aerosols and dust

• Ground cover

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

Arrhenius, 1896

Doubling CO2 = 5-6 ˚C warming

Cylon invasion happens here

Reid, 1999; by way of www.oar.noaa.gov

Sunspots and early 20th century temperatures

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