B2 Greenhouse effect

Greenhouse effect

B2.1 Emissivity

e = ratio of power rad. per unit area compared to that of a BB at same T:
e = P_{rad by object} / P_{emit/rad by BB (= σAT⁴)}
P_{rad by object} = eσAT⁴

Perfect BB has e=1

B2.2 Albedo

a = ratio scat power to total iincident power of reflected rad =
^{tot scat P}/_{tot incid P}
earth av = 0.3: 30% of incident is reflected back
no unit…

 𝑒+𝛼=1

B2.3 Solar constant

Intensity of suns rad arriving perp to earth atmosphere at mean distance from sun.
Average = S = 1.36×10³ Wm^-2. Average bc. eliptical orbit, sun output varies (sunspot)
On earths surface, I = ^S/₄

B2.4 Solar Power distribution

Incoming; 1400 Wm^-2
Absorbed by atmosphere, reflected by clouds, reflected earth, absorbed earth.

B2.5 Main Greenhouse Gases

Water Vapour: H₂O: evap from oceans
CO₂: volcanoes, wildfires, respiration
Methance: CH₄: decompositions
Nitrous oxide: N₂O: soils/oceans
Ozone: O₂ O₃ (mostly harmful rad. GOOD!!!)

B2.6 Greenhouse effect

When shortwave rad from sun hits earth, reradiated back as longwave. GHG absorb this rad, trapping it in atmosphere (not lost to space).

Need a little bit of absorpsion to keep earth at habitable T’s, however, dont want increase so higher T_av

B2.7 Absorption of radiation

Incoming shortwave: 25% abs (UV by Ozone)
Outgoing longwave: 80% abs (by other GHG’s)

Absorbs photons of certain infrared wavelengths etcetc…

B2.8 Enhanced GHE

Human activities increase [GHG] thus more absorption. 100ppm to 420 ppm of CO₂. Average global T increases. Evidence: CO₂ levels vs average T graphs.

More CO₂: burning FF, wood. Deforestation
More CH₄; decay organic matter
More N₂O: burn FF, artificial fertilizers

B2.9 Energy balance

σT⁴ = ^S/₄ hypothetically because E_in = E_out
I_in = (1-a)^S/₄ (245Wm^-2). (4 bc pir² vs 4pir²)
I_out = σT⁴ (disregarding reflection so e=1), so
245 = σT⁴ , so T = 256K = -17^oC, however, in reality way larger because greenhouse effect: 15^oC is actual T.