Balancing Earth's energy budget

Variability of the solar constant

Does geothermal heat flux play a significant role in Earth's surface energy balance?

With this energy balance, temperature is … -18°C

Global mean T = 288 K = 15°C

What accounts for this difference?

Photons in the atmosphere

Atomic absorption (and emission)

Electrons can absorb a photon of a given energy to jump to an energy level precisely ΔE more energetic

As the electron falls back down, it emits a photon (with the same λ) in a random direction.

Rayleigh (elastic) scattering

Particles (atom, molecule, aerosol) are much smaller than λ.

Emission & absorption of solar radiation

• Shortwave radiation almost fits ideal blackbody emission
• Deviation from blackbody → atomic absorption & emission
• Atomic absorption in solar atmosphere
• Scattering and molecular absorption in Earth's atmosphere

Molecular vibration ↔ Photons

	Radial	Latitudinal	Longitudinal
Symmetric
Assymetric

Molecules must be electrically lopsided for their bonds to vibrate.

Incoming shortwave (visible) radiation and outgoing longwave (infrared) radiation

For reference...

Radiative forcing and the atmosphere

External radiative forcings …

Orbital cycles

Solar cycle

Internal radiative forcings…

Gasses, Aerosols

Radiative forcing within the atmosphere

• Focus: troposphere, tropopause, (lower) stratosphere
• The atmosphere is made up of… gasses and aerosols
• Troposphere is well-mixed.
  • Why? Convection
  • Why? Thermally unstable.

Adiabatic Lapse rate

Wallace&Hobbs (2006), Ch. 10

Scenario	Lapse rate
No convection (radiative equil.)	16 K/km
Dry convection	10 K/km
Moist convection (real world)	6.5 K/km

latent heat of condensation

vapor ↔ liquid + energy

Radiative Convective Equilibrium

The equilibrium state of an atmospheric column for which any net loss or gain of radiant energy is balanced by the vertical transport of latent or sensible heat. (AMS)

… an idealization of the climate system in which there is a balance between radiative cooling of the atmosphere and heating by convection (Wing+ 2018)

Emission "through" the troposphere

• Model surface T ~ 320 K (tropics)
• Atmospheric window
• What do the "depths" of absorption features tell us?

skin temperature / altitude

Water vapor and sensitivity to greenhouse forcing

Adapted from: AGU Special Report, 1995

• Lower troposphere has more water vapor → more opaque to IR
• Upper troposphere has less water vapor → more transparent to IR
• Which is more sensitive to GHG forcing?

Enhanced greenhouse effect: warmer surface shifts the adiabat

What does this do to the height of the tropopause?

Greenhouse warming in troposphere → stratosphere cooling

Aerosol forcings

Aerosols:

• Dust
• Salts
• Soot: black, organic carbon
• Volcanic aerosols
• Industrial aerosols

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Aerosol-radiation interaction:

1. Directly: reflection, scattering
2. Indirectly: seed cloud formation as "condensation nuclei"

NOAA

Forcings and global mean temperature

Sum of forcings → effective radiative forcing → GMT

Radiative forcings determine Earth's energy budget

Would you say that H₂O is bad?

CO₂ is not bad!

Our budget represents Earth system mean

What accounts for variability?

What accounts for variability?

What accounts for variability?

Atmospheric CO₂ (1 s = 1 day)