CO2: Warming or Cooling?

Ingemar Johansson vs Floyd Patterson Yankee Stadium 1959

The lapse rate (decrease of temperature with altitude) of 6.5 C/km sets the Earth surface temperature to 15 C from a top of the atmosphere TOA at -18 C (at an altitude of 5 km) with a total warming of 33 = 5 x 6.5 C.

The role of the atmosphere is to transport 180 W/m2 absorbed by the Earth surface from insolation, to the TOA for radiation to outer space. The atmosphere thus acts like an air conditioner keeping the Earth surface at 15 C under radiative forcing.

Since the TOA temperature of -18 C is determined by constant insolation, the lapse rate determines the Earth surface temperature: Increasing lapse rate means warming and decreasing lapse rate means cooling.

A basic question in climate science thus concerns what physics determines the lapse rate. There are two main candidates, both setting up an initial lapse rate of 10 C/km to be moderated to the observed 6.5 C/km:

1. radiation according to Planck's Law
2. thermodynamics: convection + evaporation/condensation + gravity according to the equations of fluid dynamics.

Radiation:

• Radiation according to Planck's Law sets the lapse rate to 10 C/km (by dQ = 4 dT with dQ = 180 W/m2 and dT = 45 C)
• thermodynamics enters to reduce the lapse rate to 6.5, because a lapse rate of 10 is unstable
  
• radiation sets the main lapse rate with thermodynamics as secondary moderator.

Thermodynamics:

• An isentropic (adiabatic) lapse rate of 9.8 C/km is determined by thermodynamics without convection (still air) and without radiative forcing and heat transfer
• radiative forcing drives heat transfer by convection + evaporation/condensation which reduces the lapse rate to 6.5
• thermodynamics without heat transfer sets the main lapse with radiatively forced convective thermodynamics as secondary moderator combined with radiation for residual heat transfer acting on the lapse rate set by thermodynamics.

The two main scenarios are thus:

1. primal radiation + secondary thermodynamics = radiation
2. primal thermodynamics + secondary radiation = thermodynamics.

Consider now the effect of increased atmospheric CO2:

1. radiation: increased lapse rate: warming
2. thermodynamics: more heat transfer by convection: decreased lapse rate: cooling.

Which scenario is closest to reality? Radiation or thermodynamics? Warming or cooling?

Stay tuned to get an answer or think for yourself, maybe with inspiration from Basic Thermodynamics of the Atmosphere.

The basic idea is thus that increased heat transfer causes

• increased lapse rate - warming, if radiation dominates
• more vigorous convection/phase change - decreased lapse rate - cooling, if thermodynamics dominates,

which expresses a fundamental difference between heat transfer by radiation/conduction and

by convection/phase change.