- isothermal opaque atmosphere: surface temp: - 18 C
- isothermal transparent atmosphere: surface temp: - 18 C
- isentropic opaque atmosphere: surface temp: + 32 C
- thermodynamic semitransparent real atmosphere: surface temp: + 15 C.
fredag 18 juni 2010
The Atmosphere as Refrigerator 2
The previous posts lead us into the following basic scenarios of global climate with top of the atmosphere TOA temperature always - 18 C:
We understand that an isothermal atmosphere at - 18 C is possible both in the case of a fully transparent atmosphere without greenhouse gases GHG and fully opaque atmosphere filled by GHG. The real case at 15 C is somewhere between these extremes with a semi-transparent atmosphere with thermodynamics including latent heat/evaporation/condensation.
Isothermal (lapse rate = 0) and isentropic (lapse rate = 10 K/km) thermodynamic equilibrium states are possible without heat transport from the Earth surface to TOA. Convective heat transport tends to reduce the lapse rate. A reduced lapse rate connects to decreased radiative heat transport.
A simple calculation based on observed incoming = outgoing radiation = 240 W/m2 and a temperature drop dT of say 30 K from the Earth surface to TOA, gives heat transport by radiation = 4 x 30 = 120 Watts (by dQ = 4 dT), which fits with observed heat transport of 120 W by convection-evaporation/condensation (reducing the lapse rate by observed 3 K/km). This corresponds to a semi-opaque atmosphere absorbing 60 W and letting through 180 W to the Earth surface, and transporting back 120 W by convection and 120 W by radiation to TOA for radiation of 240 W to outer space at - 18 C.
We observe that in this model, increase of convective heat transport may reduce the lapse rate further and thus decrease the surface temperature. A balancing decrease of radiative heat transport fits with a smaller dT and a decrease of surface temperature. Less radiative heat transport may thus fit with increasing GHG. As noted in previous posts, the net result could be global cooling by more GHG!
Thus, not even the sign of climate sensitivity is clear, warming or cooling, not to speak of its magnitude: Whether increasing GHG will increase or decrease surface temperature will depend on the effect on incoming surface radiation and the thermodynamical heat transport including evaporation/condensation. In particular, the common belief that doubled CO2 will cause a basic global warming of 1 C, may lack scientific rationale.
Compare with Basic Thermodynamics of the Atmosphere derived from basic properties of turbulent solutions of the Navier-Stokes equations.
Also compare with Roy Spencer's dicussion of the role of PDO in global temperature variations based on the simplest possible thermodynamic model. Spencer shows that even such a simplest model can be made to fit with observations quite well, and then indicates much smaller climate sensitivity that the simple radiative model used by IPCC to predict a basic climate sensitivity of 1 C upon doubling of atmospheric CO2 (augmented to 1.5 - 4.5 C by various feedbacks).
The conclusion is that any climate model must include thermodynamics, and the natural model is then the Navier-Stokes equations with gravitation and radiation.