In a previous analysis, I derived a rigorous formula for planetary temperature. That derivation characterized the greenhouse effect using the normalized greenhouse effect parameter, , where is the outgoing longwave radiation flux, and is the surface-emitted longwave radiation flux. However, is only one possible parameter that can be used to characterize the greenhouse effect. In this analysis, I will offer another (equally rigorous) version of the planetary temperature formula, using an alternate way of expressing the greenhouse effect.
Table of Contents
- Greenhouse effect temperature difference
- Non-equilibrium temperature formula
- Equilibrium temperature formula
- Small changes
- Greenhouse effect forcing
- Simple response to forcing
- Surface Effective Radiative Emission Temperature
Greenhouse effect temperature difference
The greenhouse effect temperature difference is defined as:
Non-equilibrium temperature formula
From the prior analysis of longwave radiation:
The overbar notation is used to denote quantities suitable for using in global time-averaged equations.
From prior analysis, conservation of energy may be expressed as
where the global TOA energy imbalance, , is related to the global energy storage rate, , and the non-solar heating rate, , by:
Combining the preceding equations allows one to solve for the global mean surface temperature, :
The above formula for surface temperature must always be satisfied, whether or not there is equilibrium.
Equilibrium temperature formula
The energy storage term can be expanded as where is the thermal-energy storage rate and is the non-thermal-energy storage rate. Temperature will be stable if the thermal energy storage rate is zero. Hence, the equilibrium temperature is given by:
To consider the effect of small changes, it is useful to rearrange the formula for conservation of energy (or surface temperature) as follows:
A similar equation needs to be satisfied for the variables , , , , , , and . If we expand that similar equation and retain only first-order change terms, and subtract off the above equation, one obtains:
Rearranging and using the prior equation for leads to:
Solving this for yields:
Greenhouse effect forcing
A “forcing” in the greenhouse effect is typically characterized by the change to when no other parameters are allowed to change. Given the above formula for “small changes”, one can infer that if changes by an amount , then this will be related to the change in by:
Simple response to forcing
If a forcing results in a change in by an amount , then the temperature subsequently changes to cancel out this imbalance, with no other variables changing, then one finds:
Surface Effective Radiative Emission Temperature
When working with some datasets, it may be convenient to work with the surface “effective radiative emission temperature” (or, more concisely, “effective temperature”) rather than the actual surface temperature. The local surface effectve temperature is given by:
The surface effective temperature may be useful to work with because it can be determined by knowing the surface longwave emissions, , even if the emissivity of the surface is unknown.
The general (non-equilibrium) equation for surface effective temperature is:
The small-changes equation for the surface effective temperature is: