This is a simple calculation of the extent of the present greenhouse effect using the Stefan-Boltzmann equation

From space the Earth is a planet which emits 235 W per square metre [W/m2 or W m-2]. If this emission is considered to be the broad spectrum emitted by an ideal blackbody, the emission temperature of the Earth can be calculated from the Stefan–Boltzmann law:

E = sT4

E is the emitted flux, s the Stefan–Boltzmann constant (this has a value of 5.67 × 10-8 J s-1 K-4) and T is the absolute temperature in Kelvins [i.e., degrees Celsius + 273.2] of the emitting surface. Applied to an emission of 235 W m-2 this gives an emission temperature of 253.7 K (–19.5°C).

The various temperature records at the thousands of irregularly placed measuring sites around the world indicate a mean surface temperature of 15°C, or 288.2 K.

That the surface temperature is higher than the emission temperature by:

15 + 19.5 = 34.5°C gives an estimate for the global warming that we enjoy and which sustains all life on Earth.