The blackbody temperature of Earth is supposed to be -18C or 255K. This can be derived by reworking Stefan Boltzmann law to: TK = (S*(1-a)/4*rho) ^0,25 http://www.lwr.kth.se/Grundutbildning/1B1292/Compendium_online/ch05s02s01.html in which S is solar influx, we use 1367 w/m2 a is albedo: 0.3 rho is the Stefan Bolzmann constant: 5.667E-8 The factor 1/4 is caused by the difference in influx surface. The cross section of Earth (pi*r2) is intercepting the solar radiation but it has been equally divided over the surface of the Earth (4*pi*r2). I think the latter reasoning is wrong. You can average out the influx but that doesn not average out the temperatures because the relationship is not lineair. I may be wrong of course but If I assume the earth to be 180 slices of a sphere and calculate the radiation temperature for every degree of lattitude seperately, then I get an average blackbody temperature of about -21.1C or 251.9K. Before I show the (simple) calculations, I like to invite everybody to attempt the calculation likewise since independent duplication is the best confirmation. What would be the consequences on all climate models etc if the blackbody temperature is three degrees lower than always assumed?