I saw a claim today that without the greenhouse effect, Earth's surface would be on average at a chilling -18°C (note: this is not a climate change thread).(adsbygoogle = window.adsbygoogle || []).push({});

I set about trying to reproduce this result, so at first I assumed that Earth was a perfect blackbody, and that in order to be in equilibrium, it would have to radiate away as much power as it received. So I took the solar irradiance of ~1400 W/m^{2}and divided it by 2 (since I figured that the surface area over which it could be radiated away again would be twice the surface area over which it was received). Then I took this irradiance (or 'flux' in astronomy parlance) and divided it by the Stefan-Boltzmann constant in order to get the fourth power of the surface temperature that the Earth would have to have in order to have this surface flux (as a blackbody). The resulting surface temperature was T = 60°C.

Then I decided it was silly to assume that all of the incident solar radiation was absorbed, so I looked up the Albedo (reflectivity) of Earth on Wikipedia. Two numbers were stated: geometric Albedo of 0.367 and Bond Albedo of 0.306. Being too lazy to read more about them, I just tried them both. Assuming them to be the fraction of radiation reflected, I scaled my required output flux by (1-Albedo) and got results of 30°C and 22°C for the lower and higher albedos respectively. Neither of these is -18°C. What am I missing (aside from the obvious that Earth is not a blackbody). Shouldn't this method give something reasonably close? I assumed that an equally crude estimate was applied to arrive at the -18°C in the first place.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Earth as a Blackbody

**Physics Forums | Science Articles, Homework Help, Discussion**