Flux question

[Brad_Ad23]

Ok, here's the question.

Without the greenhouse gases in earth's atmosphere the temperature would be 250K. With them it is 40K higher (290K). Using the Stefan-Boltzman equation for flux ( F = σT4) what is the percentage of infrared radiation leaving the earth's surface taht is absorbed by the greenhouse gases?


I spent awhile on this and the best I could come up with is 18%. But if someone else knows how to do this, help will be appreciated. Thanks!

[HallsofIvy]

What is the σ? Post that and show how you used it to get 18%.

[Brad_Ad23]

σ is the Stefan-Boltzman constant... 5.67 x 10-8 J/m2K4.

I basically calculated the flux F1 for T = 250K and F2 for T = 290.
Then I did xF1= F2-F1 and solved for x.

[HallsofIvy]

Okay, now what did you get for F1 and F2? I don't get 18%, I get 81%!

[Brad_Ad23]

Hmm, yes I got 81% too. But for some reason it looks like I then did 1-x.

Is the basic setup even right for approaching this problem?