A simple energy-balance estimate: the Earth absorbs solar power of about 240 W/m^2 averaged over its surface. (a) If the Earth radiated as a blackbody with no atmosphere, what surface temperature would balance this? (Use sigma = 5.67 x 10^-8.) (b) Compare with the actual average of 288 K and explain the difference. Solution: (a) sigma T^4 = 240 gives T^4 = 240 / (5.67 x 10^-8) = 4.23 x 10^9, so T = 255 K = -18 C. (b) The actual 288 K (+15 C) is 33 C warmer: the natural greenhouse effect of water vapour, CO2 and other gases traps outgoing infrared and keeps the surface warmer than the bare blackbody value.