The sun's intensity at the distance of the earth is 1370 W/m2. 30% of this energy is reflected by water and clouds; 70% is absorbed.
What would be the earth's average temperature (in ?C) if the earth had no atmosphere? The emissivity of the surface is very close to 1. (The actual average temperature of the earth, about 15 ?C, is higher than your calculation because of the greenhouse effect.)