 By comparison , Earth 's present global equilibrium temperature is 255 K ( − 18 ° C ) , which is raised to 288 K ( 15 ° C ) by greenhouse effects . However , when life evolved early in Earth 's history , the Sun 's energy output is thought to have been only about 75 % of its current value , which would have correspondingly lowered Earth 's equilibrium temperature under the same albedo conditions . Yet Earth maintained equable temperatures in that era , perhaps with a more intense greenhouse effect , or a lower albedo , than at present . 
