Various global warming scenarios for the next century suggest that Earth's surface temperature might increase by 3 degrees. Here we estimate what change in external forcing would be required to produce a similar change (all else staying constant - a poor assumption as we will see later).

Calculate what change in the mean distance between sun and Earth would change Earth's emission temperature (T_e) by 3 degrees.

Compare this change to the present variation of Earth's orbit between perihelion and aphelion (http://en.wikipedia.org/wiki/Apsis ). If your orbital mechanics needs a tune up, go to http://csep10.phys.utk.edu/astr161/lect/history/kepler.html for a quick lesson.