According to simulation models, a magnetic shield could help Mars achieve half the atmospheric pressure of Earth in a matter of years. With protection from solar winds, frozen CO2 at Mars’s polar ice caps would start to sublimate, or turn directly into gas from a solid. The greenhouse effect would start to fill Mars’s thin atmosphere and heat the planet, mainly at the equator, at which point the vast stores of ice under the poles would melt and flood the world with liquid water.
http://www.popularmechanics.com/space/moon-mars/a25493/magnetic-shield-mars-atmosphere/
