This diagram shows how the melting of ice is a positive feedback loop. If temperatures rise, then ice begins to melt. If the ice melts, there is less to reflect sunlight; the albedo is decreased and the color of this area of the planet is darker. This means that more sunlight is absorbed and temperatures will increase leading us back to the starting point on the diagram. This idea is expanded upon at the listed website.
Image courtesy of Dr. Perovich and UCAR as well as the SHEBA project. It can be found at http://data.eol.ucar.edu/codiac_data/sheba/data/perovich/ICEWEB/feedbac1.htm