Additional info from the National Snow and Ice Data Center
The sun's rays strike the polar regions at a more grazing angle than over equatorial regions, where the rays strike at a more direct angle. The sun's angle is the primary reason why the polar regions are cold and the equatorial regions are warm.
Sea ice is white, so nearly all of the sunlight that hits the sea ice surface is reflected back into space; thus, it has a high albedo. High albedo helps keep the polar regions cold, because the sunlight reflected back into space does not warm the surface. When the climate changes enough to warm the Arctic and to melt sea ice, the polar regions have less of a reflective surface. More heat is absorbed, which causes more melting, which amplifies the warming. This cycle is technically known as a “positive feedback loop” that ultimately alters the circulation of the atmosphere — although the outcome is far from positive.