Whether a surface is bright and reflects light or dark and absorbs light affects how hot the surface gets. The term albedo refers to the ability of a surface to reflect away solar radiation, and thus heat.
Dark surfaces, like water and dark asphalt, have low albedo‚they reflect only a small portion away from Earth. Dark surfaces absorb most of the sunlight.
Most of the energy is then re-radiated as heat to the atmosphere, where greenhouse gases trap the heat near Earth's surface.
Light-colored surfaces, like glaciers and meadows, have high albedo. Most of the sunlight, and thus most of the heat, is reflected away from the planet's surface.
Only a small portion is re-radiated as heat to the atmosphere.
As surfaces with high albedo are replaced by those with low albedo, not only does the planet warm, but a positive feedback loop can be triggered.
When higher temperatures melt sea ice, water is uncovered, replacing a high-albedo surface with a low-albedo surface.
This activates a positive feedback loop: As the exposed water absorbs sunlight and releases more heat into the atmosphere, more ice melts and more water is exposed, which then absorbs more sunlight, releasing even more heat, and so on.