The fraction of solar radiation reflected by some surface to the incident radiation is called the albedo of that surface. When the albedo of the earth's surface is lower, the amount of solar radiation absorbed by the surface is smaller, and, as a result of energy balance, the temperature of the surface and of the lower atmosphere of the region is expected to be lower. Also, as a result of energy transport by the atmosphere and the ocean, the effect may be of the global scale.
Even though the difference of surface albedo between the ocean and the land, or between vegetated land and bare soil may not be ignored, it is snow and ice that the most remarkably change it. Snow and ice have usually higher albedo than water surface, soil or vegetation. Snow and ice exist at lower temperature than liquid water does, and they act to decrease temperature through the changes of albedo. Thus, snow and ice make a positive feedback to temperature. This feedback, however, works only in the temperature range where existence of both solid and liquid phases of H2O is possible. Over the land, the feedback also requires supply of H2O from the atmosphere, because the source of snow packs, glaciers and ice sheets are falling snow.
We should also note the various time scales of snow and ice. Generation and dissipation of sea ice and snow packs occur at the time scale of one year or less. Antarctic and Greenland Ice sheets have a time scale of ten thousand or hundred thousand years. Time scales of mountain glaciers are thousands of years.
The role that the cryosphere play in the climate is not only through albedo. For example, ice sheets act as boundary like mountains to atmospheric circulation (item `h' mentioned in Subsection 4.2.2). Sea ice act as thermal insulator between the ocean and the atmosphere above (`i'). Variation of the production rate of sea ice is related to variation of deep ocean circulation (`j').