In some regions, extreme warming has been detected. The warming is not universal, some cooling has occurred in the North Atlantic and central North Pacific and is known to be a consequence of changes in the atmospheric circulation. The cryospheric regions, or regions where water is found in solid form, are among the most sensitive to temperature change. Unlike other substances found on Earth, ice and snow exist relatively close to their melting point and frequently change phase from solid to liquid and back again. Consequently, consistent and prolonged warming trends should result in observable changes to the Earth's cryosphere.
Scientists have the opportunity to monitor temperature by observing such phase changes.
Water changing from solid to liquid and back often results in dramatic visual changes across the landscape as various snow and ice masses shrink or grow. In the following sections, snow cover, glaciers, sea ice, and the related parameter sea level are discussed. In all cases scientists attempt to monitor both the areal extent and mass of these snow and ice bodies. Certainly areal extent is easier to determine than mass. Various forms of remote sensing, from both aircraft and satellite, allow us to look down on surfaces at varying spatial scales and over time to determine if the snow or ice covered area is expanding or contracting.