Scientists have thought that global warming will increase the availability of surface water—freshwater resources generated by precipitation minus evapotranspiration—in wet regions, and decrease water availability in dry regions. This expectation is based primarily on atmospheric thermodynamic processes. As air temperatures rise, more water evaporates into the air from the ocean and land. Because warmer air can hold more water vapor than dry air, a more humid atmosphere is expected to amplify the existing pattern of water availability, causing the "dry-get-drier, and wet-get-wetter" atmospheric responses to global warming.