The main impact of climate change is predicted to be an increase in global mean temperature over most land surfaces. Temperature records over the last 100 years indicate a warming of surface temperatures, with the most pronounced increases observed over the last 25 years. Climate models are fairly consistent in projecting the continuation of this trend through the 21st century. According to the Intergovernmental Panel on Climate Change (IPCC), temperatures are likely to increase by 2°F to 11.5°F, with a best estimate of 3.2°F to 7.2°F, by 2100, relative to 1980–1990 temperatures.
Warming of surface waters has also been observed since 1955. This warming trend is also projected to continue for both salt and fresh water bodies. In addition to causing changes in the hydrological cycle, increasing tempertures are expected to result in diminished snowpack, increased evapotranspiration, and decreased runoff.
Combined, the range of impacts associated with warmer temperatures will alter seasonal water supplies. Warmer freshwater temperatures will also reduce dissolved oxygen levels, promote algal blooms, increase bacteria and fungi content, concentrate pollutants, and cause other adverse impacts on both water quality and habitat viability for fish and other aquatic species.
Higher temperatures are also likely to lead to a global increase in drought conditions, as less summer rainfall and increased evaporation combine to reduce surface water availability.
Increases in temperature could impact water resource managers in a variety of ways, including a decrease in water supplies due to evapotranspiration and an increase in urban and agricultural demand. Additionally, warmer temperatures will impact water quality, perhaps necessitating new or upgraded treatment approaches to meet water quality standards.