Much concern has been expressed about possible future climate change caused by burning fossil fuel and other modern human activities that increase carbon dioxide and other trace greenhouse gases in the atmosphere. World weather records indicate an overall warming trend during the last century, with a surge of warming prior to 1940 (which cannot be attributed to greenhouse gases) and a more recent rise during the 1980s. The extent to which this latest rise is real or an artifact of instrument location (heat island effect of growing cities) or a temporary anomaly is debated among climatologists. For now, most of the projections of future climate change are derived from computer climate simulation studies. Not yet well-represented in the simulation models are cloud effects, which can have a large influence on the study results.
The studies generally indicate a global average temperature rise of about 2 to 5 degrees Celsius over the next century, or about 3oC as an average, for a doubled-CO2 atmosphere. Figures for regional changes are less dependable because of regional weather influences.
Although studies assume a doubling of atmospheric carbon dioxide content, the same effect would be produced by some combination of increased CO2 and trace greenhouse gases, such as methane and chlorofluorocarbons, which, in total, produce the same effect as doubled CO2. Carbon dioxide in the atmosphere has increased from an estimated 280 parts per million about 200 years ago to roughly 315 ppm in 1960 and about 355 ppm in 1993.
Although the climate models also show precipitation, there is less confidence in those results. The most important hydrologic parameter affecting water resources is regional precipitation, and model results are not considered reliable enough to use for any decisions. Some researchers have examined scenarios with ranges of precipitation, for example 10 percent drier or wetter, to obtain insights into how sensitive water systems are to these changes.
Sea level rise is inferred largely from projected temperature increases and is less certain. Causes would be thermal expansion as the ocean warms and melting of permanent ice fields and glaciers. Average projections of sea level rise call for about 1 foot by the middle of the next century, which would represent a strong increase over the roughly 0.5-foot rise estimated for the past 100 years.
For California, if global warming occurs, the most likely impact would be a shift in runoff patterns, with less and earlier runoff from snowmelt and more winter runoff from the higher mountain areas. This change in runoff directly relates to the temperature; the warmer temperatures would mean higher snow levels during winter storms, more cool-season runoff, and less carryover into late spring and summer (assuming precipitation remains the same).
If average temperatures warm by 3oC and this change applies to winter season storm systems, it would lift average snowline levels by about 1,500 feet. Compared to today, the portion of California's winter precipitation stored in the mountain snowpack would decrease significantly. The impact in the northern Sierra Nevada would be larger than in the higher elevation southern Sierra Nevada. Preliminary estimates (assuming the same average precipitation amounts and patterns) indicate that this shift would reduce the average April to July snowmelt runoff by about one-third. A corresponding increase in runoff would be expected during the winter, when it often would have to be passed through major reservoirs as flood control releases. There would be some loss in water supply yield if the shift in snowmelt runoff occurs.
If sea level rises, it could have a major impact on California water transfers through the Sacramento-San Joaquin Delta. There are two primary problems: (1) a slight increase in ocean salinity intrusion due to deeper channels and, partly because of less uncontrolled spring runoff, a longer season of relatively low Delta outflows, and (2) problems with levees protecting the low-lying land. Both problems would degrade the quality and reliability of fresh water transfer supplies pumped at the southern edge of the Delta with existing facilities and operations.
There is a general relationship between rainfall intensity and the warmness of the climate. Other factors being equal, warm air holds more water vapor than cool air. Lifting of the air, either orographically by a mountain range, by convective activity (thunderstorms), or by a weather system front, then has the potential for greater precipitation intensity. Also, higher snow levels in the Sierra Nevada mean more direct rain runoff and less snow accumulation. Major floods on California's rivers are produced by slow-moving Pacific storm systems which sweep moist subtropical air from the southwest into California. When these moisture-laden air streams run into the mountains, copious amounts of rain and runoff result as the southwesterly winds are lifted to cross the Sierra Nevada and coastal mountain ranges (orographic effect). Whether the southwesterly winter storm winds would be stronger or weaker if global warming occurs has not been determined.
These three potential impacts and other possible changes will probably be slow
to develop because climate change is expected to be gradual. The uncertainty
about potential changes is high, and there should be time for confirmation
of these changes and time to adapt. It is useful to monitor climate changes,
however, and determine how they may affect current water supply systems.
Back to Sidebars Index in Bulletin 160-93
Back to Volume I Index in Bulletin 160-93