The greenhouse effect of global warming poses grave environmental dangers to California, including permanent water shortages, scientists warned a congressional hearing in San Francisco. Too much carbon dioxide and other gases in the atmosphere would mean dirtier air, less fresh water, changes in fish and wildlife, and greater use of pesticides and herbicides. Climatic changes associated with global warming, such as higher ocean temperatures and decreased snowfall, may already be occurring, the scientists told the House Interior Committee’s water and power resources subcommittee. Warmer winters would mean less snowmelt into reservoirs to be stored for use during longer, hotter and drier summers. Sea levels rising from 1 1/2 to 9 feet in the next century would push salt into the Sacramento-San Joaquin Delta, draining wetlands and rice fields and decreasing waterfowl species. “It’s something we should do something about right now,” said Roger Revelle, a University of San Diego scientist.