Good News for the Sunset and Pacifica?

The latest projections by the Union of Concerned Scientists in Washington, D.C., suggest that in a worst-case scenario, San Francisco can expect 55 heat- wave days -- three or more consecutive days of temperatures above 79 degrees -- a year by the 2050s and up to 135 such days a year by the 2090s, compared with only 10 to 15 heat-wave days in the 1990s.

These higher temperatures will translate into more deaths among old people, infants and people with heat-sensitive medical conditions, the authors said. By this reckoning, San Francisco would wind up experiencing at least 1, 300 and up to as many as 4,500 more heat-related deaths in the decade of the 2090s -- or three to 10 times as many as occurred in the 1990s.

The scenario suggests San Francisco will be transformed into something akin to an oven inside a deep freeze whose door keeps swinging open and shut.

"You'll get a lot of temperature variation, heat waves alternating with rapid cooling," said Moser, a staff scientist with the National Center for Atmospheric Research in Boulder, Colo. "That's a situation the body won't get used to."

She said this was one of many projected harmful impacts of climate change that might be avoided, or at least reduced, if steps were taken now to cut the amount of greenhouse gases polluting the atmosphere.

The best- and worst-case scenarios in the report reflect different assumptions on how much the release of such gases is cut back in coming years. One scenario assumes continued reliance on burning fossil fuels and rapid increase in heat-trapping gases all this century; the other assumes a transition to cleaner energy sources, which would allow greenhouse gas emissions to peak at mid-century, then taper back to current levels by 2100.

The report calls for "aggressive vehicle global warming standards" and was issued in advance of a key meeting of the California Air Resources Board on Sept. 23 in Los Angeles.