San Francisco is in a declared state of emergency, its power and water supplies threatened by one of the largest fires on record in California. The blaze is 250 kilometres to the east of the city, on the fringes of Yosemite National Park. It is a grim warning of profound changes that may lie ahead, as the western US comes to terms with a new ecology of fire, wrought by climate change.

The Rim Fire is already the seventh largest in California since records began in the 1930s, having so far torched more than 700 square kilometres.

The state of emergency for San Francisco reflects the fire’s close proximity to the Hetch Hetchy reservoir, which provides San Francisco with most of its water and generates hydroelectric power for the city’s General Hospital, transit system and airport. The emergency declaration will free funds to secure alternative supplies in the event of major disruption.

Tinderbox conditions
Wildfires have always been a part of life in the US west, but they are on the rise as climate change takes hold. In California’s Sierra Nevada mountains, the main problem is the earlier onset of spring. “The snow melts earlier, especially at lower elevations,” says Michael Wehner of the Lawrence Berkeley National Laboratory, a lead author for the US National Climate Assessment. That gives forests longer to dry out, producing tinderbox conditions by late August.

By threatening San Francisco’s power and water, the Rim Fire serves a warning that wildfires can have effects far beyond the area they burn. These will include a surge in air pollution as wildfire activity increases, a new study suggests.

Xu Yue of Yale University has calculated that by the middle of the century, as wildfire activity grows with climate change, total summertime soot pollution will rise by up to 27 per cent in the western US, while aerosols of organic compounds will increase by as much as 70 per cent (Atmospheric Environment, doi.org/nkq).

Both are important contributors to PM2.5, a class of microscopic particles that trigger cardiovascular and respiratory problems. Still, it is hard to predict exactly what the health effects of this increased pollution would be, as that depends on whether smoke plumes from major fires engulf major population centres. “Any detriment in air quality is worrisome,” says Sarah Henderson of the British Columbia Centre for Disease Control in Vancouver, Canada, who studies pollution from forest fire smoke.

Another concern is that scorched forest may not recover – at least not to its former state. Mixed conifer forest, like the area now ablaze, is slowly being replaced at lower elevations by shrub land, which is better adapted to drier conditions. Events like the Rim Fire will accelerate this process, suggests Matthew Brooks of the US Geological Survey’s Yosemite Field Station in Oakhurst, who is studying fire ecology in the Sierra Nevada.

This, in turn, will reduce the ability of wild lands to mitigate global warming by pulling carbon dioxide from the atmosphere. “You’ve replaced a big sponge with a smaller sponge,” Brooks notes.