Fresh flowers are placed in Santa Rosa, California on Sunday. Credit:AP "People say I was prescient by what I predicted for 2025," he said. "The sad joke is I should have said 2015. It is frightening how quickly we got here." California knows extreme weather. Throughout its history, it has endured natural disasters, like floods and heat waves. But many scientists say the wildfires of the past week are not completely natural. Park Williams, a Columbia University research scientist, said the fingerprint of climate change "is definitely there". Daniel Swain, a climate scientist at the University of California, Los Angeles, agreed. "Increasing temperature plays a significant role in making these fires more explosive, and covering ground more quickly," Swain said. From April to September across northern California, mean temperatures were the highest in 123 years, according to John Abatzoglou, a University of Idaho geography professor who grew up in California and specialises in wildfires and climate change.

An aerial view shows the devastation of the Coffey Park neighbourhood, in Santa Rosa, California. Credit:AP "Climate has enabled fire activity across the west this summer [and now fall]," he said in email. First the wet winter spurred robust vegetative growth. Then the record temperatures dried out that brush faster than in a "normal year", he said. What is the new normal? For California, it is reflected in the weather patterns of past several years. Six years of drought was followed by record winter snow and rain, followed by record heat from April through September. Santa Rosa hit 43 degrees Celsius on September 1, a record high for the date. Five weeks later parts of the city caught fire, destroying hundreds of homes and businesses. A firefighter holds a water hose while fighting a wildfire on Saturday. Credit:AP Worse extremes can be expected in coming decades. Droughts, heat waves, reduced snowpack, winter storms, and sea-level rise are expected to intensify, say scientists who monitor climate change. How much will involve both natural weather patterns and levels of greenhouse gas emissions that are building in the atmosphere, warming the planet.

Sacramento and the Central Valley face risks on several fronts. The number of extreme heat days - 40 degrees or higher - is expected to rise 6 to 10 times by end of the century in cities such as Sacramento and Fresno, according to Cal-Adapt.org, a data collaboration involving the California Energy Commission, the University of California, Berkeley, and other partners. An American flag hangs on a burnt home in Santa Rosa on Sunday. Credit:AP This year, the northern California city of Redding baked under 72 days of 27 degrees or higher, surpassing the previous record of 69 days set a half century earlier. Studies of past heat waves in California show they can cause hundreds of deaths in a single month. A 2009 state study of the July 2006 heat wave found that as many as 500 Californians may have died that month, and that each fraction of a degree increase in temperature from day to day led to a 9 per cent increase in daily deaths. Santa Clara firefighter Terry Sanders and son Isaac, 11, who lost their home in a wildfire, comfort each other on Saturday. Credit:AP

Swain said not every summer will produce record heat, just as every winter will not necessarily be dry. Sacramento's delta breeze will still make a regular summer appearance, just as San Franciscans will bundle up in the June fogs. "We will still have natural climate variability. Some years will be hotter than we are used to, and some years cooler," he said. "But the hot years will be outside the realm of our previous experience." Under most climate change projections, the Sierra Nevada will receive less snow, particularly at lower elevations. That will result in wildfires starting earlier in the season and extending longer, a pattern already documented. It also will heighten the risks for California's mountain communities, such as Lake Tahoe and Mammoth. California has has enacted the nation's most aggressive laws to reduce greenhouse gas emissions, and government and non-profit groups are collaborating on ways to adapt to the changing climate. One of those is the Resilient by Design challenge, an effort in and around San Francisco to respond to sea-level rise and other expected impacts of climate change. Yet sea-level rise is one of the most challenging aspects of climate change to forecast. "It is one of these insidious things where uncertainty is not our friend," Swain said.

Until recently, scientists expected the Bay Area and coastal California to face gradual sea-level rise of 60 to 90 centimetres by the end of the century. But ice sheets in Greenland and Antarctica are melting faster than expected. It is now possible it could see 1.8 metres of sea level rise or higher by 2100, Swain said. With that kind of sea rise, Bay Area airports would be underwater, unless fortified by expensive sea walls. Freeways, residential neighbourhoods and some Silicon Valley campuses would be threatened. Saltwater would intrude deeper into the Sacramento-San Joaquin Valley, rendering current water supplies undrinkable for more than 20 million Californians. Swain and other scientists said it is already too late to prevent impacts of climate change, given the level of greenhouse gases already in the atmosphere. But there is still opportunity to avoid the most serious consequences. That would involve retooling of the international carbon economy beyond what is envisioned in the Paris Agreement, which US President Donald Trump has threatened to abandon. This year's hurricanes, flooding and wildfires should serve as a warning, said former US defence secretary Leon Panetta, who leads the Panetta Institute at the University of California, Monterey Bay. Panetta said he could smell the smoke from the North Bay fires, despite being more than 240 kilometres 150 miles south of Santa Rosa. Because of the drought and other impacts of climate change, he said, "It's a new normal ... what we are witnessing today".

Blue skies Glimpses of blue skies gave hope on Sunday to firefighters battling the deadliest wildfires in California history, which have killed at least 40 people and reduced whole neighbourhoods in the state's wine country to ash. Two of the three deadliest blazes were more than half contained by Sunday, making it safe enough for law enforcement to begin inspecting some evacuated areas in hard-hit Sonoma County, according to the county sheriff's office. Only after those inspections were complete would they begin to decide when it would be safe for residents whose homes were not among the 5,700 structures destroyed by more than a dozen separate wildfires, which ignited a week ago and have since consumed an area larger than New York City. "The skies are blue," Don Martini, a 69-year-old retired carpenter, said after waking up at the Sonoma Raceway campgrounds, where he had spent the previous four days with only smoke and smog overhead. "I haven't seen a blue sky since this whole thing started."

Some at the raceway evacuation center hoped to return to their homes on Sunday. But the fast-moving fires north of San Francisco remained a danger, with thousands more people ordered to leave their homes on Saturday as the death toll crept upward. Hundreds of people remain unaccounted for, and entire neighborhoods have been turned to ashes. McClatchy