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Global warming is intensifying and will result in more disastrous fires, like the ones that have ravaged California, and other weather catastrophes unless governments act now to reduce carbon emissions, according to a stark new assessment of the impact of climate change released Friday.

The fourth National Climate Assessment, put together over 2½ years by 300 federal and independent scientists, predicts more hurricanes, tornadoes, floods, fires, water shortages, heat waves and drought across the globe unless governments take action to stop the release of greenhouse gases and halt the relentless heating of Earth.

The report, prepared by 13 federal agencies, mandated by Congress and made public by the White House, used California’s catastrophic fires, including the deadly Carr Fire near Redding this year and the Wine Country fires last year, to help make the point.

In 2015, 10.1 million acres burned across the U.S., the most acreage since record keeping began in 1960, the report said, but the recent California fires were the costliest, destroying well over $1 billion worth of property.

The assessment was apparently completed before the Camp Fire in Butte County became the deadliest and most destructive wildfire in state history, but the report made clear that fire and other environmental horrors will be more common and more frequent across the country in the future if fossil fuels, which make up 85 percent of U.S. emissions, aren’t reduced.

The 1,656-page analysis was unambiguous that climate change is here and getting worse. It said warming temperatures, melting ice, rising sea levels and fire are likely to take a terrible toll on the U.S. economy, reducing it by as much as 10 percent by century’s end.

That would mean annual losses of hundreds of billions of dollars by the end of the century from heat-related deaths, sea level rise and infrastructure damage.

“The impacts of climate change are already being felt in communities across the country,” said the report. Besides damaged infrastructure, it said, the impacts include more disease-carrying pests such as mosquitoes and ticks, increases in instances of human allergies, including asthma, and decreases in crop productivity and livestock health.

There is “clear and compelling evidence that global average temperature is much higher and is rising more rapidly than anything modern civilization has experienced,” said David Easterling, a director at the National Oceanic and Atmospheric Administration, during a news conference Friday. “This warming trend can only be explained by human activities, especially emissions of greenhouse gases into the atmosphere.”

Average temperatures have increased globally by about 1.8 degrees since 1901, according to the report. Efforts to reduce emissions “have not yet approached the levels necessary,” Easterling said.

The analysis is remarkable in how it so resolutely contradicts President Trump, who has referred to human-caused global warming as a hoax and has taken steps to prevent and dismantle environmental regulations, including limits on tailpipe and power plant emissions, exactly the opposite of what the report urges.

Trump also announced U.S. withdrawal by November 2020 from the Paris Agreement, a pledge by nearly all the nations of the world to cut carbon emissions. He has also blamed California wildfires on the lack of forest management and suggested in a tweet Wednesday that the cold snap in the Northeast shows that global warming doesn’t exist.

The National Climate Assessment is supposed to be issued every four years but has come out only four times since passage of the 1990 law calling for the analysis. The last one came out in 2014. Then, in August 2017, the Trump administration disbanded the federal advisory panel for the National Climate Assessment, a group that was supposed to help policymakers and private-sector officials incorporate the analysis into long-term planning.

The scientists who were made available Friday for a news conference would not comment on Trump’s position on climate change or whether his administration had anything to do with the timing of the release the day after Thanksgiving, when most of the public is preoccupied with family.

The White House did not comment on the report, which is the second volume of the assessment. The first volume was issued by the White House last year.

It follows an alarming report released last month by the Intergovernmental Panel on Climate Change, which predicted an economic and humanitarian crisis by 2040 at the current rate.

The Bay Area could be hit particularly hard, according to a report released in June by the Union of Concerned Scientists. It said 4,100 homes in San Mateo County and nearly 4,400 in Marin County could be underwater by 2045 because of climate-driven sea level rise.

Orange County, Santa Clara County and Los Angeles County could also see significant losses.

Across California, 20,472 residential properties, worth $15 billion, will be at risk over the next 30 years, the report said.

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com. Twitter: @pfimrite