Latest Climate Report Warns of Major Upheaval to California Way of Life The dire projections come from the state’s Climate Change Assessment, which analyzes the impacts of changing weather patterns on the nation’s most-populous state. In just a few decades, summers in California will not only be much hotter, but will also stretch for months longer.

Floods will become more widespread and extreme. Exposure to wildfire smoke will kill thousands more people every year. Those are the dire projections of California’s Fifth Climate Change Assessment, a report produced by California every few years that analyzes the impacts of changing weather patterns on the nation’s most-populous state.

The future laid out in the report, which was published on Monday, touches on familiar warnings: more wildfires, bigger storms, longer droughts.

But it illustrates in unsettling detail just how quickly the climate is being reshaped, and reveals what is likely to be major upheaval to long-established ways of life in California. According to the report, average temperatures are likely to increase in California by about 4 degrees Fahrenheit by the middle of the century.

Current warming in California is unprecedented compared with the past century and estimates for the past millennium, the report shows. More and more, warm weather will arrive in California earlier in the year and persist later, with summer conditions lasting for 135 days on average by the middle of the century, compared with an average of 90 days between 1981 and 2010, according to the report.

That prolonged summer will have widespread impacts, including on fire risk, tourism, agriculture and wildlife. The warmer conditions will also mean a 40 percent reduction by midcentury in California’s snowpack in the Sierra Nevada, which feeds the state’s reservoirs.

What once fell as snow will instead fall as rain and immediately become runoff. This will lead to more severe droughts, according to the report. Many Californians have already lived through the beginnings of these trends.

The state this year logged its warmest August on record. And several of the most destructive fires in state history have happened in the last decade, including the wildfires last year in Los Angeles that were among the costliest disasters in U.S. history.

Abby Edwards, senior deputy director for the Governor’s Office of Land Use and Climate Innovation, which produced the assessment, said she hoped the report would alert communities and decision makers about where to focus their efforts as the climate continues to change. The report, which weaves together existing and new, state-commissioned research, contains multiple scenarios based on different greenhouse gas emissions levels in the coming years, as well as recommendations for how to adapt to changing climate patterns.

“The future is not predetermined,” Ms. Edwards said in an interview. The report comes as the Trump administration has been undermining California’s position as a national leader in environmental protection.

As the world’s fourth-largest economy, California’s environmental regulations have influenced car and consumer goods manufacturers, and many of the state’s green policies have been adopted by other states in what’s known as the “California effect.”Among the recent moves from the Trump administration: It ordered an oil company to restart a pipeline off Santa Barbara that caused one of the state’s largest oil spills in 2015; it announced $75 million for a new coal export terminal in Oakland that city officials have tried to stop for nearly a decade; and it has signaled that it could curb the power of the California Coastal Commission, which has been entrusted with protecting the state’s 840-mile shoreline for more than a half-century. “As the Trump administration turns its back on science, California is doubling down on climate action, on protecting our communities, and on building a safer future,” Gov. Gavin Newsom said in a statement.

The state’s assessments began in 2006 under former Gov. Arnold Schwarzenegger as a way to evaluate the specific threats of global warming on the state.

This year’s assessment was the most extensive yet. Beyond the statistics, the report also laid out how a changing climate could threaten beloved ways of life in California, such as skiing in the Sierra Nevada and going to the beach.

A longer summer means a shorter winter for California, and warmer temperatures are expected to blanket lower-elevation ski resorts with rain instead of snow.

The snowpack is expected to fall by 60 percent by 2100. The report also found that without major intervention, as much as three-quarters of California’s beaches could disappear — either seasonally or permanently — by 2100 because of coastal erosion.

In that same time frame, sea level rise is expected to threaten $78 billion worth of property because of coastal flooding. The state’s agricultural industry is expected to take a major hit as well. Many crops require a certain number of hours of cold temperature to develop properly, and warmer temperatures could decrease fruit yields in some parts of the state by as much as 40 percent, according to the report.

At the same time, water shortages could mean that the number of acres that farmers irrigate could decline dramatically, including by up to 40 percent in the San Joaquin Valley, a major agricultural hub.

Soumya Karlamangla is a Times reporter who covers California.

She is based in the Bay Area.