California Climate Assessment Documents Faster Drought, Flood, Heat and Wildfire Impacts

California’s climate assessment is a record of what’s already happening

California is publishing its Fifth Climate Change Assessment, the latest in a series the state has produced since 2006. State law now requires an assessment at least every five years. These reports pull together the best available climate science over several years, with input from communities and Tribes, then send the findings to independent experts for review. That process matters: the assessment isn’t a forecast from a single model, but a synthesis of research meant to help the state understand what climate change is doing and prepare for what comes next. I coordinated the San Joaquin Valley regional report for California’s Fourth Climate Change Assessment—the first focused specifically on the Valley—before joining UCS. We warned then of longer droughts, shrinking mountain snowpack, more dry wells, extreme heat, and abrupt swings from drought to flood. Since then, much of that has happened, sometimes more often and more severely than we expected.

The impacts have arrived faster than the planning cycle

From 2020 to 2022, drought left thousands of household wells without water, especially in disadvantaged communities. Then came a winter of atmospheric rivers: nine in three weeks. Floodwaters spread across California and brought Pa’ashi, or Tulare Lake, back to life. Roads, farms, and homes were submerged in a basin that had been artificially drained a century earlier. This year, California’s snowpack reached just 18% of normal, helping intensify water stress across much of the West. Climate change made those snow conditions about four times more likely. Our latest research finds that emissions from major fossil fuel companies are dramatically altering the water cycle. The wildfire record tells a similar story. Nine of California’s 10 largest fires have occurred in the past decade. The Dixie and August Complex fires alone burned nearly 2 million acres (810,000 hectares). Across the state, about 14 million acres (5.7 million hectares)—roughly 14% of California—burned over those ten years. UCS research attributes 37% of the forest area burned in the western United States and southwestern Canada since 1986 to emissions from 88 major fossil fuel and cement producers. In 2025, the Palisades and Eaton fires became the costliest wildfire disaster on record. Estimated losses reached as much as $250 billion, with more than 16,000 buildings destroyed. Thirty-one people died directly; estimates put indirect deaths at about 440. Climate change made the atmospheric conditions that raise wildfire risk 35% more likely and 6% more intense.

These aren’t isolated California stories. Hurricane Helene killed more than 200 people in the United States in 2024, most of them in North Carolina; climate change made its rainfall about 10% heavier and its winds 11% stronger. Phoenix recorded 645 heat deaths in 2023, a record, during a month-long heat wave scientists said would have been “virtually impossible” without climate change. The 2021 Pacific Northwest heat dome killed hundreds in Oregon, Washington, and British Columbia. Without human-caused climate change, such heat would have been at least 150 times less likely. If global temperatures rise another 1.4°F (0.8°C), heat domes that might now occur once in a thousand years could return every five to ten years. The international toll is just as stark. Climate change made the monsoon rains that flooded Pakistan in 2022 up to 50% heavier, affecting more than 33 million people. It made the 2023 rainfall that burst two dams above Derna, Libya, up to 50 times more likely, killing thousands. And it made the 2024 rains that killed more than 230 people in Valencia, Spain, about 21% heavier. By mid-2026, marine heat waves covered nearly 82% of the world’s oceans, with consequences for ecosystems and fisheries that are difficult to predict. The oceans have absorbed about 90% of the extra heat trapped by climate change. That’s a slow-burning fuse: ocean heat is already fueling stronger storms and marine heat waves, and it will keep raising sea levels for centuries even if emissions stopped today. Knowing what’s coming gives communities a chance to prepare. What has many scientists deeply concerned is the loss of a key source of that knowledge: the current U.S. administration disbanded the author team for the Sixth National Climate Assessment, the country’s main resource for understanding climate risks by region and sector. A threat doesn’t disappear when a government stops measuring it. We simply see it later, when it costs more—and people with fewer resources often pay first, sometimes with their lives. In 2022, the White House estimated climate change could add up to $2 trillion a year (2 million million dollars) to the federal budget by the end of the century. California’s assessments are designed to help prevent or limit precisely this kind of loss.

What’s different in the new assessment

Four changes stand out. It tracks warming levels as well as calendar years. The reports ask not only what California might look like in 2050, but what happens when the planet warms by 3.6°F (2°C). That gives planners a clearer way to connect the scale of expected harm to the investments needed to protect communities. The projections are more local. New downscaling methods translate global climate models into projections for California’s valleys, mountains, deserts, and coast. Tribes and communities helped shape the research. Each regional report involved consultation with local communities and Tribes. That approach began with the San Joaquin Valley report for the Fourth Assessment. The state saw how much more closely the results reflected people’s lives, and made community engagement part of the process for every regional report. Tribes also received more support to carry out their own research this time. The work is shifting toward solutions. We have a clearer picture of how California’s climate will change and what those changes will bring. Now the focus is on measures communities want and, in some cases, are already testing.

California’s climate future in a few numbers

The assessment’s summary draws together findings from across the state. These are the figures Californians should keep in view:

• Temperatures will climb. Average temperatures are projected to rise 3.5°F to 4.3°F (1.9°C to 2.4°C) by 2050, and 5.7°F to 8°F (3.2°C to 4.4°C) by 2100, depending on how much global heat-trapping emissions continue to grow.
• Mountain snowpack is shrinking. April snowpack, one of the state’s major water reserves, is projected to fall about 40% by mid-century and about 60% by the end of the century.
• Dry and wet extremes will swing harder. About one in five Californians—roughly 7 million people—already lives in an area at risk of flooding.
• Wildfire smoke will cause more deaths. California is projected to see the country’s largest increase in deaths from wildfire smoke, with more than 5,000 additional deaths a year by 2050.
• Heat waves will last longer. Under high emissions, the longest heat wave of the year could grow from about three days statewide today to as long as 19 days in the Sierra Nevada and 17 days in the inland deserts by century’s end. People experiencing homelessness make up 0.5% of California’s population but account for 13% of heat-related hospitalizations.
• Rising seas will reach more homes and businesses. With sea level rising 0.8 feet (0.25 meters) by mid-century and 3.3 feet (1 meter) by 2100, a once-in-a-century storm could flood coastal areas where 166,000 people live and properties worth $60 billion by mid-century. By 2100, the figures could reach 310,000 people and $122 billion in property.
• The costs are already showing up. Average home insurance rates rose 85% from 2020 to 2026. Wildfire costs accounted for about 5 of the 6.2 cents per kilowatt-hour increase in electricity prices between 2019 and 2024.

Across all nine regions, the people facing the greatest exposure and with the fewest resources to recover include farmworkers, Tribes, rural communities, renters, people without housing, and low-income families.

In the San Joaquin Valley, water links the risks

I live in the San Joaquin Valley and have returned as a coauthor of its regional assessment. This is one of North America’s most transformed landscapes, burdened by deep structural problems and injustice. UCS works extensively here because relatively low-cost measures could bring major gains in climate resilience, prosperity, and well-being. The Valley is home to 4.4 million people, most of them in disadvantaged communities, as well as dozens of Tribes that survived genocide and dispossession over the past two centuries. It is also the most profitable agricultural region in the United States. But it has lost 95% of its original wetlands and riparian forests, along with much of its natural protection against drought, flooding, and extreme heat. Groundwater pumping has become so severe in some areas that the land has sunk by more than an inch (2.5 centimeters) a month during droughts. That subsidence permanently reduces how much groundwater the Valley can store—water the region will need to protect future food and water supplies. Here’s what the regional report found:

• The Valley is warming fast. Temperatures have risen nearly 3°F (1.7°C) in the past half-century and could rise another 5.5°F (3°C) by 2100. That puts farmworkers and other outdoor workers, who help feed the country, at direct risk from dangerous heat.
• Winters are losing their chill. Much of the Valley has lost 160 to 400 hours of winter cold over four decades, and models project about 20% fewer cold hours by mid-century. That threatens crops such as walnuts, pistachios, cherries, and other stone fruits.
• Water is becoming harder to predict. Snow melts earlier, droughts grow more severe, and the atmosphere is getting “thirstier”. The 2012–2014 drought was the worst in at least 1,200 years, even as extremely wet years become more likely.
• Groundwater, the Valley’s water savings account, is being overdrawn by about 2.2 million acre-feet (2,700 cubic hectometers) a year. Thousands of wells run dry, while nitrate, arsenic, and uranium contaminate water in communities least able to afford treatment.
• Agriculture will have to change. Projections suggest irrigated farmland in the Valley could shrink by as much as 40%. The system was built around water the region doesn’t have and a climate that no longer exists.

The same rural communities breathing some of the country’s dirtiest air are more likely to lose well water, work in dangerous heat, and receive help last. Valley fever is also increasing with the swings between wet and dry years. It can be fatal, and it hits the San Joaquin Valley and its farmworkers especially hard.

Solutions already taking shape in the San Joaquin Valley

California’s climate future will turn on who shapes the changes already underway. That’s the point behind our research: carefully repurposing farmland near communities, with residents directing the plans, can conserve water, curb nitrate and pesticide pollution, restore habitat, and create better-paying jobs. Get the choices wrong, though, and the result could be dust storms, ghost towns, and fresh inequities. The stakes are practical as well as environmental.

California’s Fifth Climate Change Assessment points to projects that offer a different path:

• Let communities lead farmland reuse. In Allensworth, the Allensworth Progressive Association is developing a community-owned agroecology center with regenerative farming, agrivoltaics, and land access for farmers of color. There’s also a short documentary about Allensworth. In Fairmead, residents are turning a 17.3-acre (7-hectare) almond orchard into a stormwater basin with native habitat and a walking trail. These community-adjacent “revitalization zones,” described in our research, can receive support from the state’s Multibenefit Land Repurposing Program, which is already funding this kind of transition.
• Use farmland to recharge groundwater. About 3.6 million acres (1.46 million hectares) of California farmland could capture floodwater and store it underground for the next drought. The condition matters: contaminated land must be kept out of the process.
• Give rivers room and manage dams with better forecasts. Dos Rios State Park, shaped in part by the perspectives of Mewuk and Yokuts tribal communities, turned a former dairy into California’s largest floodplain restoration project. Forecast informed reservoir operations, known as FIRO, can hold on to more water without increasing flood risk.
• Support Tribal stewardship. Cultural burning can cut the likelihood of catastrophic wildfire by as much as 92% over a decade. After decades of advocacy by the Yurok, Karuk, and Klamath Tribes, removal of four dams on the Klamath River reopened more than 420 miles (675 km) of salmon habitat.
• Pair clean energy with climate-smart farming. Solar panels on reused farmland and agrivoltaics can cut emissions and create jobs when communities support the projects. Cover crops, compost, and crop diversity help soil retain water and withstand heat and drought.

These measures do more than address climate impacts. They can improve daily life and economic prospects while tackling problems that run deeper than the climate crisis itself.

Why climate assessments matter for California’s future

The Fifth Assessment matters because fair, science-based resilience policy needs a clear account of the risks facing people, ecosystems, and the state’s economy. Producing an assessment at this scale takes resources that many states don’t have, though every state needs the knowledge. The country needs a strong National Climate Assessment, too. Its future is at risk following decisions by the current administration.

Science can clarify what lies ahead and identify ways to make life safer for this generation and the next. That’s why California’s Fifth Climate Change Assessment deserves thanks—and why its findings need to shape decisions on the ground.