Today, California is releasing its Fifth Climate Change Assessment. Since 2006, the State has periodically gathered the best available science on how climate change is affecting California, and current law now requires these assessments at least every five years. The assessments are based on what we call “synthesis science”, which combines the most cutting-edge climate science into a set of reports written over several years by scientists with support from community members and Tribes and independently reviewed by experts.
Before I joined UCS, I coordinated the San Joaquin Valley regional report for California’s Fourth Climate Change Assessment, the first regional assessment ever done specifically for the Valley. Back then, we wrote about what might happen: longer droughts, less snowpack, more dry wells, more extreme heat, and drought-to-flood whiplash. Since then, we have watched much of it happen, sometimes more extreme and more frequent than we expected.
What we have witnessed recently
Since the Fourth Assessment, California has been hit hard by events that climate change caused or made worse. The 2020–2022 drought left thousands of domestic wells dry, mostly in disadvantaged communities. Then a winter of atmospheric rivers, including nine in just three weeks, flooded the state and brought Pa’ashi (Tulare Lake) back, flooding roads, farms, and homes in a lakebed that was drained a century ago. And this year, the snow drought that left California’s snowpack at just 18% of average by April 1 hit much of the West, and climate change made it about four times more likely. Our recent research shows that climate change caused by major fossil fuel polluters dramatically influences our water cycle.
Nine of the 10 largest wildfires on record have occurred in the last decade, including Dixie and the August Complex fires that burned nearly two million acres combined. In the last decade, about 14 million acres have burned in California (an area equivalent to about 14% of the state burned in a decade). Our research at UCS found that emissions traced to the 88 largest fossil fuel producers and cement manufacturers are responsible for 37% of the forest area burned in the western United States and southwestern Canada since 1986.
In 2025, the Palisades and Eaton fires became the most expensive wildfire disaster on record (with losses estimated at up to $250 billion and more than 16,000 structures destroyed), killing 31 people directly and causing about 440 indirect deaths. Climate change made the fire weather 35% more likely and 6% more intense.
I guess we all have seen the effects of climate change everywhere.
In the United States in 2024, Hurricane Helene killed more than 200 people, mostly in North Carolina; climate change made its rainfall about 10% heavier and its winds about 11% stronger. In 2023, extreme heat killed 645 people in metro Phoenix, a record, in a month-long heat wave that scientists found would have been “virtually impossible” without climate change. In 2021, the Pacific Northwest heat dome killed hundreds of people in Oregon, Washington, and British Columbia; without climate change, this extreme heat dome would be at least 150 times less likely to occur, and if global temperatures increase by an additional 1.4°F, these heat domes that currently may happen once every 1,000 years, may occur every 5 to 10 years.
Around the world, climate change made the 2022 monsoon rains that flooded Pakistan up to 50% more intense, affecting more than 33 million people; made the 2023 rains that collapsed two dams above Derna, Libya, killing thousands, up to 50 times more likely; and made the rainfall that killed more than 230 people in Valencia, Spain, in 2024 about 21% heavier. By mid-2026, about 82% of the global ocean was experiencing marine heatwaves, with consequences for ecosystems and fisheries that are hard to predict. The oceans have absorbed about 90% of the excess heat trapped by climate change. That looks like a “ticking bomb”: that heat fuels stronger storms and marine heat waves today, and it will keep raising sea levels for centuries, even if we stopped all emissions now.
It is concerning, but if we know the predictions about it, we can prepare. But many scientists are very concerned about what we will not know, since the current US administration disbanded the author team for the sixth National Climate Assessment, the country’s main tool for understanding climate risks to every region and sector.
When a country stops measuring a threat, the threat does not go away. We just see it later, when it costs more, and the people with the fewest resources pay first, sometimes with their lives. In 2022, the White House Office of Management and Budget estimated that climate change could cost the federal budget up to $2 trillion a year by the end of the century.
That loss and destruction is what the California Climate Change Assessments are trying to prevent or minimize.
What makes this California assessment different from previous ones
Four things stand out:
- Climate change is analyzed by warming level, not only by date. Instead of asking only, “what will 2050 look like?”, the reports also ask, “what happens to California when the world reaches 2°C of warming?” That focuses planning on how intense the impacts will be and what investments the state should make to protect communities.
- Projections are more local. New downscaling methods translate global climate models into implications for California’s valleys, mountains, deserts, and coasts.
- Tribes and communities shaped the science. Every regional report involved consultation with communities and Tribes. We did this for the San Joaquin Valley in the Fourth Assessment, the State saw that it made the report more connected to people’s reality, and now it is part of how every report is built. Tribes also had more support to develop their own research than in the Fourth Assessment.
- The focus is shifting to solutions. We have a good idea of what kind of change is coming to California’s climate and its likely effects. Now we are focusing on the solutions that people on the ground want to see and, in some cases, are already testing.
California’s climate future, in a few numbers
The Statewide Summary brings together findings from across the assessment. A few that every Californian should know are:
- It will be much hotter. 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 the trajectory of global heat-trapping emissions.
- Our snowpack, which is an essential water reservoir, is shrinking too fast. April snowpack is projected to shrink about 40% by mid-century and about 60% by the end of the century.
- Weather whiplash will become the norm. Swings between very wet and very dry years will become more frequent. One in five Californians (about 7 million people) already live in areas at risk of flooding.
- Smoke will kill more people. California is projected to have the largest increase in wildfire smoke deaths of any state, more than 5,000 additional deaths per year by 2050.
- Heat waves will last longer. Under a high emissions scenario, the longest heat wave of the year grows from about three days across the state today to as long as 19 days in the Sierra Nevada and 17 days in the Inland Deserts by end-of-century. At present, people living unsheltered are 0.5% of Californians but represent 13% of heat-related hospitalizations.
- The coast is moving in. Sea level rise of 0.8 ft (0.2 m) by mid-century and 3.3 ft (1 m) by 2100, combined with a 100-year storm, can expose to coastal flooding 166,000 people and $60 billion in property (mid-century), and up to 310,000 people and $122 billion (2100).
- We are already paying for it. Home insurance premiums rose 85% on average between 2020 and 2026, and wildfire costs explain about 5 of the 6.2 cents per kilowatt-hour that electricity prices rose from 2019 to 2024.
Across all nine regions, the message is the same: farmworkers, Tribes, rural communities, renters, people living unsheltered, and low-income individuals and families face the most exposure and have the fewest resources to recover.
The San Joaquin Valley: water is the thread that ties everything together
I live in the San Joaquin Valley, and I coauthored the regional assessment again. The Valley is one of the most transformed landscapes in North America, with too many structural problems and injustices. That is why at UCS we focus a lot on this region, especially for all the relatively low-cost, high-reward opportunities to improve climate resilience, prosperity, and well-being.
The San Joaquin Valley is home to 4.4 million people, with a majority living in disadvantaged communities, and it includes dozens of Tribes that survived genocide and displacement over the last two centuries. The Valley is also the most profitable agricultural region in the United States. The Valley has lost 95% of its original wetlands and riverside forests, which has led to a massive loss of our natural climate resilience to cope with droughts, floods, and extreme heat. In some places the groundwater overdraft is so intense that the land has sunk more than one inch per month during droughts, permanently reducing the groundwater storage we need for future water and food security.
Here is what our report found:
- The Valley is warming fast. Temperatures have risen nearly 3°F (1.7°C) over the past half-century and could rise up to 5.5°F (3°C) more by 2100, a direct threat to the farmworkers and outdoor workers who feed the country, and also to livestock and crops.
- Winters are losing their chill. Much of the Valley has lost 160 to 400 winter chill hours over four decades, and models project about 20% fewer by mid-century, putting walnuts, pistachios, and stone fruits like cherries at risk.
- Water is becoming more volatile. Snow melts earlier, droughts are more intense, and the atmosphere is getting “thirstier”. The 2012–2014 drought was the most severe in at least 1,200 years, while extremely wet years are also becoming more likely.
- Groundwater, our “water savings account,” is overdrafted by about 2.2 million acre-feet per year, leaving thousands of dry wells and more concentrated nitrate, arsenic, and uranium in the communities least able to afford water treatment.
- Agriculture will look different. Irrigated acreage in the Valley could decline by up to about 40% under future climate projections. Our current agricultural system was built on water we do not have and on a climate that no longer exists.
The same rural communities that breathe some of the worst air in the country are the ones most likely to lose their wells, work in dangerous heat, and wait the longest for help. Valley fever, which is increasing with wet-dry whiplash and can be lethal, hits the San Joaquin Valley and its farmworkers hardest.
Solutions for the San Joaquin Valley that are already on the ground
The question is not whether the Valley’s landscape will change, but how, and who will decide. As our research shows, if we repurpose cropland strategically, in revitalization zones near communities and with communities shaping the plans, we can save water, reduce nitrate and pesticide pollution, restore habitat, and create better-paying jobs. These can all contribute to building communities that thrive even under extreme climate change. If we do it wrong, we could create dust bowls, ghost towns, and new injustices.
The Fifth Assessment documents solutions that work, in the Valley and across the state:
- Community-led land repurposing. In Allensworth, the Allensworth Progressive Association is building a community-owned Agroecology Hub with regenerative agriculture, agrivoltaics, and land access for farmers of color (watch Allensworth mini documentary if you haven’t). In Fairmead, residents are turning a 17.3-acre almond orchard into a stormwater basin with native habitat and a walking path. This approach is called creating buffers or revitalization zones around communities, and the State’s Multibenefit Land Repurposing Program funds this kind of transition that is already showing positive results.
- Managed aquifer recharge. About 3.6 million acres of agricultural land statewide could bank floodwater underground, turning the next flood into the next drought’s water supply, as long as we avoid contaminated sites.
- Floodplain restoration and smarter reservoir operations. Dos Rios State Park, guided by Mewuk and Yokuts Tribal communities, turned a former dairy into California’s largest floodplain restoration. Forecast-informed reservoir operations (FIRO) can keep more water in storage without increasing flood risk.
- Tribal stewardship. Prescribed and cultural fire can reduce the likelihood of catastrophic wildfire by up to 92% for a decade, and the removal of four Klamath River dams, after decades of advocacy by the Yurok, Karuk, and Klamath Tribes, reopened more than 420 miles of salmon habitat.
- Clean energy and climate-smart farming done right. Solar on repurposed farmland and agrivoltaics can create jobs and cut emissions with community support, while cover crops, compost, and diverse crops help farms hold more water and withstand heat and drought.
These solutions tackle climate change and also many of the root problems behind it, improving quality of life and, in many cases, our prosperity.
The importance of climate assessments for the future
California’s Climate Assessments are important to inform science-based, equitable resilience policies to help protect the State’s people, ecosystems, and economy from the impacts of climate change. Not every state has the capacity or resources needed to conduct this kind of assessment, but we all need it. We also need a robust National Climate Assessment, which is unfortunately under threat from the current administration.
Science helps us understand the challenges we face and points us toward the solutions that can help us build a safer, better future for people today and for generations to come.
That’s why I am grateful for California’s Fifth Climate Assessment.