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Ashley Siefert Nunes at UCS
asiefert@ucsusa.org
202-331-5666
WASHINGTON - Increases in potentially lethal heat driven by climate change will affect every state in the contiguous U.S. in the decades ahead, according to a new report and accompanying peer-reviewed study in Environmental Research Communications, both by the Union of Concerned Scientists, released today. Few places would be unaffected by extreme heat conditions by midcentury and only a few mountainous regions would remain extreme heat refuges by the century's end.
Without global action to reduce heat-trapping emissions, the number of days per year when the heat index--or "feels like" temperature--exceeds 100 degrees Fahrenheit would more than double from historical levels to an average of 36 across the country by midcentury and increase four-fold to an average of 54 by late century. The average number of days per year nationwide with a heat index above 105 degrees Fahrenheit would more than quadruple to 24 by midcentury and increase eight-fold to 40 by late century.
"Our analysis shows a hotter future that's hard to imagine today," said Kristina Dahl, senior climate scientist at UCS and co-author of the report "Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days." "Nearly everywhere, people will experience more days of dangerous heat even in the next few decades. By the end of the century, with no action to reduce global emissions, parts of Florida and Texas would experience the equivalent of at least five months per year on average when the 'feels like' temperature exceeds 100 degrees Fahrenheit, with most of these days even surpassing 105 degrees. On some days, conditions would be so extreme that they exceed the upper limit of the National Weather Service heat-index scale and a heat index would be incalculable. Such conditions could pose unprecedented health risks."
In the U.S., these "off-the-charts" days now occur only in the Sonoran Desert--located on the border of southern California and Arizona--where historically fewer than 2,000 residents have been exposed to the equivalent of a week or more of these conditions per year on average. By midcentury, these "off-the-charts" conditions would extend to other parts of the country, and areas currently home to more than 6 million people would be subjected to them for the equivalent of a week or more per year on average. By late century this would increase to areas where more than 118 million people--over one-third of the U.S. population--live.
"We have little to no experience with 'off-the-charts' heat in the U.S.," said Erika Spanger-Siegfried, lead climate analyst at UCS and report co-author. "These conditions occur at or above a heat index of 127 degrees, depending on temperature and humidity. Exposure to conditions in that range makes it difficult for human bodies to cool themselves and could be deadly."
Overall, the study showed that the Southeast and Southern Great Plains would bear the brunt of the extreme heat. With no action to reduce emissions, areas of states in these regions would experience the equivalent of three months per year on average by midcentury that feel hotter than 105 degrees Fahrenheit, possibly as hot as 115 degrees, 125 degrees, or worse. In this time frame, parts of those regions and the Midwest would experience "off-the-charts" heat days for the first time. By late century, communities in each state in the contiguous U.S. would experience days with a heat index exceeding 105 degrees Fahrenheit, with nearly one-third of the population enduring the equivalent of two months of such heat. Similarly, "off-the-charts" heat days would spread to communities in 47 states.
In addition, the analysis found that by midcentury with no reduction in global emissions:
According to the analysis, by late century with no reduction in global emissions:
The analysis calculated the frequency of days with heat index thresholds above 90 degrees Fahrenheit--the point at which outdoor workers generally become susceptible to heat-related illness--as well as above 100 and 105 degrees Fahrenheit, when the National Weather Service (NWS) generally recommends issuing heat advisories and excessive heat warnings, respectively. The number of high heat-index days was calculated by averaging projections from 18 high-resolution climate models between April and October. The report looked at these conditions for three possible futures. The "no action scenario" assumes carbon emissions continue to rise and the global average temperature increases nearly 4.3 degrees Celsius (about 8 degrees Fahrenheit) above pre-industrial levels by century's end. The "slow action scenario" assumes carbon emissions start declining at midcentury and the global average temperature rises 2.4 degrees Celsius (4.3 degrees Fahrenheit) by century's end. In the "rapid action scenario," global average warming is limited to 2 degrees Celsius (3.6 degrees Fahrenheit)--in line with the Paris Agreement. All population data presented here, including for future projections, is based on the most recent U.S. Census conducted in 2010 and does not account for population growth or changes in distribution.
"The rise in days with extreme heat will change life as we know it nationwide, but with significant regional differences," said Rachel Licker, senior climate scientist at UCS and report co-author. "For example, in some regions currently unaccustomed to extreme heat--those such as the upper Midwest, Northeast and Northwest--the ability of people and infrastructure to cope with it is woefully inadequate. At the same time, people in states already experiencing extreme heat--including in the Southeast, Southern Great Plains and Southwest--have not seen heat like this. By late century, they may have to significantly alter ways of life to deal with the equivalent of up to five months a year with a heat index above--often way above--105 degrees. We don't know what people would be able and willing to endure, but such heat could certainly drive large-scale relocation of residents toward cooler regions."
The report notes that the rising heat could particularly affect outdoor workers and thus sectors depending on their labor.
"By the end of the century, on most days between April and October, construction workers in parts of Florida won't be able to safely work outside during the day because the heat index would exceed 100 degrees," said Dahl. "Likewise, agricultural centers such as Illinois and California's Central Valley could struggle to keep farm workers safe, with the heat index exceeding 90 degrees and 100 degrees, respectively, for the equivalent of about three months a year. If farm workers are unable to work as a result of extreme heat, this could affect the productivity of farming enterprises."
People exposed to the same heat event can have different levels of heat-related health risk, with children, elderly adults, people with special needs, and outdoor workers having higher risks of heat-related illness and death. City-dwellers contend with the urban heat island effect--a phenomenonwhere where heat-retaining materials and surfaces drive up temperatures, particularly at night--which can increase rates of heat-related illness. Meanwhile, residents of some rural areas may face higher risk of heat-related hospitalization and death given their distance from cooling centers and medical facilities.
"Low-income communities, communities of color and other vulnerable populations may be particularly at risk when exposed to extreme heat," said Juan Declet-Barreto, climate scientist at UCS and report co-author. "Longstanding social and economic inequities have led to these communities often having more limited access to transportation, cooling centers, and health care, and they may lack air conditioning, or the financial resources to run it."
The report clearly shows how actions taken, or not taken, within the next few years to reduce emissions will help determine how hot and humid our future becomes. The longer the U.S. and other countries wait to drastically reduce emissions, the less feasible it will be to realize the "rapid action scenario" analyzed.
"The best ways to avoid the worst impacts of an overheated future are to enact policies that rapidly reduce global warming emissions and to help communities prepare for the extreme heat that is already inevitable," said Astrid Caldas, senior climate scientist at UCS and report co-author. "Extreme heat is one of the climate change impacts most responsive to emissions reductions, making it possible to limit how extreme our hotter future becomes for today's children."
Governors and state legislators have begun moving toward 100 percent clean energy and Congress is considering a range of energy and climate policies--including renewable energy standards, climate resilient infrastructure and innovation incentives, which may see bipartisan support--that could help keep the worst at bay.
"To ensure a safe future, elected officials urgently need to transform our existing climate and energy policies," said Rachel Cleetus, lead economist and policy director at UCS and report co-author. "Economists have advised putting a price on carbon emissions to properly account for damages from the fossil-fuel-based economy and signal intentions to protect the environment."
The report includes a range of preparedness recommendations for governments, including: investing in heat-resilient infrastructure; creating heat adaptation and emergency response plans; expanding funding for programs to provide cooling assistance to low- and fixed-income households; directing the Occupational Safety and Health Administration (OSHA) to set up protective occupational standards for workers during extreme heat; requiring utilities to keep power on for residents during extreme heat events; and investing in research, data tools and public communication to better predict extreme heat and keep people safe.
To view the report PDF, click here.
Spreadsheets with our data on extreme heat are available and can be sorted by city, by county, by state, by region and by population.
To get the results for your city or county by using our interactive widget, click here.
To use the interactive mapping tool, click here. The map allows you to learn more about extreme heat in specific counties. When you zoom in, the maps become more detailed.
For all other materials, including regional press releases, our methodology document and Spanish-language materials, click here.
The Union of Concerned Scientists is the leading science-based nonprofit working for a healthy environment and a safer world. UCS combines independent scientific research and citizen action to develop innovative, practical solutions and to secure responsible changes in government policy, corporate practices, and consumer choices.
"Mr. Lee Zeldin, the administrator of the Environmental Protection Agency, is making it impossible for us to regulate these life-threatening emissions," one activist said.
As smoke from Canadian wildfires triggered an air quality alert for New York City and Long Island on Sunday, activists with Climate Defiance disrupted a speech by Environmental Protection Agency Administrator Lee Zeldin in the Hamptons.
The disruption came four days after reports emerged that Zeldin's EPA was set to repeal the 2009 "endangerment finding" that greenhouse gas emissions "threaten the public health and welfare of current and future generations." It is this finding that has given the EPA the authority to regulate climate emissions under the Clean Air Act.
"We are in a climate crisis largely caused by the burning of fossil fuels," the first activist to disrupt the speech said, according to video footage shared by Climate Defiance. "And Mr. Lee Zeldin, the administrator of the Environmental Protection Agency, is making it impossible for us to regulate these life-threatening emissions."
Zeldin's speech took place at the Global Breakfast Forum, held at The Hamptons Synagogue.
"What are you going to say to your children when the Hamptons are underwater?"
Several of the young Jewish activists who disrupted the speech referenced their faith.
"The Torah commands us to be stewards of the Earth, not the oil industry," one activist said.
The audience largely responded with boos and jeers, and one attacked two of the activists with a chair, according to Climate Defiance video footage.
However, the Climate Defiance activists emphasized that Zeldin and the pro-fossil fuel Trump administration were the forces that would ultimately disrupt life and community in the Hamptons.
"History is going to remember you as a monster," one yelled out to Zeldin.
Another said: "Lee Zeldin, you have taken half of a million dollars from fossil fuels. What are you going to say to your children when the Hamptons are underwater?"
The disrupters also referenced Project 2025 and the broader Trump administration. According to the Project 2025 Tracker, Zeldin's EPA has achieved 57% of the Heritage Foundation road map's objectives.
"Lee Zeldin is carrying out the plans of Project 2025 and fossil fuels to a T," one said. "Your orange overlord does not care about any of you. All of you will be suffering from the rising seas and the worsening climate crisis."
A member of Extinction Rebellion NYC, who assisted with the protest, said in a statement: "Heritage has long been helmed by fossil fuel interests like Koch Industries, which has done some of the heaviest lifting to make sure nothing is done on climate change in the U.S. The majority of these wishes have been executed by Zeldin himself, and through Trump, who asked for $1 billion from oil companies in a dinner at Mar-a-Lago during his campaign. His Big, Beautiful Bill is a wish list directly penned in Project 2025. And when we hit 4°C of warming this century, we will know the true cost of these deadly practices."
Protesters also referenced the repeal of the endangerment finding, climate-fueled extreme weather events like Hurricane Sandy, and the smoke pollution clouding the region as Zeldin spoke.
"There is smoke in the air for another summer," one said. "This is only going to get worse and worse."
Both New York City Emergency Management and the New York State Department of Environmental Conservation issued Air Quality Health Advisories through 11:59 pm Eastern Time on Sunday as smoke poured into the region from Canadian wildfires. Air quality was listed as "unhealthy for sensitive groups," and at 11:00 am Eastern Time on Sunday, New York City had the eighth worst air quality of any city on Earth.
The smoke recalled the thick orange haze that blanketed New York and other parts of the Northeast during the record-breaking Canadian wildfire season of 2023. The climate crisis makes wildfires more frequent and extreme.
"There is nothing humane or tactical about letting a trickle of aid in after a man-made famine has started while continuing to bomb starving men, women, and children, even in so-called safe zones," one advocate said.
The Israeli military began instituting tactical pauses in its assault on certain sections of Gaza on Sunday, as part of a plan to allow what Israeli Prime Minister Benjamin Netanyahu described as "minimal humanitarian supplies" to enter the besieged enclave.
Several humanitarian organizations and political leaders described the Israeli approach as vastly insufficient at best and a dangerous distraction at worst, as Palestinians in Gaza continue to die of starvation that experts say has been deliberately imposed on them by the U.S.-backed Israeli military.
"Deadly airdrops and a trickle of trucks won't undo months of engineered starvation in Gaza," Bushra Khalidi, Oxfam policy lead for the Occupied Palestinian territory, said in a statement on Sunday. "What's needed is the immediate opening of all crossings for full, unhindered, and safe aid delivery across all of Gaza and a permanent cease-fire. Anything less risks being little more than a tactical gesture."
Israel announced a plan to institute a daily 10-hour "tactical pause" in fighting from 10:00 am to 8:00 pm local time in the populated Gaza localities of Gaza City, Deir al-Balah, and Muwasi, as The Associated Press explained.
"These actions are not pauses—they are part of an ongoing genocide that the world must act to stop."
However, on Sunday—the first day of the supposed pause—Israeli attacks killed a total of 62 people, Al Jazeera reported, including 34 who were seeking humanitarian relief. Another six people died of hunger, bringing the total death toll from starvation and malnutrition to 133, including 87 children, according to the Gaza Health Ministry.
"The Israeli government's so-called 'tactical pauses' are a cruel and transparent farce," said Council on American-Islamic Relations (CAIR) national deputy director Edward Ahmed Mitchell in a statement on Sunday. "There is nothing humane or tactical about letting a trickle of aid in after a man-made famine has started while continuing to bomb starving men, women, and children, even in so-called safe zones. These actions are not pauses—they are part of an ongoing genocide that the world must act to stop."
British Foreign Secretary David Lammy, meanwhile, called the pause "essential, but long overdue."
"This announcement alone cannot alleviate the needs of those desperately suffering in Gaza," Lammy said, as The Guardian reported. "We need a cease-fire that can end the war, for hostages to be released, and aid to enter Gaza by land unhindered."
The United Nations' World Food Program posted on social media that it welcomed the news of the pause, as well as the creation of more humanitarian corridors for aid, and that it had enough food supplies either in or en route to the area to feed the entire population of Gaza for nearly three months.
"A man-made hunger can only be addressed by political will."
Since the border crossings opened on May 27 following nearly three months of total siege, WFP has only been able to bring in 22,000 tons of food aid, about a third of the over 62,000 tons of food aid needed to feed the population of Gaza each month.
While it welcomed the pause, WFP did add that "an agreed cease-fire is the only way for humanitarian assistance to reach the entire civilian population in Gaza with critical food supplies in a consistent, predictable, orderly, and safe manner—wherever they are across the Gaza Strip."
Joe English, emergency communications specialist for UNICEF, emphasized that the limited pauses proposed by Israel were not the ideal conditions for treating serious malnutrition.
"This is a short turnaround in terms of the notice that we have, and so we cannot work miracles," English told CNN.
English explained that, while UNICEF can treat malnutrition, children who are malnourished require a course of treatments over an extended period of time in order to fully recover, something only truly possible with a cease-fire, which would allow the U.N. to reestablish the 400 aid distribution points it had set up across Gaza before the last cease-fire ended in March.
"We have to be able to reach people and also to reach people where they are," he said. "We can't be expecting people to continue to traverse many miles, often on foot, through militarized areas, to get access to aid."
In addition to bringing in food aid through trucks, Israel, Jordan, and the United Arab Emirates all began air-dropping aid over the weekend. However, this method has been widely criticized by humanitarian experts as ineffective and even dangerous.
"The planes are insulting for us. We are a people who deserve dignity."
"Airdrops will not reverse the deepening starvation. They are expensive, inefficient, and can even kill starving civilians. It is a distraction and screensmoke," U.N. Relief and Works Agency for Palestinian Refugees in the Near East Commissioner-General Philippe Lazzarini wrote on social media on Saturday.
"A man-made hunger can only be addressed by political will. Lift the siege, open the gates, and guarantee safe movements and dignified access to people in need," Lazzarini wrote.
Palestinians in Gaza also complained about the air drops.
"From 6:00 am until now we didn't eat or drink. We didn't get aid from the trucks. After that, they said that planes will airdrop aid, so we waited for that as well," Massad Ghaban told Reuters. "The planes are insulting for us. We are a people who deserve dignity."
In a reminder of what is at stake in effectively delivering aid to Gaza, the World Health Organization (WHO) warned on Sunday that "malnutrition is on a dangerous trajectory in the Gaza Strip, marked by a spike in deaths in July."
WHO continued:
Of 74 malnutrition-related deaths in 2025, 63 occurred in July—including 24 children under 5, a child over 5, and 38 adults. Most of these people were declared dead on arrival at health facilities or died shortly after, their bodies showing clear signs of severe wasting. The crisis remains entirely preventable. Deliberate blocking and delay of large-scale food, health, and humanitarian aid has cost many lives.
WHO said that the search for lifesaving aid was itself deadly: "Families are being forced to risk their lives for a handful of food, often under dangerous and chaotic conditions. Since 27 May, more than 1,060 people have been killed and 7,200 injured while trying to access food."
Israeli solders have reported that they had been ordered to fire on Palestinian civilians seeking aid.
In the face of Israel's atrocities, CAIR's Mitchell called for decisive action: "No more statements. Our government, Western nations, and Arab Muslim nations must act immediately to end the genocide, allow unfettered humanitarian aid into Gaza, secure the release of all captives and political prisoners, and hold Israeli leaders accountable for war crimes. Every moment of inaction contributes to the unimaginable suffering of everyone in Gaza."
"All across the country we showed that when our families stick together, we are powerful," one organizer said.
Tens of thousands of people in more than 225 towns and cities across the U.S. came out on Saturday as part of the Families First National Day of Action to protest Trump administration and Republican policies that defund the safety net while funneling unprecedented amounts of cash toward immigration enforcement.
The day of action came around three weeks after the U.S. House passed and President Donald Trump signed a budget bill that would strip 17 million of Americans of their health insurance and 2 million of their food aid while making Immigration and Customs Enforcement the highest-funded federal law enforcement agency in U.S. history.
"Yesterday marked the 35th anniversary of the Americans with Disabilities Act. And we are just days away from the 60th anniversary of Medicaid and Medicare at the end of this month. These policies represent a promise we made to each other: that no matter the ups and downs of life, our ability to take care of our families, from one generation to the next, should be supported," Ai-jen Poo, executive director of Caring Across Generations and president of the National Domestic Workers Alliance, told Common Dreams on Sunday.
"But a big ugly budget bill just passed," Poo continued, "that breaks that promise by making historic cuts to programs like Medicaid, Medicare, and SNAP, by using our tax dollars to stoke fear and rip families apart simply due to their immigration status. This is not what families want, and those who passed it must know that the vast majority of us want our tax dollars to go to healthcare and food, a safety net for families, supporting public funds for families, health, food, and the economic security for all of us, not billionaires."
"To show our power and resolve for a better future we came out in the thousands all across the country."
Families First is a coalition made up of over 75 organizations including Caring Across Generations, National Domestic Workers Alliance, MoveOn, Community Change Action, MomsRising, Planned Parenthood, People's Action Institute, Family Values @ Work, Families Over Billionaires, Fair Share America, Working Families Power, and labor unions like the Service Employees International Union (SEIU); American Federation of State, County, and Municipal Employees; American Federation of Teachers; and the National Education Association.
"To show our power and resolve for a better future we came out in the thousands all across the country, hosting over 225 events where we peacefully protested, to show the intergenerational face of those of us prepared to hold the ones who passed this bill accountable every day, and to take action. From spelling out the word 'familia' on the beach in California, taking a Medicaid Motorcade through the state of Indiana, to a rally in D.C. on the National Mall at the seat of power," Poo said.
Here are some highlights from Saturday's day of action.
On the National Mall across from the U.S. Capitol building, organizers capped a 60-hour vigil opposing Medicaid cuts with a rally at 12:00 pm ET.
Jennifer Wells, the director of economic justice at Community Change, spoke at the rally on the important role that Medicaid and the Supplemental Nutrition Assistance Program (SNAP) played in her life.
"I'm here both as an advocate and organizer and as someone who has lived the realities we're fighting to change, as a person who has been directly shaped by the programs that are currently under attack," Wells said. "I was a Medicaid kid, I was a SNAP kid. These programs kept me and my mom and my brother healthy, alive, and moving forward when we had nothing to fall back on."
Families gathered in Newark's Military Park to protest the budget cuts.
"Congress is helping the rich get richer while cutting healthcare, education, and support for working families," New Jersey Citizen Action wrote on social media. "We're making sure everyone knows who's responsible. We're fighting for a country where every child is cared for, no one goes hungry, and we all have access to the healthcare we need to live."
The Indiana Rural Summit planned a "Motorcade for Medicaid" to drive by rural hospitals across the state.
"We're using the event as a touchpoint to demonstrate the importance and value of local hospitals that are at risk of closing because they have historically relied on Medicaid for financial viability," organizer Michelle Higgs told The Republic. "We want to amplify the voices of those who are impacted, whether they're disabled, have a chronic illness, or are elderly."
Union members took to the streets from Miami, Florida to Seattle, Washington.
SEIU members marched in cities including Tampa; Orlando; Miami; Washington, D.C.; Allentown, Pennsylvania; New York City, Boston; and Las Vegas. Meanwhile, hundreds of union workers protested in downtown Seattle.
In Connecticut, SEIU members marched to the Brennan Rogers Magnet School, which closed due to a state funding shortfall.
"Cleaners, healthcare workers, construction workers, we are the ones that make this country run and we ask for no special privileges in return. but we are under attack," Ciro Gutierrez, a 32BJSEIU Connecticut commercial member, said.
Reflecting on the day of action, Poo concluded: "All across the country we showed that when our families stick together, we are powerful. When we share our stories, we break through. When we stand side by side—from small towns to big cities—we can't be ignored. And we won't be divided."