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THIS HEAT WAVE WOULDN’T HAVE HAPPENED IN 1776  
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Andrea Thompson
July 3, 2026
Scientific American
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_ The heat wave sending temperatures into the triple digits this July
Fourth is a prime example of climate change loading the dice. Such
heat and high humidity would have been “virtually impossible” when
the U.S. was founded _

Thermometer, Immo Wegmann

 

July is always the hottest month for the U.S., but Julys today are
significantly hotter than they were when Benjamin Franklin, Thomas
Jefferson and the other Founding Fathers declared the U.S. an
independent nation. The brutal heat wave
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smothering the eastern half of the country as it celebrates its 250th
birthday is stark reminder of just how profoundly the climate has
changed since the American Revolution.

In that time the U.S. has warmed by about 3 degrees
Fahrenheit—faster than the overall global average rise of 2.5
degrees F. That warming is driven by the greenhouse gases accumulating
in the Earth’s atmosphere from the burning of fossil fuels that
began on a massive scale with another revolution, the Industrial one.
In the mid-18th century, carbon dioxide concentrations in the
atmosphere were around 280 parts per million. This year they surpassed
430 ppm. And historically the U.S. has been the world’s largest
greenhouse gas emitter
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The warming of the planet creates noticeable changes in the weather we
experience every day, though it varies from place to place: some areas
of the U.S. are warming faster than others; some are getting wetter;
others are becoming drier. Winter snows have declined while downpours
have increased, and the hurricanes that batter our shores are becoming
stronger and causing larger floods.

Ed Hawkins, National Centre for Atmospheric Science, UoR. CC by 4.0
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But as it’s summer, let’s look at how heat—with one of the
clearest links to global warming —has changed. Summer has of course
always been the warm season, and heat waves have always happened. But
according to the temperature record going back to the late 1800s, one
thing is clear: summer temperatures are higher across the country
overall and heat waves last longer, are more intense and happen more
frequently than in the past. The 50 largest U.S. cities experience
twice as many heat waves they did in the 1980s. Take Philadelphia,
where the Declaration of Independence was signed and the Constitution
was ratified: the average July temperature there has risen by 4.4
degrees F just since 1970. And kids growing up there today experience
four more heat waves every summer than their grandparents did.

Back in the 1770s, there would have been roughly equal odds of setting
records for heat or cold, but now heat records are twice as likely.
The heat wave sending temperatures into the triple digits for this
July Fourth weekend
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is a prime example of an event for which climate change has loaded the
dice. Such heat and high humidity would have been “virtually
impossible” when the U.S. was founded, according to scientists with
the World Weather Attribution research group.

“On America’s 250th birthday, our study gives a clear reality
check,” WWA team member Theodore Keeping, extreme weather and
wildfire researcher at Imperial College London said in statement about
their analysis of the heat wave.

Because heat is the deadliest weather phenomenon
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authorities have warned those celebrating to take precautions—avoid
being outside at the hottest times of day, stay in air conditioning as
much as possible, and stay hydrated and aware of signs of heat illness
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Climate scientists and environmental advocates say that the event
underscores the urgent need to rein in emissions.

“Climate change is here,” WWA researcher Friederike Otto, also of
ICL, said in the press statement. “It’s already impacting the
things we enjoy in our everyday lives, and it will continue to get
worse the longer we drag out the inevitable transition to net zero
emissions.”

_ANDREA THOMPSON_
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senior desk editor for life science at Scientific American, covering
the environment, energy and earth sciences. She has been covering
these issues for nearly two decades. Prior to joining Scientific
American, she was a senior writer covering climate science at Climate
Central and a reporter and editor at Live Science, where she primarily
covered earth science and the environment. She has moderated panels,
including as part of the United Nations Sustainable Development Media
Zone, and appeared in radio and television interviews on major
networks. She holds a graduate degree in science, health and
environmental reporting from New York University, as well as a B.S.
and an M.S. in atmospheric chemistry from the Georgia Institute of
Technology. Follow Thompson on Bluesky [email protected]_
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_Founded 1845, __Scientific American_
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is the oldest continuously published magazine in the United States. It
has published articles by more than 200 Nobel Prize winners._

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covers the most important and exciting research, ideas and knowledge
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