How hot was it on July 4, 1776 ?
Tony Pann, Meteorologist, WBAL Baltimore, on Facebook:
PHILADELPHIA WEATHER JULY 4th, 1776: the image (above) is a picture of Thomas Jefferson’s actual weather notes from July of that year! (monticello.org) At 1 PM on July 4th, it was 76 degrees and dry…rather cool for this time of the year.
Heat and humidity as severe, prolonged and far-reaching as this week’s would have been “virtually impossible” in the Northeast and eastern Canada before humans began warming the planet, a team of scientists said on Friday.
Emissions of carbon dioxide and other greenhouse gases from the burning of oil, gas and coal have trapped more of the sun’s heat at Earth’s surface, raising temperatures worldwide for more than a century. Summer hot spells are nothing new, but because of the excess heat around the planet caused by global warming, they can produce higher temperatures today than they once did.
To estimate how much climate change increased the likelihood of this week’s sweltering conditions, the scientists analyzed records of a measurement of heat stress called “wet bulb globe temperature,” a figure that accounts for humidity, wind and direct sunlight. Given that the heat wave is still unfolding, the researchers combined weather observations and forecast data.
The scientists are affiliated with World Weather Attribution, a collaboration that examines extreme weather events to pinpoint the degree to which they were influenced by global warming. Their findings have not yet been published in a peer-reviewed academic journal.
Last month, the same group of researchers analyzed the recent searing heat in Western Europe and concluded that climate change had also fueled that hot spell.
“On America’s 250th birthday, our study gives a clear reality check,” said Theodore Keeping, a climate scientist at Imperial College London who worked on the analysis. “The climate the country has today is fundamentally different to the one it had when the founding fathers signed the Declaration of Independence.”
Some events at London Climate Action Week were canceled because of the stifling temperatures.
Among the casualties was a session titled “Extreme Heat: Improving governance and strengthening action around the world,” which was scheduled for Wednesday.
Organizers said proceeding would have been a health hazard.
“The event venue, like most buildings in London, does not have any cooling mechanisms in place, and we cannot risk the wellbeing of speakers or guests by subjecting everyone to very unpleasant indoor conditions in addition to hot journeys to the venue,” they said in a statement.
In short, a session about extreme heat got canceled because of extreme heat.
“It is deeply ironic,” said Bob Ward, policy director of the Grantham Research Institute on Climate Change and the Environment at the London School of Economics, and one of the sponsors of the event.
A prominent example of recent extreme heat is the 2021 PNW heatwave, which provides important context for understanding extreme heat and its drivers in the US. A study by Fleishman et al., 2025 analysed over 70 research papers covering the heat wave that occurred across the Pacific Northwest of the United States and Canada in late June 2021. This study found that anthropogenic climate change has increased the mean daily temperatures, although it remains unclear whether trends in extreme daily maximum temperatures are increasing more rapidly than the trend in extreme daily mean temperature. The likelihood of such an event has increased substantially as compared to the pre-industrial era, ranging from 700% to infinity.
Furthermore, the likelihood of similar summer heatwaves is expected to increase significantly under continued warming.
Some events at London Climate Action Week were canceled because of the stifling temperatures.
Pick a week in May, perhaps?