How Has The Climate Changed Since America's First 4th of July?

The Founding Fathers who gathered in Philadelphia to adopt the Declaration of Independence on July 4, 1776 picked a nice day to do their work. It was a Thursday, and the temperature at 6:00 a.m. was 68°F, going up to a warmish but still pleasant 76°F at 1:00 p.m., according to daily records kept by Virginia’s Thomas Jefferson.  The planetary metabolism at the time was set more for such balmy days than it was for the increasingly suffocating summers we experience in the 21st century. It was in 1867 that scientists would first define the epoch that includes the late 1700s as the Holocene—a period that began 11,700 years ago and is still ongoing. The Holocene was originally temperate, with atmospheric carbon levels measuring about 280 parts per million (ppm)—enough to keep the Earth warm but not stifling. About a billion acres of North America—or 46% of the continent—were covered in carbon-absorbing trees, further helping to regulate the climate. If anything, the planet was calibrated for cold. The first Independence Day occurred during the period known as the Little Ice Age, which ran from 1300 to 1850 and saw temperatures in North America falling 1°C to 2°C (1.8° F to 3.6°F) below thousand-year averages. “It was quite a bit colder [than average] in the 17th century,” says Kyle Harper, professor of classics and letters at the University of Oklahoma and a faculty member at the Santa Fe Institute. “The 18th century is a little less extreme, but it’s still part of the Little Ice Age. The 19th century starts to get even colder for a little bit. And then, of course, it turns around.”  That turnaround—a wholesale reshaping of our world’s climate—has been attributable in large measure to humans, and it’s what makes today’s Independence Day so different from the one 249 years ago. The Little Ice Age that preceded the majority of climate-altering human activity was caused largely by clusters of volcanic eruptions, which released a sun-shielding haze into the atmosphere, along with four solar minimums—or periods of reduced solar activity—occurring on and off from 1280 to 1830. “The sun is not a totally constant star,” says Harper. “The power of the solar dynamo itself is changing.” Those factors helped lead to a shift in the Atlantic current, which furthered the cooling. Cool temperatures were not constant during the Little Ice Age, of course. As always, day to day weather is very different from decade to decade or century to century climate, and there were plenty of scorchers in America’s early years. “Some of those summers in the 1770s and 1780s were still really hot,” says Harper. “In 1787 when they were drafting the Constitution in Philadelphia it was hellishly hot.” Humanity would make that heat more common—and more intense. In 1760, the Industrial Revolution—a period of explosive factory-building and carbon-burning—began in Europe and North America, pouring greenhouse gasses into the sky and countering the natural forces keeping the Earth relatively cool. At the same time, great swaths of forested land around the world were being cleared and put to the torch to make room for agriculture. That practice, known as slash-and-burn farming, actually began 12,000 years ago, though it didn’t get started in earnest in North America until 1500 when European settlers arrived. Since then more than 25% of the continent’s forestland has disappeared. In the Amazon, the figure is about 20%. Not only does that take hundreds of millions of acres of carbon-absorbing trees out of circulation, it also pumps more carbon into the skies as unwanted trees and surrounding brush are incinerated. “Trees are a huge carbon stock,” says Harper. “You take something that was alive and had a lot of carbon in it and you burn it and that releases carbon dioxide into the atmosphere. The percentage of trees that we’ve cumulatively cut down definitely affects the Earth system.” Across the arc of the past two and a half centuries, those slash-and-burn practices, along with fossil fuel-burning factories and internal combustion engines have released an estimated 1.5 trillion tons of CO2 into the air, according to the National Oceanic and Atmospheric Administration (NOAA). That has swamped the skies, with CO2 levels rising from 280 ppm in Colonial times to 422.8 ppm in 2024, according to NOAA. In turn, temperatures have soared. Last year, the Earth was 2.65°F (1.47°C) warmer than it was when formal record-keeping began in the late 1800s, according to NASA and NOAA—and the problem is only forecast to get worse. “What does one degree mean? What does two degrees mean?” asks Harper. “Two degrees, when you’re talking about a global average, is a massive change. And beyond that, you talk about four degrees—it’s really like a different planet.”  Nearly 250 years ago, a small group of men on a little patch of that planet raised the flag of a new country. Today, that country—and the 194 others around the globe—face an existential peril the American colonists could not have foreseen.