The Founding Fathers who convened in Philadelphia to ratify the Declaration of Independence on July 4, 1776, chose a pleasant day for their task. It was a Thursday, and the temperature at 6:00 a.m. was 68°F, increasing to a moderately warm but agreeable 76°F by 1:00 p.m., according to daily records kept by Virginia’s Thomas Jefferson.
The Earth’s climate at that time was more accustomed to such mild days than to the intense heat we experience in the 21st century. It was much later that scientists would first define the era encompassing the late 1700s as the Holocene—a period that began 11,700 years ago and continues to this day. The Holocene was initially temperate, with atmospheric carbon levels measuring approximately 280 parts per million (ppm)—enough to keep the Earth warm but not oppressive. Vast areas—or 46% of the continent—were covered in carbon-absorbing trees, further aiding in climate regulation. If anything, the planet was calibrated for colder conditions. The inaugural Independence Day occurred during the period known as the Little Ice Age, which spanned from 1300 to 1850 and saw North American temperatures fall 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,” states 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 reversal—a complete transformation of our world’s climate—has been largely attributable to human activity, and it is 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 marked by clusters of volcanic eruptions, which released a sun-shielding haze into the atmosphere, along with solar minimums—or periods of reduced solar activity—occurring intermittently from 1280 to 1830. “The sun is not a totally constant star,” Harper explains. “The power of the solar dynamo itself is changing.” Those factors helped lead to a shift in the Atlantic current, which further promoted the cooling.
Cool temperatures were not constant during the Little Ice Age, naturally. As always, daily weather is very distinct from climate patterns spanning decades or centuries, and America’s early years certainly had numerous scorchers.
“Some of those summers in the 1770s and 1780s were still really hot,” Harper states. “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, emitting greenhouse gases into the sky and counteracting the natural forces maintaining the Earth’s relative coolness. Concurrently, vast stretches of forested land around the world were being cleared and set ablaze to make way for agriculture. That practice, known as slash-and-burn farming, actually began 15,000 years ago, though it didn’t gain significant traction in North America until European settlers arrived in 1500. Since then, 90% of the continent’s forestland has vanished. In the Amazon, 17% of the rainforest has been destroyed. This not only removes hundreds of millions of acres of carbon-absorbing trees from circulation but also pumps more carbon into the skies as unwanted trees and surrounding brush are incinerated.
“Trees are a huge carbon stock,” Harper says. “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.”
Over the span of the past two and a half centuries, these 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). This has inundated the skies, with CO2 levels increasing from 280 ppm in Colonial times to 422.8 ppm in 2024, as per 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 commenced in the late 1800s, according to NASA and NOAA—and the problem is only predicted to worsen.
“What does one degree mean? What does two degrees mean?” Harper asks. “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 tiny segment of that planet raised the flag of a new country. Today, that country—and the world—face an existential peril the American colonists could not have foreseen.