Earth’s climate has always changed, driven by slow shifts in the planet’s orbit around the Sun. But the rapid warming happening now was started by humans burning fossil fuels, beginning around the mid-1800s with the Industrial Revolution. The difference between past climate shifts and today’s is speed: natural cycles play out over tens of thousands of years, while human-driven warming has reshaped the atmosphere in under two centuries.
Earth’s Climate Has Always Shifted
Long before humans existed, Earth cycled between ice ages and warm periods. These shifts were driven by three slow, predictable changes in how Earth moves around the Sun. The shape of Earth’s orbit stretches and compresses over time (eccentricity). The tilt of Earth’s axis wobbles between steeper and shallower angles (obliquity). And the direction the axis points gradually rotates like a spinning top (precession). Together, these are called Milankovitch cycles, named after the Serbian scientist who calculated their effects a century ago.
These orbital changes don’t add or remove much total sunlight. Instead, they redistribute where and when sunlight hits Earth’s surface, which is enough to trigger or end ice ages. Between one and three million years ago, ice ages arrived roughly every 41,000 years, matching the tilt cycle. About 800,000 years ago, the pattern shifted to a 100,000-year rhythm, aligning with changes in Earth’s orbital shape. The point is that natural climate change is real, but it operates on timescales of millennia, not decades.
How Scientists First Connected CO2 to Warming
The idea that certain gases trap heat in the atmosphere dates back to the 1850s. In 1856, an American scientist named Eunice Foote ran a simple experiment: she placed thermometers inside glass cylinders filled with different gases and set them in the sun. Carbon dioxide and water vapor heated up significantly more than other gases. Foote was the first person to notice this effect and to suggest that changes in the amount of these gases in the atmosphere could alter the climate.
Three years later, the Irish physicist John Tyndall conducted more detailed experiments confirming the same principle. Their combined work established the basic mechanism behind what we now call the greenhouse effect: certain gas molecules absorb heat energy that would otherwise escape to space, warming the atmosphere from within. Without any greenhouse gases at all, Earth would be a frozen, uninhabitable planet. The problem isn’t the greenhouse effect itself. It’s how much extra greenhouse gas humans have added.
The Industrial Revolution Changed Everything
The modern warming trend traces back to roughly 1850, when industrialization began scaling up in Britain and soon spread across Europe and the United States. Coal-powered factories, railways, and steam engines released carbon dioxide that had been locked underground in fossil deposits for millions of years. The United Kingdom was the world’s largest CO2 emitter in 1850, the first year reliable emissions data exists.
From 1850 through the mid-20th century, emissions grew nearly without interruption, fueled by industrialization and population growth across the Western world. Then the post-World War II economic boom, the rise of petroleum, and the global spread of industrial economies accelerated the trend dramatically. By 2022, global CO2 emissions were 182 times higher than they were in 1850. That’s not a gradual increase. It’s an exponential one.
As of early 2025, the atmosphere contained about 427 parts per million (ppm) of carbon dioxide, measured at NOAA’s Mauna Loa observatory in Hawaii. Before industrialization, that number hovered around 280 ppm for thousands of years. In geological terms, we’ve added the equivalent of millions of years’ worth of natural carbon release in less than 200 years.
It’s Not Just CO2
Carbon dioxide gets the most attention because it’s the largest contributor to warming and it lingers in the atmosphere for centuries. But methane and nitrous oxide play significant roles too. Methane traps far more heat per molecule than CO2, though it breaks down faster, lasting about 12 years in the atmosphere. Nitrous oxide persists for roughly 121 years and is even more potent molecule for molecule.
The sources of these gases differ. CO2 comes primarily from burning fossil fuels and, to a lesser extent, from deforestation that eliminates trees which would otherwise absorb carbon. Methane comes largely from agriculture (especially livestock and rice cultivation), energy extraction, and landfills. Developing countries, particularly in Asia, have become the largest methane emitters in recent decades, driven by rapid industrialization and agricultural expansion. Nitrous oxide rises mainly from fertilized soils and certain industrial processes.
Deforestation has been a quieter but persistent contributor since well before the oil age. Clearing forests for farmland releases stored carbon directly and removes the planet’s capacity to pull CO2 back out of the air. This was a significant source of emissions throughout the 1800s and remains one today, particularly in tropical regions.
How We Know Humans Are the Cause
The most comprehensive assessment comes from the IPCC, the international body that synthesizes climate research from thousands of scientists worldwide. Their latest report calculates that human activities warmed the planet by approximately 1.07°C between the 1850-1900 baseline and the 2010-2019 decade. The observed warming during that period was 0.9°C to 1.2°C, meaning human influence accounts for essentially all of it. Natural factors like volcanic eruptions and solar variability contributed somewhere between negative 0.1°C and positive 0.1°C over the same period. In other words, natural forces were roughly a wash.
Greenhouse gas emissions alone would have warmed the planet even more, by an estimated 1.0°C to 2.0°C. The reason actual warming came in lower is that other human-caused pollution, particularly aerosols from burning fossil fuels, reflects some sunlight back to space and partially masks the full greenhouse warming. This creates an uncomfortable paradox: cleaning up air pollution, which is necessary for public health, will likely reveal warming that was previously hidden.
Where the Heat Actually Goes
Most people think of climate change as an air temperature problem, but the atmosphere is only absorbing a fraction of the extra heat. The oceans store about 90% of the excess energy trapped by greenhouse gases. NOAA has tracked global ocean heat content in the upper 700 meters since 1955, and the trend has been rising steadily since about 1970.
This matters for several reasons. Warmer oceans fuel stronger hurricanes, accelerate ice sheet melting from below, and expand physically as they heat up, raising sea levels even before any ice melts. The ocean also acts as a buffer, meaning that even if emissions stopped tomorrow, the heat already stored in the deep ocean would continue warming the planet for decades. It’s a slow-moving but massive source of committed warming that’s already locked in.
Natural Cycles vs. Human Influence
When people ask “what started climate change,” they sometimes wonder whether what’s happening now could just be another natural cycle. The short answer is no, and the evidence is clear on this point. Natural orbital cycles operate over 41,000 to 100,000 years. The current warming has occurred in about 150 years. Natural forcings over the industrial era have contributed virtually zero net warming. And the chemical fingerprint of the extra CO2 in the atmosphere matches fossil carbon, not volcanic or oceanic sources.
Earth’s climate system has natural variability built in, including El Niño cycles, volcanic cooling events, and solar fluctuations. These create year-to-year noise in the temperature record. But the underlying trend, the consistent upward push over decades, is driven by the accumulation of greenhouse gases from human activity. The physics Eunice Foote demonstrated in 1856 with a glass tube and a thermometer is the same physics reshaping the planet today, just operating at a scale she never could have imagined.