Yes, carbon dioxide (CO2) is a greenhouse gas, and it is the single most important one driving climate change. CO2 traps heat that would otherwise escape from Earth’s surface into space, warming the planet in a process known as the greenhouse effect. The global average concentration of CO2 in the atmosphere reached 422.8 parts per million (ppm) in 2024, more than 50% higher than the 280 ppm level that held steady for nearly 6,000 years before the Industrial Revolution.
How CO2 Traps Heat
Sunlight passes through the atmosphere and warms the Earth’s surface. The surface then radiates that energy back upward as infrared radiation, a form of heat energy. CO2 molecules absorb specific wavelengths of this outgoing infrared radiation rather than letting it pass through to space. When a CO2 molecule absorbs infrared energy, it vibrates and re-emits the energy in all directions, sending some of it back toward the ground. This keeps the lower atmosphere warmer than it would be otherwise.
Other gases do the same thing. Water vapor, methane, and nitrous oxide are all greenhouse gases. But CO2 is used as the universal baseline for comparing them. The EPA assigns CO2 a “global warming potential” of 1, and every other greenhouse gas is measured against it. Methane, for instance, traps far more heat per molecule over a 20-year window, but CO2 matters most in the long run because of its sheer volume in the atmosphere and its persistence. CO2 emissions cause concentration increases that last thousands of years.
When Scientists First Proved It
The heat-trapping property of CO2 was demonstrated experimentally in 1856 by Eunice Newton Foote, an American scientist. She filled glass cylinders with different gases, placed thermometers inside, and set them in the sun. The cylinder containing carbon dioxide warmed the most and held its temperature long after she moved it into shade. “An atmosphere of that gas would give to our earth a high temperature,” she wrote, describing the greenhouse effect decades before the term existed.
Three years later, the Irish physicist John Tyndall ran more sophisticated experiments using infrared radiation specifically, confirming that CO2 and water vapor absorb heat energy radiated from warm surfaces. Tyndall is widely credited as the father of climate science, though Foote’s earlier work established the core observation.
How Much CO2 Is in the Atmosphere Now
NOAA’s Mauna Loa Observatory in Hawaii has tracked atmospheric CO2 since 1958. Before the Industrial Revolution began in the mid-1700s, CO2 levels were consistently around 280 ppm. By 2022, levels had crossed 421 ppm. Daily readings from March 2025 show concentrations approaching 430 ppm, and the number continues climbing by roughly 2 ppm per year.
That represents a 50% increase over pre-industrial levels, a shift that took less than 300 years. For context, ice core records show CO2 stayed between about 180 and 280 ppm for hundreds of thousands of years before humans began burning fossil fuels at scale.
Why CO2 Levels Keep Rising
The natural carbon cycle moves enormous amounts of CO2 between the atmosphere, oceans, soil, and living things. Plants absorb it during photosynthesis, oceans dissolve it, and decomposition and volcanic activity release it. For millennia, these flows roughly balanced out.
Burning coal, oil, and natural gas adds carbon that had been locked underground for millions of years back into the atmosphere. Deforestation compounds the problem by removing trees that would otherwise pull CO2 out of the air. The result is a net surplus: more CO2 enters the atmosphere each year than natural processes can remove. That surplus accumulates, which is why concentrations keep climbing even though the ocean and land still absorb large amounts.
Effects Beyond Warming
Rising CO2 does more than heat the atmosphere. The ocean absorbs about 30% of the CO2 released by human activity. When CO2 dissolves in seawater, it forms carbonic acid, making the water more acidic. Since the Industrial Revolution, the pH of surface ocean water has dropped by 0.1 units, which translates to roughly a 30% increase in acidity. That shift threatens shell-forming organisms like corals, oysters, and certain plankton species that form the base of marine food chains.
Higher CO2 concentrations also intensify the water cycle. A warmer atmosphere holds more moisture, which fuels heavier rainfall events in some regions and more prolonged droughts in others. These cascading effects are why CO2 receives so much attention relative to other greenhouse gases: its volume, its longevity in the atmosphere, and the breadth of its consequences make it the primary driver of the climate changes unfolding today.