CO2 is increasing, and the rate of increase is accelerating. Atmospheric carbon dioxide has risen from about 280 parts per million (ppm) before the Industrial Revolution to well over 420 ppm today. Not only is the concentration climbing every single year, but the annual jump in 2024 was the largest on record.
How Fast CO2 Is Rising
NOAA’s Global Monitoring Laboratory tracks atmospheric CO2 year by year. Over the past decade, the average annual increase has hovered between 2 and 3 ppm, but 2024 broke that pattern with a jump of 3.77 ppm, the highest single-year growth rate ever recorded. For context, here’s what the last ten years look like:
- 2015: +2.94 ppm
- 2016: +2.84 ppm
- 2017: +2.14 ppm
- 2018: +2.39 ppm
- 2019: +2.49 ppm
- 2020: +2.33 ppm
- 2021: +2.39 ppm
- 2022: +2.29 ppm
- 2023: +2.71 ppm
- 2024: +3.77 ppm
Even 2020, when global economic activity slowed dramatically during the pandemic, still saw CO2 rise by 2.33 ppm. The temporary dip in emissions that year barely registered in the atmosphere because existing CO2 lingers for centuries, and the reduction was small relative to total accumulated levels.
Why It Keeps Going Up
The core driver is fossil fuel combustion: burning coal, oil, and natural gas for electricity, transportation, heating, and industrial production. Deforestation and other land-use changes add to the total by releasing stored carbon and reducing the number of trees pulling CO2 out of the air. Cement manufacturing and other industrial chemical processes also contribute. Despite the global expansion of renewable energy, total fossil fuel use has continued to grow, which is why atmospheric concentrations keep climbing rather than leveling off.
Nature does absorb a significant share of what humans emit. The ocean alone takes in roughly 31% of the CO2 released into the atmosphere. Forests, soils, and other land ecosystems absorb another large chunk. But these natural “sinks” can only handle so much. The remaining CO2, roughly 40 to 45% of what’s emitted, stays in the atmosphere and accumulates year after year.
How We Know: The Keeling Curve
The longest continuous record of atmospheric CO2 comes from the Mauna Loa Observatory in Hawaii, where scientist Charles David Keeling began measurements in 1958. The resulting graph, known as the Keeling Curve, is one of the most important datasets in climate science. It shows an unmistakable upward trend over more than six decades, from about 315 ppm in 1958 to above 420 ppm now.
The curve also shows a small zigzag pattern layered on top of the upward trend. This reflects the seasonal breathing of the planet: during spring and summer in the Northern Hemisphere, plants absorb CO2 as they grow, pulling concentrations down slightly. In fall and winter, decaying leaves and reduced photosynthesis allow CO2 to tick back up. Globally, this seasonal swing averages about 0.4 ppm from peak to trough, though it’s more pronounced in the mid-latitudes where large forests are concentrated. That seasonal dip is tiny compared to the year-over-year increase, so the overall direction never reverses.
The Pre-Industrial Baseline
Scientists know what CO2 levels looked like before industrialization by analyzing tiny air bubbles trapped in Antarctic and Greenland ice cores. For roughly 10,000 years before the mid-1700s, atmospheric CO2 held remarkably steady at about 280 ppm. The current level represents a roughly 50% increase over that baseline, and nearly all of it has occurred in the last 150 years. The pace of change is far faster than anything found in the ice core record, which stretches back hundreds of thousands of years.
What Rising CO2 Does
Carbon dioxide traps heat in the atmosphere by absorbing infrared radiation that would otherwise escape into space. As concentrations rise, more heat is retained, raising global average temperatures. This warming drives a cascade of effects: more intense heatwaves, shifting rainfall patterns, shrinking glaciers, and rising sea levels.
The ocean pays a separate price. When seawater absorbs CO2, chemical reactions make the water more acidic. This process, called ocean acidification, threatens coral reefs, shellfish, and the broader marine food web. The ocean’s ability to keep absorbing CO2 at its current rate is not guaranteed either. Warmer water holds less dissolved gas, so as ocean temperatures climb, the carbon sink could weaken, leaving more CO2 in the atmosphere and further accelerating the cycle.
Is There Any Sign of a Slowdown?
Not in the atmosphere. While some individual countries have reduced their emissions over the past decade, global emissions have continued to edge upward. The 2024 growth rate of 3.77 ppm suggests the opposite of a slowdown, though part of that spike may reflect natural variability on top of the human-driven trend (El Niño events, for instance, can temporarily boost the growth rate by reducing how much CO2 tropical forests absorb). Even so, the underlying trajectory is clear: CO2 has risen every year since measurements began, and the average rate of increase over the last decade is roughly 70% faster than it was in the 1980s.
For atmospheric CO2 to stop rising, global emissions would need to fall to a level that natural sinks can fully absorb. For concentrations to actually decrease, emissions would need to drop even further, or carbon would need to be actively removed from the atmosphere at a massive scale. Neither scenario is close to reality at present.