Every May, the Keeling Curve reaches its annual peak. The Northern Hemisphere's vegetation is still weeks away from the full photosynthetic draw-down of summer, so atmospheric carbon dioxide sits at its highest concentration of the year. In 2026, that peak has already produced the highest daily reading in the 68-year history of the measurement: 433.95 parts per million, recorded at the Mauna Loa Observatory on May 1, 2026.
The finalized May 2026 monthly average from NOAA's Global Monitoring Laboratory and the Scripps Institution of Oceanography will be published in the first week of June. Based on the April 2026 monthly average of 431.12 ppm and the trajectory of daily readings, analysts expect the final figure to land between 432 and 434 ppm, representing a year-over-year increase of approximately 2.3 to 3.0 ppm over the May 2025 peak of 430.5 ppm.
What the Keeling Curve Measures | A 68-Year Baseline
The Keeling Curve is the longest continuous instrumental record of atmospheric carbon dioxide concentration in the world. Charles David Keeling began taking measurements at Mauna Loa Observatory in Hawaii in March 1958, when the baseline reading was 316 parts per million. His choice of Mauna Loa was deliberate: the observatory sits at 3,400 meters elevation, above the boundary layer where local pollution sources interfere, and in the middle of the Pacific Ocean far from industrial centers. The readings represent a clean sample of well-mixed global air.
Keeling observed immediately that the concentration rose every Northern Hemisphere spring and fell every summer, tracking the breathing cycle of terrestrial vegetation. He also observed, over multiple years, that the baseline was rising. Both observations have continued without interruption for 68 years. The seasonal oscillation has grown slightly larger over time as forests absorb more carbon in summer. The long-term baseline has risen from 316 ppm in 1958 to more than 433 ppm today, an increase of 37 percent.
The 2026 Numbers in Context
The May 2025 finalized peak of 430.5 ppm (NOAA) and 430.2 ppm (Scripps) represented the first time in recorded human history that atmospheric CO2 crossed the 430 ppm threshold at seasonal peak. The year-over-year jump of 3.5 ppm from May 2024 to May 2025 was among the largest single-year increases in the record, driven by continued fossil fuel emissions and carbon releases from tropical ecosystems stressed by the preceding El Niño cycle.
The 2026 trajectory shows a more moderate pace. The April 2026 monthly average of 431.12 ppm sits 1.48 ppm above the April 2025 reading of 429.64 ppm. A moderate La Niña pattern in late 2025 and early 2026 has partially suppressed the tropical carbon release that amplified last year's spike. The net result is a smaller but still significant year-over-year increase.
| Year | May Peak (NOAA) | Year-Over-Year Change | Climate Context |
|---|---|---|---|
| 2026 | Pending June release | Est. +2.3 to +3.0 ppm | Highest daily reading 433.95 ppm on May 1. Moderate La Niña dampening. |
| 2025 | 430.5 ppm | +3.5 ppm | First crossing of the 430 ppm threshold. Post-El Niño carbon release amplified. |
| 2024 | 426.7 ppm | +2.7 ppm | Intense global heat anomalies accelerated carbon releases from tropical ecosystems. |
| 2023 | 424.0 ppm | +3.0 ppm | Start of the record-breaking global acceleration phase. |
| 2022 | 421.0 ppm | +1.9 ppm | Crossed the 420 ppm milestone for the first time. |
| 1958 baseline | 316 ppm | Start of record | First measurements by Charles David Keeling at Mauna Loa. |
Why the Annual Peak Occurs in May
The seasonal rhythm of the Keeling Curve is driven almost entirely by Northern Hemisphere vegetation. The Northern Hemisphere contains about 67 percent of Earth's total land area and the vast majority of its temperate forests, boreal forests, and agricultural land. In winter and early spring, those forests are dormant or just waking up. Plants are not absorbing significant CO2. Decomposition and respiration in soils continue releasing carbon year-round.
By June, full-canopy photosynthesis kicks in across North America, Europe, and Asia. Billions of trees and plants begin drawing CO2 out of the air faster than it is being emitted by soil and decomposition. The Keeling Curve bends downward. By September or October, as Northern Hemisphere vegetation enters senescence and stops photosynthesizing, the drawdown halts and the curve turns upward again.
This seasonal oscillation, sometimes called the planet breathing, has an amplitude of roughly 6 to 8 ppm between the May peak and the fall minimum. The fact that the minimum is rising at nearly the same rate as the maximum confirms that the underlying atmospheric accumulation is occurring faster than seasonal vegetation can compensate.
What 433 ppm Means in Paleoclimate Context
Ice core records from Antarctica and Greenland allow scientists to reconstruct atmospheric CO2 concentrations going back approximately 800,000 years. For the entirety of that record, CO2 concentrations fluctuated between roughly 180 ppm during glacial maxima and 280 ppm during interglacial warm periods. The current concentration of more than 433 ppm has no precedent in at least 800,000 years of Earth's climate history, and most paleoclimate researchers believe it has no precedent in the last 3 to 5 million years.
The rate of increase is equally without precedent. Natural transitions between glacial and interglacial periods raised CO2 by 80 to 100 ppm over thousands to tens of thousands of years. The current increase of approximately 135 ppm above pre-industrial levels has occurred in approximately 170 years, a rate of change roughly 100 times faster than any comparable natural transition in the ice core record.
The Ozone Connection | Why This Matters for This Site
CO2 does not directly deplete stratospheric ozone. But rising atmospheric CO2 has measurable effects on stratospheric chemistry and temperature that interact with ozone recovery in important ways. As CO2 increases, the troposphere warms but the stratosphere actually cools, because CO2 at high altitude radiates heat to space more efficiently than it absorbs it. Stratospheric cooling accelerates the formation of polar stratospheric clouds, particularly over Antarctica, which are the surfaces on which chlorine chemistry destroys ozone most efficiently.
This means that even as the Montreal Protocol successfully reduces stratospheric chlorine loading, climate change is creating conditions that could slow or complicate ozone layer recovery, particularly in the Arctic. The two planetary boundary issues, ozone depletion and climate change, are not independent. They are chemically coupled through stratospheric temperature. For coverage of ozone layer recovery data, see our ozone layer recovery analysis for 2026.
For context on how atmospheric science tools are tracking planetary changes, see our coverage of JWST's atmospheric characterization work and the 2026 Arctic sea ice minimum data. The real-time Mauna Loa CO2 data is published daily by the NOAA Global Monitoring Laboratory. The Scripps record is maintained at scrippsco2.ucsd.edu.
Frequently Asked Questions
- What is the Keeling Curve?
The Keeling Curve is the continuous instrumental record of atmospheric carbon dioxide concentration at Mauna Loa Observatory in Hawaii, begun by scientist Charles David Keeling in 1958. It is the longest uninterrupted record of its kind and the primary data source for tracking the long-term rise of atmospheric CO2 from human activity.
- Why does CO2 peak every May?
The Northern Hemisphere contains the majority of Earth's land area and temperate forests. In winter and spring, dormant vegetation is not absorbing CO2 from the air while soil decomposition continues releasing it. The seasonal peak occurs in May, just before Northern Hemisphere forests reach full photosynthetic activity and begin drawing CO2 back down through the summer months.
- What was the CO2 level before industrialization?
Pre-industrial atmospheric CO2 concentration was approximately 280 parts per million, stable for roughly 10,000 years during the current interglacial period. The current level of more than 433 ppm represents an increase of more than 53 percent above that baseline in approximately 170 years.
- When will the official May 2026 monthly average be published?
NOAA and Scripps Institution of Oceanography typically publish the finalized monthly average for May in the first week of June. Based on daily readings and the April 2026 average of 431.12 ppm, the May 2026 figure is expected to land between 432 and 434 ppm.
- Does CO2 affect the ozone layer?
Not directly. But rising CO2 cools the stratosphere, which promotes polar stratospheric cloud formation. Those clouds accelerate ozone-destroying chlorine chemistry, meaning climate change and ozone depletion are chemically coupled through stratospheric temperature, even though the two problems have different root causes.