Why does carbon dioxide in the air peak every May?
― A man who kept measuring the Earth's "breath" atop a Hawaiian mountain
Carbon dioxide in the air isn't constant all year round. It peaks every year around May and hits its lowest point around October. That's because the forests and grasslands of the Northern Hemisphere inhale it in summer and exhale it in winter. The first person to measure this "breathing of the Earth" did so with a device he built himself as a young researcher.
For some reason, news that "CO₂ levels have hit a record high" seems to come mostly from spring into early summer.
You rarely hear that kind of story in autumn. Yet it's not as if emissions suddenly drop in autumn alone.
This isn't just a coincidence. The air itself rises and falls on the same rhythm, year after year.
Only two reasons for the May peak and October dip
When leaves open in spring, plants pull in huge amounts of CO₂ through photosynthesis. From May to September, the level in the air steadily drops.
Once leaves fall, the uptake almost stops. Meanwhile, the microbes and fungi breaking down fallen leaves keep releasing CO₂ all winter.
And most of the world's land lies in the Northern Hemisphere. So the planet's overall breathing rhythm follows the Northern Hemisphere's seasons. Let's start with the story of the person who first confirmed this.
The young man who measured air while living in a tent
In the early 1950s, no one could yet measure CO₂ in the air with real accuracy. Readings varied wildly from one researcher to another, and were thought to differ greatly by place and day.
A young American researcher named Charles Keeling built his own device capable of measuring CO₂ with great precision. Around 1955, he is said to have camped in a coastal California forest, collecting air in bottles every few hours.
What he found was strange. Near the forest, readings rose at night and fell during the day. But in the well-ventilated afternoon air, the value settled to nearly the same number no matter where he measured. Keeling concluded that the Earth must have a "baseline" value that nobody yet knew.
A slow breath, seen from atop a Hawaiian mountain
To measure that "baseline" value, he needed a place far from forests and factories. The site chosen was the observatory on Mauna Loa, a mountain on Hawaii Island in the middle of the Pacific, at an altitude of about 3,400 metres. Measurements began in March 1958.
Once a full year of data was in, the value climbed toward spring, peaking in May. From there it fell through summer, bottomed out around October, and began climbing again. Move the slider in Figure 1 to see how it changes month by month.
Keeling took this as evidence that Northern Hemisphere plants were absorbing CO₂ in summer. The air atop the mountain is well mixed, carrying the imprint of forests on continents thousands of kilometres away. That's how a single observatory could reveal the breathing of an entire hemisphere's forests.
The jagged breathing sat on top of a rising slope
As measurements continued, something else became clear. Each year's May peak was a little higher than the previous year's May. The October dip, too, was higher than the year before.
Look at Figure 2. While repeating its jagged breathing pattern, the whole line climbs a slope. The value, roughly 315 ppm in 1958, had passed 420 ppm by the 2020s. That's the portion of CO₂ from burning coal, oil, and the like that forests and oceans couldn't fully absorb, left over in the air.
This upward-sloping graph is called the "Keeling Curve," after him. It's considered the first clear numerical record showing that human activity is changing the atmosphere.
Research that keeps measuring the same thing year after year tends to be seen as "surely we know enough by now," and struggles to attract funding. The Mauna Loa observations are said to have nearly been halted several times for lack of funds. Because they continued anyway, both the breathing zigzag and the long-term slope became visible. The observations have continued even after Keeling's death in 2005.
Measurements have been taken at the South Pole since around the same time, but the seasonal swing there is said to be very small, because the Southern Hemisphere is mostly ocean with few forests. By contrast, in Alaska, near the forest-covered Arctic, the swing is reported to be more than twice as large as at Mauna Loa.
Summary
Carbon dioxide in the air peaks in May because Northern Hemisphere plants stop absorbing it in winter, letting only the decomposition-released portion accumulate. In summer, when leaves are thick, forests absorb it again, bringing the level to its lowest around October. The rhythm of the Northern Hemisphere, which holds most of the world's land, becomes the breathing rhythm of the whole planet. Keeling was the first to measure this, atop a mountain in Hawaii. And the same record also showed that, zigzag and all, the air itself is changing year after year.
Spring air is thickest with the breath forests exhaled all winter.
And beneath that breathing, the slope climbs a little higher every year.
For how the ocean absorbs CO₂, see "The Ocean Is Absorbing CO₂ ― But What Happens Next?", and for why such a tiny amount changes temperature, see "Why Does CO₂ — Just 0.04% of the Air — Change the Temperature?". For another story of a small observatory's record moving the world, this time in Antarctica, see "Why Was the Ozone Hole Found at a Small Antarctic Station Before Satellites Found It?".
- Open the Japan Meteorological Agency's page on "year-on-year changes in CO₂ concentration" and look at the monthly graphs for Japanese observation points such as Ryori in Iwate Prefecture.
- Find the highest and lowest months within a year. As at Mauna Loa, it should be high in spring and low from late summer into autumn.
- Note the day a nearby park tree buds and the day its leaves fall. Compare these dates with when the graph starts falling and starts rising.
The further north an observation point is, the larger the swing tends to be. It's also worth comparing Yonaguni Island in Okinawa Prefecture with Ryori.
Want to go deeper? ― Terms, formulas, and how this connects to the textbookWe mark which level each section belongs to, from junior-high science to university-level courses
- JHSCovered in junior-high science
- HSCovered in high-school Biology, Earth Science, or Chemistry
- HS+Advanced high-school content, or textbook sidebar material
- UnivContent not taught in high school — university-level ecology, atmospheric chemistry, or geochemistry
- ResearchNot yet settled even at the university level — what researchers are actively investigating now
JHSTerms: this phenomenon has a name
- Keeling Curve: the graph of atmospheric CO₂ measured continuously at the Mauna Loa Observatory since 1958.
- Seasonal variation: the up-and-down cycle repeating every year. At Mauna Loa, the peak is in May and the low point is around October.
- Photosynthesis vs. respiration/decomposition: the process by which plants take in CO₂, versus the process by which living things release it. Which one wins out shifts with the season.
JHSHSCheck it with a formula: how much carbon did summer forests absorb?
Let's estimate, from the size of the yearly swing seen at Mauna Loa, how much carbon Northern Hemisphere plants absorbed on balance in summer. Since the air is well mixed, multiplying the change in concentration by "the total number of molecules in the atmosphere" gives the amount of carbon gained or lost.
| In symbols | ΔC = Δx × ( M ÷ m ) × 12 |
| In words | Carbon lost = change in concentration × total moles of air (mass of atmosphere ÷ mass of one mole of air) × mass of one mole of carbon |
| Where this comes from | The basic idea of molar amounts. Concentration is "how many molecules out of a million air molecules," so multiplying by the total number of air molecules gives the number of CO₂ molecules — and each one contains exactly one carbon atom. |
| Meaning of symbols | Δx is the change in concentration (in ppm), M is the total mass of the atmosphere (about 5.1×10¹⁸ kg), m is the mass of one mole of air (about 0.029 kg), and 12 is the mass of one mole of carbon (in grams) |
| Result of the formula above, per 1 ppm | said to be about 2.1 billion tonnes of carbon |
| Mauna Loa's yearly swing | about 6 ppm |
| CO₂'s weight relative to carbon's weight | 44 ÷ 12, about 3.67 times |
| CO₂ emitted by humans per year, from fossil fuels etc. | said to be about 37 billion tonnes |
| Value in 1958 and around 2024 | about 315 ppm and about 420 ppm (a 66-year gap) |
| Carbon lost in summer (billion tonnes) | 6 × 2.1 = 12.6 |
| Converted to CO₂ weight (billion tonnes) | 12.6 × 3.67 ≒ 46.2 |
| Compared to humans' annual emissions (times) | 46.2 ÷ 37 ≒ 1.25 |
| Concentration rise over 66 years (ppm) | 420 − 315 = 105 |
| Average yearly rise (ppm) | 105 ÷ 66 ≒ 1.6 |
Just from Mauna Loa's seasonal swing, Northern Hemisphere plants work out to absorbing roughly 46 billion tonnes of CO₂ on balance in summer — a scale that exceeds humanity's yearly emissions. But nearly the same amount is exhaled back out from autumn through winter. The breathing nets out to almost zero each year; it's human emissions that push the slope upward. Note that plants' actual total uptake is far larger than this — what we're seeing here is only the difference between "inhale" and "exhale."
HSHS+Why does the Northern Hemisphere win out, and why May?
HSPhotosynthesis minus respiration is called "net primary production"; subtracting what's released by the decomposition of fallen leaves and soil gives "net ecosystem production." In winter, photosynthesis nearly stops while decomposition continues, so this value turns negative. CO₂ keeps accumulating until leaves open in spring and the value turns positive again. That's why the peak comes at the end of spring, in May.
HS+Roughly two-thirds of the world's land lies in the Northern Hemisphere. The Southern Hemisphere's opposite season cancels some of this out, but because the North has vastly more forest, its rhythm dominates. Also, since it takes about a year for air to mix between hemispheres, the swing appears further smoothed out and smaller at Southern Hemisphere observation sites.
UnivHow the precise measurements work, and the carbon cycle
Keeling determined the CO₂ level using a "manometer," which precisely measures gas volume and pressure, and used it as a reference to calibrate a "non-dispersive infrared absorption" analyser — a method that exploits CO₂'s property of absorbing infrared light at specific wavelengths. His measurements became the foundation for estimating the "carbon cycle," the exchange of carbon among the atmosphere, oceans, and land ecosystems. The fraction of human-emitted CO₂ that remains in the air is called the "airborne fraction," and is thought to be roughly half.
📖 For the derivation of the formula and further reading: Keeling Curve (Japanese Wikipedia) / Carbon Cycle (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why the breathing swing is getting bigger. At high northern latitudes, the seasonal swing is reported to have grown larger than it was in the 1960s. Whether this is due to longer growing seasons in northern forests, increased farmland production, or some mix of the two, is still debated.
- How long forests and oceans will keep absorbing. Currently, forests and oceans absorb roughly half of what humans emit. Whether this fraction will fall as warming progresses is one of the most uncertain parts of future projections.
- Year-to-year variation in the rate of increase. In El Niño years, the rate of increase in the air tends to speed up. Exactly how much is due to drought and wildfires in tropical forests is still being worked out in detail.
In other words, even this article describes things "as far as we currently understand them." The record from a single observatory is still raising new questions.
Connections to the textbook (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| JHS | Science: "Plant structure and function," "Nature and humans" | Photosynthesis and respiration, the role of decomposers |
| HS | Biology Basics: "Ecosystems and matter cycling"; Chemistry Basics: "Molar amounts" | Net primary production, checking it with a formula |
| HS+ | Earth Science: "Atmospheric circulation" | Why the swing differs between north and south |
| Univ | Atmospheric chemistry, ecology, geochemistry | Infrared absorption measurement, the carbon cycle, airborne fraction |
| Research | Earth system science | The widening seasonal swing, the future of absorption |
| ― | Everyday connections | Why "record high" news clusters in spring, local trees budding and shedding leaves |
- Japan Meteorological Agency, "Year-on-Year Changes in Carbon Dioxide Concentration" (気象庁)
- NOAA Global Monitoring Laboratory, "Trends in Atmospheric Carbon Dioxide"
- Scripps Institution of Oceanography, "The Keeling Curve"
- Keeling, C. D. (1960). The concentration and isotopic abundances of carbon dioxide in the atmosphere. Tellus, 12(2), 200–203.
- Keeling, C. D. (1998). Rewards and penalties of monitoring the Earth. Annual Review of Energy and the Environment, 23, 25–82.
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding. The graphs show an approximate shape based on observed trends; for actual monthly values, please check the public data from the Japan Meteorological Agency and NOAA.