NCEA Level 1 · Year 11 · Science · 2 minutes
The Keeling Curve: how we know CO₂ is rising
A yearly zigzag from plants, a steady climb from fossil fuels: the Keeling Curve.
Key terms
- Parts per million (ppm)
- A way to measure a gas: out of every million molecules of air, this many are carbon dioxide.
- Photosynthesis
- How plants take in carbon dioxide, which is why levels dip in the northern summer.
- Respiration
- A process that, along with decay, puts carbon dioxide back into the air when photosynthesis slows in winter.
- Fossil fuels
- Coal, oil and gas, which took millions of years to build up underground and release carbon dioxide when burned.
- Greenhouse gas
- A gas such as carbon dioxide that absorbs heat radiated from Earth's surface and gives some of it back down.
- Keeling Curve
- The graph of carbon dioxide measured at Mauna Loa since 1958, showing a yearly zigzag on a rising trend.
Transcript
Every breath you take holds a little more carbon dioxide than the air your grandparents breathed as kids. We know that, because one scientist kept measuring the air, for decades.
In nineteen fifty eight, Charles David Keeling started measuring carbon dioxide on Mauna Loa, a volcano in Hawaii. Up there, the air is clean, far from factories and traffic. His first reading: three hundred and thirteen parts per million.
Parts per million, or ppm, means out of every million molecules of air, about three hundred and thirteen were carbon dioxide. Tiny, but it matters.
Within his first year, he saw a pattern: a peak around May, and a low around October. That's the Northern Hemisphere's plants breathing. In the northern summer, they take in carbon dioxide for photosynthesis. In winter, photosynthesis slows down, and respiration and decay put it back.
But underneath the zigzag, the whole line kept climbing. Every single year since, the lowest point has been higher than the year before.
In twenty twenty five, Mauna Loa averaged about four hundred and twenty seven parts per million. That's more than a third above Keeling's first reading. And it's about fifty percent above the level before the Industrial Revolution, around two hundred and eighty.
So where's it coming from? Mostly from burning fossil fuels: coal, oil and gas. That carbon took millions of years to build up underground. Combustion releases it as carbon dioxide faster than plants and oceans can soak it up, so the extra stays in the air.
Why does it matter? Carbon dioxide is a greenhouse gas. It absorbs heat radiated from Earth's surface, then gives it out again in all directions, some of it back down. More carbon dioxide means more heat trapped, and a warmer planet.
And it's not just Hawaii. At Baring Head, near Wellington, scientists have measured carbon dioxide since nineteen seventy two: the longest record in the Southern Hemisphere. It has climbed from about three hundred and twenty six to about four hundred and twenty four.
So, for your exam. One: the yearly zigzag comes from photosynthesis and respiration. Two: the rising trend comes mainly from burning fossil fuels. Three: more carbon dioxide strengthens the greenhouse effect. All from one curve, started by one careful scientist, and still climbing.
Sources
Every fact in the video, and where it comes from.
- 1aps.org/apsnews/2025/02/keeling-measurements-co2-mauna…
- Charles David Keeling's first CO2 reading at the Mauna Loa Observatory, Hawaii, was 313 ppm on 29 March 1958; Mauna Loa was chosen for its high elevation and remote location (clean, well-mixed air, little influence from industry, traffic or local plants). Keeling died in 2005; the Scripps CO2 Program continues under his son Ralph Keeling.
- The early Mauna Loa record showed CO2 rising to a peak in May and falling to a minimum in October, which Keeling attributed to plants withdrawing CO2 for growth in summer and returning it in winter.
- Respiration happens all year; the seasonal swing comes from photosynthesis exceeding respiration and decay in the northern growing season and slowing in winter (photosynthesis vs ecosystem respiration).
- 2en.wikipedia.org/wiki/Keeling_Curve
- The early Mauna Loa record showed CO2 rising to a peak in May and falling to a minimum in October, which Keeling attributed to plants withdrawing CO2 for growth in summer and returning it in winter.
- 3climate.gov/news-features/understanding-climate/climate-ch…
- The seasonal cycle is driven by Northern Hemisphere vegetation: photosynthesis draws CO2 down in the northern growing season; respiration and decay release it in the northern autumn and winter. The annual maximum is usually in May (NOAA monthly data: May in 61 of 67 complete years; minimum in September or October).
- Before the Industrial Revolution (mid-1700s) atmospheric CO2 was 280 ppm or less; CO2 is rising mostly because of fossil fuels people burn for energy; CO2 is a greenhouse gas that absorbs and radiates heat.
- Respiration happens all year; the seasonal swing comes from photosynthesis exceeding respiration and decay in the northern growing season and slowing in winter (photosynthesis vs ecosystem respiration).
- 4today.ucsd.edu/story/annual-carbon-dioxide-peak-passes-anothe…
- The seasonal cycle is driven by Northern Hemisphere vegetation: photosynthesis draws CO2 down in the northern growing season; respiration and decay release it in the northern autumn and winter. The annual maximum is usually in May (NOAA monthly data: May in 61 of 67 complete years; minimum in September or October).
- 5gml.noaa.gov/webdata/ccgg/trends/co2/co2_mm_mlo.txt
- The seasonal cycle is driven by Northern Hemisphere vegetation: photosynthesis draws CO2 down in the northern growing season; respiration and decay release it in the northern autumn and winter. The annual maximum is usually in May (NOAA monthly data: May in 61 of 67 complete years; minimum in September or October).
- Curve drawn from NOAA Global Monitoring Laboratory monthly means for Mauna Loa (March 1958 – September 2026; data to April 1974 from C. D. Keeling, Scripps Institution of Oceanography; Dec 2022 – Jul 2023 from Maunakea after the Mauna Loa eruption).
- 6gml.noaa.gov/webdata/ccgg/trends/co2/co2_mm_mlo.txt (own ca…
- In the NOAA/Scripps Mauna Loa monthly record, every complete year from 1959 to 2025 has a lower annual minimum than the year after it (each year's lowest month is higher than the previous year's).
- 7gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_mlo.txt
- Mauna Loa annual mean CO2 for 2025 was 427.35 ppm (1959: 315.98 ppm). 427 is about 36% above 313 and about 52% above 280.
- Mauna Loa annual mean CO2 was about 316 ppm in 1959 (315.98) and 427 ppm in 2025 — so air today holds clearly more CO2 than in the 1950s–60s.
- 8science.nasa.gov/earth/earth-observatory/the-carbon-cycle
- Fossil fuel carbon took millions of years to accumulate; burning it releases carbon much faster than natural processes remove it; land plants and the ocean have taken up about 55% of the extra carbon; greenhouse gases absorb heat radiated by Earth and re-emit some of it, warming the surface.
- 9gml.noaa.gov/ccgg/trends/ (NOAA GML definitions)
- ppm (parts per million) is a mole fraction: the number of CO2 molecules per million molecules of (dry) air.
- 10earthsciences.nz/about-us/celebrating-100-years-of-science/trac…
- Baring Head, just south of Wellington, has measured CO2 since 1972 (first measurements 14 December 1972), when levels were about 326 ppm; recently around 424 ppm. It is the longest continuous CO2 record in the Southern Hemisphere. Run by NIWA, which merged into Earth Sciences New Zealand in July 2025.
- 11niwa.co.nz/news/internationally-significant-co2-site-cele…
- Baring Head, just south of Wellington, has measured CO2 since 1972 (first measurements 14 December 1972), when levels were about 326 ppm; recently around 424 ppm. It is the longest continuous CO2 record in the Southern Hemisphere. Run by NIWA, which merged into Earth Sciences New Zealand in July 2025.
Image and sound credits
- CO₂ data: NOAA Global Monitoring Laboratory and Scripps Institution of Oceanography (C. D. Keeling), Mauna Loa monthly means, March 1958 – September 2026 (gml.noaa.gov/ccgg/trends)
- Charles David Keeling and John Chin at Mauna Loa Observatory, 1997: John Miller, NOAA, public domain, via Wikimedia Commons
- Charles David Keeling receives the National Medal of Science, 2001: National Science Foundation, public domain, via Wikimedia Commons
- Mauna Loa Observatory: NOAA Research, public domain, via Wikimedia Commons
- Mauna Loa Observatory, north flank of Mauna Loa: Brian Vasel, NOAA, public domain, via Wikimedia Commons
- CO₂ infrared analyser and air-sample flask at Mauna Loa Observatory, 1982: Cdr John Bortniak, NOAA Photo Library, public domain
- “Forest, Siberian taiga, Krasnoyarsk Sea, Russia” by Vyacheslav Argenberg, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0), via Wikimedia Commons
- “Yallourn brown coal fired power station” by Joel200716, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0), via Wikimedia Commons
- “Baring, Pencarrow Heads & Miramar Peninsula” by Aidan from Wellington, New Zealand, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0), via Wikimedia Commons
- Lighthouse at Baring Head, 1949: Whites Aviation, Alexander Turnbull Library (WA-), CC BY 4.0, via DigitalNZ (natlib.govt.nz/records/22330618)
- Map data: Natural Earth (public domain)
- Charts and diagrams drawn in code from the data above
- Paper textures: ambientCG (CC0) · Torn paper edges generated with AI
- Sound effects: Freesound contributors (CC0)
- Music: generated with ACE-Step · Voice: Gemini TTS
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