Měnící se rovnováha CO₂ a O₂ v atmosféře

A long-term trend in increasing concentrations of carbon dioxide (CO₂) and declining concentrations of oxygen (O₂) shows that humans have been causing changes in the atmosphere composition by their activity – primarily by burning fossil fuels.

CO₂ concentrations¹ oscillate during the year, but are increasing in the long term: about 26 ppm just in the last decade (2015–2025). Concentrations were around 315 ppm in 1960, and reached around 425 ppm in 2025 – which means they have risen by 35% in only 65 years.

The graph also shows a trend in O₂ concentrations²: how much it changed each year compared to the reference year (1991). O₂ concentrations also oscillate during the year but are declining in the long term: around 55 ppm just in the last decade.

How are CO₂ and O₂ concentrations measured?

The method for exact determination of CO₂ concentrations (to 0.1 ppm, i.e. 0.00001%) was developed by Charles Keeling in 1952. He was surprised by the results of his measurements at first as the concentrations appeared to be changing randomly, depending on the direction of the wind. Then he realized that his results in San Francisco were influenced by nearby forests (photosynthesis) and local factories (combustion) and that concentrations must be measured at a place which is far enough from such factors. So he moved to the central Pacific: to Mauna Loa in Hawaii. It was not until he got to this remote location that his measurements started to make sense – concentrations proved stable in Hawaii. Keeling noticed that concentrations oscillate during the year – falling from May to October and rising in the remaining part of the year. He was observing Earth’s “breathing”.

During photosynthesis, plants capture CO₂ from the air and produce O₂. Conversely, they use O₂ and release CO₂ during respiration.

CO₂ + H₂O → O₂ + carbohydrates

Most of the world’s forests are located in the Northern Hemisphere. In summer, when deciduous trees are in full leaf, photosynthesis prevails; plants capture CO₂ and use the carbon to build their trunks and foliage. In winter, photosynthesis stops and plants begin to release CO₂ through respiration, as they burn energy to maintain their cells, even in a dormant state. Meanwhile, soil microbes decompose fallen leaves, further releasing CO₂ back into the air.

Keeling observed this oscillation between summer and winter but also noticed a long-term increase in CO₂ concentrations which he believed was caused by the burning of coal, oil, and natural gas.

During combustion, O₂ is consumed and CO₂ released. When coal is burned, the chemical reaction is simple: C + O₂ → CO₂.

When natural gas is burned, even more O₂ is consumed as water vapour is produced as well: CH₄ + 2 O₂ → CO₂ + 2 H₂O

The fact that rising CO₂ concentrations are caused by combustion was proven by Keeling’s son Ralph, who found a precise method to measure concentrations of oxygen in 1988. His results show an unnatural long-term decline in O₂ concentrations. Subsequent research, including isotopic analysis, confirmed that the rise in atmospheric CO₂ is attributable to fossil fuel combustion.³

Footnotes

1. The dataset of averaged monthly CO2 concentrations as measured in Mauna Loa Observatory in Hawaii is accessible on NOAA, Global Monitoring Library, Trends in CO2, CH4, N2O, SF6. ↩

2. The dataset of averaged monthly O2 concentrations as measured in Mauna Loa Observatory in Hawaii is accessible via the Scripps O2 Program. Dataset accessed 2026-01-22. ↩

3. E.g. Graven, H., Keeling, R.F., Rogelj, J. (2020). Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future. Global Biogeochemical Cycles, 34 (11). DOI: 10.1029/2019GB006170. ↩