Causes: Carbon dioxide CO2
Image: Patrick Hendry
Summary
The element carbon (C) is in the rock and soil, the oceans, all living things, and the atmosphere. When living things die and are buried, depending on different conditions, some of of the carbon becomes calcium carbonate, which over millions of years turns into limestone. Others plants and animals become fossilized, eventually turning into coal, oil, and natural gas. When these ‘fossil fuels’ are burned, they emit the gas carbon dioxide (CO2) into the atmosphere.
While Earth’s climate has changed for many reasons, the relationship between CO2, temperature, and sea levels is well understood, well documented, and goes back many millions of years. This is because CO2 acts like an incredibly powerful planetary thermostat. During cold glacial epochs (colloquially called ‘ice ages’) the amount of CO2 in the atmosphere was approximately 180ppm (parts per million). During warm interglacial epochs CO2 was around 280ppm. This extra 100ppm of CO2 doesn’t seem like much, but itcorresponded to an increase in temperature of 6-8°C and as much as 120m higher sea levels.
This played out most recently at the end of the Last Glacial Maximum 23,000-19,000 years ago (Fig. 1). Due to Earth’s eccentric orbit around the sun over the next 7,000 years the planet warmed (Fig. 1 Last Deglacial Transition). Atmospheric CO2 increased 83ppm from 188ppm to 271ppm. Sea levels rose 125m as the ice sheets that covered Europe, North America, and parts of South America retreated. The two remaining polar ice sheets across Antarctica and Greenland acted as an air conditioning system that helped regulate and stabilize Earth’s complex weather systems. This stability enabled the growth and expansion of human civilization during the Holocene Epoch. Over the following 9,200 years, CO2 increased just 10ppm to reach 280ppm in the year 1800.
But over the next 200 years to the year 2000, carbon that had taken 200 million years to accumulate underground as fossilised dead plants and animals was burned to power the Industrial Revolution. This released staggering volumes of greenhouse gases, primarily CO2 into the atmosphere. It also powered the mass destruction of CO2-absorbing natural ecosystems, replacing them with greenhouse-gas emitting industrial-scale agriculture. As of 2026, atmospheric CO2 has passed the higher than the Mid-Pliocene Warm Period, when temperatures were 4C higher than today, ans sea levels were 25m higher (Fig. 1).
Around half the excess carbon dioxide that we’ve emitted into the atmosphere has been absorbed by the superhero of climate change: our oceans. However, this is is also changing the chemistry of the ocean, making it more acidic.
What’s covered in this page:
- Interactive graph showing changes to CO2 over time
- The carbon cycle: overview
- The calcium carbonate part of the carbon cycle
- The fossil fuel part of the carbon cycle
- How coal was formed
- How oil and ‘natural’ gas is formed
- How we know humans, not volcanoes are responsible for all this extra CO2
- How rising temperatures are melting permafrost and releasing ancient CO2
- Isn’t more CO2 great for growing more food?
- How many tonnes of carbon does it take to add 1ppm of carbon dioxide to the atmosphere?
- References and further reading
The carbon cycle: overview
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The first minute or two explains the carbon cycle, the rest of the video explains the impact of land use and intensification.
Plants need CO2 to grow, releasing O2 (oxygen) as a waste product. Animals and people need O2, and breathe out CO2 as a waste product. Because CO2 is a greenhouse gas it regulates Earth’s temperature. When there’s less CO2 in the atmosphere, more heat from the sun escapes from the atmosphere and Earth cools. When there’s more CO2 in the atmosphere, the opposite happens: Earth warms.
How the carbon moves around the planet, from deep in the oceans, through plants, animals, and the atmosphere is called the carbon cycle.
There are four main stages, however, there is no ‘start’ or ‘stop’ point, as the cycle is continuous:
Photosynthesis
Plants on land and in the ocean draw in CO2 (carbon + oxygen) from the atmosphere or seawater and use solar energy + H2O (water) to make carbohydrates (C6H12O6), which they use to grow. They release some CO2 along with an unwanted bi-product, (O2) into water and air, which animals and people use in respiration.
Respiration
Animals (including people) take in the O2 made by plants, and exhale CO2, which goes into the atmosphere. Plants use some of this for photosynthesis. However, during respiration plants also release about half the CO2 that they took up. As temperatures increase, the amount of CO2 they release also increases (see the tab ‘Isn’t more CO2 good for plants?)
When plants and animals die, if they’re not eaten, they decompose in the soil or fall to the bottom of the ocean (about 18% of our bodies are carbon, and, like our bones and teeth, coral and the shells of marine animals is made of calcium carbonate). Some gasses from decomposition, including CO2, escape into the atmosphere, but depending on where these plants and animals die and how quickly they are buried, quite a bit of the carbon is locked away underground. Over tens or hundreds of millions of years, it can become oil, coal, or limestone.
The world’s soils contain more carbon than terrestrial vegetation and the atmosphere combined. – Nature Geoscience
When fossil fuels are burned for energy (combusted), oxygen (O2) is used and CO2 is released. The more combustion occurs, the more O2 is taken out of the atmosphere. Humans are burning staggering quantities of fossil fuels for energy, so equally staggering amounts of CO2 is being released into the atmosphere, while O2 levels are falling (Fig. 2). The amount being lost is tiny—about nineteen O2 molecules out of every 1 million O2 molecules in the atmosphere each year. This small loss provides the chemical evidence that points the finger at humans burning fossil fuels, but it doesn’t affect our ability to breathe.
The calcium carbonate (limestone) part of the carbon cycle
The fossil fuel part of the carbon cycle
How coal was formed
During the 60-million year-long Carboniferous Period 386-299 million years ago, carbon dioxide in the atmosphere was ~800ppm. The climate was very warm and also wet (warmer air carries more moisture so there’s a lot more rain). Life had evolved to live in these conditions, so plants and animals thrived.
The tectonic plates that would eventually form the super-continent Pangaea were colliding. Mountains were being pushed up, which forced the crust downward beneath soggy tropical wetland regions. Over millions of years dead trees fell into the swampy ground. Here, they couldn’t be decomposed through normal processes that use oxygen. Instead, they turned into peat and eventually, coal.
This unique combination of colliding tectonic plates, a warm wet climate, and trees that had slowly evolved to thrive in these conditions, is not likely to be repeated in Earth’s future. Even if similar circumstances arose it would take hundreds of millions of years for life to adapt and the cycle to begin.
This is why coal is regarded as a non-renewable resource; it’s certainly not renewable in human terms.
How oil and ‘natural’ gas is formed
The conditions for oil formation were less unique than that of coal, however it is still regarded as a non-renewable fossil fuel because the process takes millions of years—far faster than oil is being extracted. Not all marine animals become oil, however. The vast bulk of them are are metamorphised into limestone (calcium carbonate).
How do we know volcanoes are not responsible for all this extra CO2?
Two reasons:
CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C versus 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio—about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with the atmosphere, the average 13C/12C ratio of the atmosphere decreases. – Eric Steig, isotope geochemist, University of Washington, Seattle.
CO2 from melting permafrost
Unlike ice, permafrost doesn’t ‘melt’ once temperatures rise above 0°C. Permafrost falls apart, and the organic material decomposes, just as frozen meat or vegetables left outside a freezer will decompose if not eaten.
Permafrost can be as thin as <1m and as thick as >1,000m. It covers approximately 22.79 million km² (about 24% of the exposed land surface) of the Northern Hemisphere.
If decomposition occurs in an environment where there’s oxygen, then CO2 is released. Some of this may be used by plants. If the environment is anaerobic (lacks oxygen), methane is released; this goes directly into the atmosphere.
See the page on melting permafrost and burning ice (methane clathrates) for more details.
Across nine million square miles at the top of the planet, climate change is writing a new chapter. Arctic permafrost isn’t thawing gradually, as scientists once predicted. Geologically speaking, it’s thawing almost overnight. As soils like the ones at Duvanny Yar (background photo and Fig. 7) soften and slump, they’re releasing vestiges of ancient life—and masses of carbon—that have been locked in frozen dirt for millennia. – Craig Welsh, National Geographic 2019 Photos: Katie Orlinsky; see her entire photo essay on melting permafrost.
Photos: Katie Orlinsky
Isn’t more CO2 great for growing more food?
This is often called the CO2 ‘fertilisation effect’
The speeding-up of photosynthesis—known as ‘CO2 fertilisation’—is well-known to be an important outcome of higher CO2 concentrations, along with increased water use efficiency. This is because as CO2 in the atmosphere increases, plants don’t lose as much water through their leaves because the number of their stoma decreases. So it might seem that drier conditions shouldn’t have such a large impact.
However, there are 5 things wrong with this assumption:
1. The nutritional values of plants is declining as carbon dioxide levels inceease. So animals and people will need to eat larger quantities (more calories) in order to get the same nutritional benefit as before. Physicist and science educator Dr. Derek Muller explains in the Video 3.
2. Orchard crops (apples, pears, Kiwi fruits, avocados, grapes, cherries etc) can grow only within a certain optimal temperature range suited to each species. It can take several years for orchards to reach maturity. An orchard planted today may not survive a future climate by the time it reaches maturity. Or it may be less productive for fewer year and more prone to pests and diseases, making them too expensive to maintain. For a more detailed explanation see the NZ Science Blog on this subject.
3. Diseases thrive in a warming climate, and this includes plant diseases. An orchard crop may be productive for fewer year and more prone to pests and diseases, making them too expensive to maintain. For a more detailed explanation see the NZ Science Blog on this subject.
As we have seen, pathogens tend to migrate to follow suitable climates, as long as their hosts are present. This means that as humans respond to climate change with altered agricultural practice, crop diseases are likely to keep pace. – Dr Helen Fones 2020
4. Rapid plant growth is leading to increased warming and droughts: As the climate warms, warmer spring temperatures are also arriving earlier. In the Arctic and sub-Arctic, plant leaves are coming out sooner each year. This early ‘leaf-out’ is triggering an array of feedback effects including increased surface warming in the Northern Hemisphere Arctic:
We identify warming hotspots in the Canadian Arctic Archipelago (~0.7 °C), east and west edges of Siberia (~0.4 °C) and southeastern Tibetan Plateau (~0.3 °C). …With continued warming, positive feedbacks between climate and leaf phenology are likely to amplify warming in the northern high latitudes. – Xu et al 2020
The Arctic is not alone in experiencing the not-so beneficial impacts of rapid springtime plant growth. Early and rapid spring growth across Europe in 2018 led to plants sucking up large quantities of water from the soil. By summer, the ‘legacy effect’ amplified summer drought conditions in areas that already were heat and/or water stressed.
Spring conditions led to an enhancement of photosynthesis early on in the growing season, but at the cost of strong soil-water depletion. In the crop-dominated areas in central Europe, increased growth in spring made ecosystems more vulnerable to drought in summer. – Dr Anna Bastos 2020
The assumption that tropical forests across Africa and the Amazon will continue to absorb endless quantities of CO2 also has recently been brought into question. While forests that have not yet been burned for agriculture continue to grow larger and absorb CO2:
Across Africa and the Amazon…higher temperatures and stronger drought conditions are slowing plant growth—and killing trees. – Hubai et al, 2020
5. The hotter it gets, the more CO2 plants release back into the atmosphere
Currently, around 25 per cent of carbon emissions from the use of fossil fuels is being taken up and stored by plants, which is good, as it helps reduce the concentration of greenhouse gases in the atmosphere.Our work suggests that this positive contribution of plants may decline in the future as they begin to respire more as the world warms. – Prof. Atkin, ANU
Photosynthesis peak at about 18C in many of the world’s leafy areas, then went into decline. However, respiration kept increasing, so plants were breathing out greenhouse gas faster, while taking progressively less in. With enough heat, they’d flip from sinks to sources. – Gibson, Stuff article on the above research
See also:
- 2025: The Guardian: Plants losing appetite for carbon dioxide amid effects of warming climate
- 2021: Ortiz-Bobea et al; Anthropogenic climate change has slowed global agricultural productivity growth, Nature Climate Change 11 pp 306-312
- 2020: Yale Climate Connections; More CO2 in the atmosphere hurts key plants and crops more than it helps:
- 2019: Yuan et al; Increased atmospheric vapor pressure deficit reduces global vegetation growth, Science Advances 5|8
