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Causes: carbon dioxide

Causes: carbon dioxide

(Image: Mike Marrah)

Carbon Dioxide (CO2)


  • Carbon is in the rock and soil, the oceans, all living things, and the atmosphere. How it cycles between them is called the carbon cycle (Figs. 2 & 3).
  • Carbon is a very powerful planetary climate stabiliser. It acts like a control nob on the planet’s thermostat. When combined with oxygen to become the gas CO2 in the atmosphere, it keeps Earth warm. This happens as living things take carbon from the atmosphere and ocean and use it to grow bones and teeth and shells. When they die, some carbon is locked underground as calcium carbonate and turned into limestone (Fig. 3). Other process have turned it into coal and oil (Figs. 2, 6 & 7). When this happens over many millions of years, the level of CO2 in the atmosphere goes down, so Earth cools. At other times and over equally long periods volcanoes have released enough CO2 into the atmosphere to warm Earth.
  • This natural cycle has slowly moved the climate between ‘hothouse’ an ‘icehouse’ states since the planet formed ~4.6 billion years ago.
  • In less than 200 years humans have moved more carbon from the ground into the atmosphere than it took natural process hundreds of millions of years to do.
  • We know this because carbon comes in three forms (isotopes): C12, C13, and C14. Their ratio in the atmosphere is a chemical fingerprint that points squarely at humans. (Find out how Kiwi kids helped to measure this during Covid lockdown: Radio NZ Podcast Grass and the science of CO2)
  • More CO2 may mean plants grow faster and bigger, but they are also less nutritious, and by taking in too much water, they create a positive feedback effect leading to greater warming and more intense droughts.
  • Once temperatures reach a certain peak, plants release more CO2 than they absorb.
  • The oceans have absorbed around 50% of the CO2 resulting in ocean acidification and concurrent dramatic impacts on oceanic ecosystems.

Fig. 1: Instructions for this interactive graph (Credit: The Institute.)

  • Mouse over anywhere on the graph to see the changes in global atmospheric carbon dioxide over the last thousand years.
  • To see details for time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, and release it.
  • To see how this compares to the past 771,000 years, click on the ‘time’ icon on the top left.
  • Compare this to rising global temperatures by clicking the planet/thermometer icon at the top left corner.
  • To return the graph to its original position, double-click the time icon to the left of the thermometer/planet icon

The annual ups and downs in the graph are because plants accumulate carbon in the spring and summer and release some back to the air in autumn and winter. As the northern hemisphere has more land and more plants, carbon dioxide levels go up in winter. Annual measurements of carbon dioxide are an average of these ups and downs.

Fig. 2: The 'fossil fuel' part of the carbon cycle. Depending on conditions at the time, some dead plants and animals don't decompose. See Figures 4 and 5 for more detailed explanation of how some become oil and coal. NOTE: The processes described in Figures 2 and 3 work simultaneously. They have been included here as separate images to better illustrate the processes.
Fig. 2: The ‘fossil fuel’ part of the carbon cycle. Depending on conditions at the time, some dead plants and animals don’t decompose. See Figures 4 and 5 for more detailed explanation of how some become oil and coal. NOTE: The processes described in Figures 2 and 3 work simultaneously. They have been included here as separate images to better illustrate the processes.
Fig. 3: The ‘calcium carbonate’ part of the carbon cycle. When marine animals including corals die, their skeletons and shells are buried and compressed. Some ultimately become the metamorphic rock limestone. This is moved deep underground (subducted) where tectonic plates grind against one another. Eventually, it becomes the main source of CO2 erupted by volcanoes. This mixes with rain to become slightly acidic, which in turn weathers rocks and carries carbon to the ocean where it is used by marine animals and corals, completing the cycle. Some 8% of carbon emissions is from the production of cement, which is made from limestone.(Image: University of Illinois)

The carbon cycle

Plants need CO2 to grow, releasing O2 (oxygen) as a waste product. Animals and people need O2, and they breath 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 (Figs. 2 and 3).

There are four main stages, however, there is no ‘start’ or ‘stop’ point, as the cycle is continuous:

Plants on land and in the ocean draw in CO2 (carbon + oxygen) from the atmosphere or sewater and use solar energy + H2O (water) to make carbohydrates (C6H12O6), which they use to grow. They release some CO2 along with an unwanted biproduct, (O2) into water and air, which animals and people use in 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.

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 (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. 4).

How do we know volcanoes are not releasing all this CO2?

Two reasons. Volcanoes erupt CO2 along with other chemicals and rock that’s been melted deep underground. Much of this rock formed hundreds of millions of years earlier and is recycled by subduction (plate tectonics) (Fig. 3). Because rock is melted by heat, not combustion (burning), no oxygen is used, so oxygen levels should have stayed the same if volcanoes are to blame.

Secondly, the CO2 from volcanoes has a different isotopic ratio(4) than the CO2 from burning fossil fuels (Video 1).

Fig. 4: Decline in oxygen measured at Cape Grim, Tasmania. The location was selected to measure Earth’s atmosphere in 1976 because winds from Antarctica and the Indian Ocean hit no significant land masses in the way. The ups and downs in the graph are because there is a natural annual summer/winter ‘cycle’ in the atmosphere.

Video 1: Professor Richard Alley explains how the chemistry of the atmosphere clearly points to humans burning fossils fuels.

Carbon dioxide from melting permafrost

Fig. 5: 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 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, September 2019 issue of National Geographic magazine. Photo: Katie Orlinsky (see her entire photo essay on melting permafrost).

Permafrost is a combination of soil, sediment, and the remains of dead plants and animals that stay at or below 0°C for at least two years. Unlike ice, it 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.

If decomposition occurs in an environment where there’s oxygen, then carbon dioxide gas 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 (Video 2).

Video 2: Yale University presentation on the effect of melting permafrost.

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.

Melting permafrost is the result of a feedback effect of climate change, whereby anthropogenic forcing is  triggering natural forcing.

The IPCC has determined that the maximum ‘safe’ temperature Earth can reach is less than 2°C on average, and preferably no more than 1.5°C for disastrous impacts to be avoided.

However, the Arctic has already warmed much faster than anywhere else on Earth; it’s average temperature is now 4°C above what it was in 1960.

In NOAA’s 2019 report card, it’s estimated that melting permafrost is contributing 600 million metric tonnes of net carbon per year into Earth’s atmosphere (Video 3).

Video 3: NOAA summary of the state of the Arctic 2019.

“By 2100, near-surface permafrost area will decrease by 2-66% for RCP2.6 and 30–99% for RCP8.5. This could release 10s to 100s of gigatonnes of carbon as CO2 and methane to the atmosphere for RCP8.5.”  IPCC, 2019

How ‘fossil’ fuels are part of the ‘slow’ carbon cycle

Fig: 6: 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 has 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 (now the Appalachians in the US) 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. 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. Even if similar circumstances arose in the future, it would take hundreds of millions of years. This is why coal is regarded as a non-renewable resource; it’s certainly not renewable in human terms.
Fig. 7: Oil and natural gas. The conditions for oil formation were less unique, 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. Today, limestone is used to make cement, which is one of the major contributor to greenhouse gasses.

More CO2 does not mean plants grow better

The CO2 ‘fertilisation effect’ isn’t helping; 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, in theory plants don’t lose as much water through their leaves because the number of their stoma decreases. So drier conditions shouldn’t have such a large impact.

However, their nutritional values are declining. So animals and people will need to eat larger quantities (and that means more calories) in order to get the same nutritional benefit as before (Video 4).

Video 4: Dr. Derek Muller explains what’s causing our food to become less nutritious.

Moreover, orchard crops (apples, pears, Kiwi fruits, grapes 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, for example, may not survive a future climate by the time they reach maturity. Or they may be less productive for fewer year and more prone to pests and diseases moving in, 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

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-Artic, 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 those areas of the forests that have not yet been burned for agriculture continue to grow larger and absorb CO2, the rate of uptake is declining and plants and trees are showing signs of heat stress:

“Across Africa and the Amazon…higher temperatures and stronger drought conditions are slowing plant growth—and killing trees.” – Hubai et al, 2020

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


Less carbon dioxide cools Earth

  Sometimes a little too much. See ‘snowball Earth‘.

Natural Forcings:

Those that happen through natural changes. Anthropogenic Forcings are those due to human activities. Click here to learn about the main forcings and how they work (links to another page on this website).

Declining oxygen levels in the atmosphere:

The amount being lost is tiny—about nineteen O2 molecules out of every 1 million O2 molecules in the atmosphere each year. This loss provides the chemical evidence that points the finger at humans burning fossil fuels, but it doesn’t affect our ability to breathe.

Chemical fingerprint:

It has long been known that Carbon comes in three forms (isotopes): C12 (the most common), C13, and C14.

“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.

For more technical information see: Stuiver et al (1984): 13C/12C ratios and the transfer of biospheric carbon to the atmosphere. Journal of Geophysical Research: Atmospheres 89/D7, 11731-1748.

References and further reading