Skip to content

Causes: Greenhouse gas – carbon dioxide (CO2)

Causes: Greenhouse gas – carbon dioxide

(Image: Mike Marrah)

Greenhouse gas: carbon dioxide (CO2)     and the carbon cycle

Summary

  • 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 in the tabs below).
  • Carbon is a powerful 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 processes turned the dead plants and animals into coal and oil (Figs. 2, 6 & 7 in the tabs below). When this happens over millions of years, the amount of carbon underground goes up, while it goes down in the atmosphere so Earth cools. At other times and over equally long periods volcanoes (and the odd comet) have released enough CO2 into the atmosphere to warm Earth.
  • This natural cycle has slowly moved the climate between warm ‘hothouse’ and cold  ‘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): 12C, 13C and 14C. Their relative ratio in the atmosphere is a chemical fingerprint (see the volcano tab below) that points squarely at humans (Kiwi kids helped measure this).
  • 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 making them more acidic.

(Image: Mike Marrah)

GHG: carbon dioxide (CO2)     & the carbon cycle

Summary

  • 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 powerful 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 processes 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): 12C, 13C and 14C. Their relative 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)

Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time 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 because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs.
  • Instructions for interactive graphs (Credit: The 2°Institute.)
  • Mouse over anywhere on the graphs to see the changes over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time 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 because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs.
  • Plants need CO2 to grow, releasing O2 (oxygen) as a waste product. Animals and people need O2, and they 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 (See Fig. 2 the ‘fossil fuel’ part and Fig. 3 the ‘limestone’ part, in the tabs below).

    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?)

    Decomposition
    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
     
     
    Combustion
    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. (Forests are also being burned, but the amount from fossil fuels for energy is orders of magnitude greater.)

    The graph above shows the 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.

Melting permafrost: image – Katie Orlinksy, National Geographic

More information