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To start life there has to be $\ce{CO2}$.

$\ce{Solar energy + 6CO2 + 6H2O -> C6H12O6 (sugars) + 6O2}$

$C_6H_{12}O_{6} + O_2 \longrightarrow H_2O + {CO}_2 + \text{Energy}$

So to start this cycle, life needs $\text{CO}_2$. Where did the $\text{CO}_2$ came from?

$C + O_2 + \text{Hot temperature in the Earth} \longrightarrow {CO}_2$

Volcanoes erupt ${CO}_2 \longrightarrow {CO}_2$ in the Atmosphere.

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  • $\begingroup$ I'd question the assumption that you need CO2 to start life. For instance, there are chemosynthetic bacteria that don't depend on photosynthesis or solar energy. $\endgroup$
    – jamesqf
    Commented Apr 8, 2019 at 3:35
  • $\begingroup$ you are thinking of it backwards, CO2 is normal, with quite a few ways to form naturally, it is an oxygen atmosphere that is weird and needed life. $\endgroup$
    – John
    Commented Feb 19, 2022 at 12:48
  • $\begingroup$ Let me rephrase it differently. When there is no life, how is CO2 then formed? A process of the right temperature and process? $\endgroup$
    – Marcel Plan B
    Commented Feb 19, 2022 at 19:24
  • $\begingroup$ Venus seems to manage CO$_2$ without life very well. $\endgroup$
    – Vogon Poet
    Commented Feb 20, 2022 at 2:36

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The answer is Volcanos. There might be other inorganic processes capable to produce $\text{CO}_2$, but on Earth, the main inorganic source of $\text{CO}_2$ are volcanoes.

In some period of Earth's history, there is evidence of large glaciations events, some of them are thought to have been triggered by the lack of $\text{CO}_2$ (like the Snowball Earth and Permo-carboniferous glaciations), and volcanoes are with no doubt the main candidate to have been responsible of restoring the warm climate by feeding large amounts of $\text{CO}_2$ into the atmosphere over millions of years.

$\text{CO}_2$ is an important component of rocky planets, but it is mostly dissolved (in large quantities) in the magma that exist (or once existed) in the interior of such planets. However, when the magma rises close to the surface, the drop in pressure reduce the solubility of $\text{CO}_2$ and it form bubbles of gaseous $\text{CO}_2$, that can then be expelled into the atmosphere trough volcanic eruptions, that in addition to teprha, and rocks include large amounts of volcanic gases of which $\text{CO}_2$ is one of the main components. Probably that process of exsolution of $\text{CO}_2$ from hot molten rocks was happening even before the existence of volcanoes as we know them.

The same process happened once in Mars and Venus, whose atmospheres are composed mostly by $\text{CO}_2$ (96% and 96.5% respectively), suggesting that abundant $\text{CO}_2$ was also present in the early Earth's atmosphere.

Currently, it is estimated that volcanoes input to the atmosphere about 100 million tons of carbon in average per year in the form of $\text{CO}_2$, as illustrated by this NASA figure of the Carbon cycle:

enter image description here (Originally from here)

You can see that many processes move carbon around, but the main reservoir by far is the crust, and that reservoir is tiny compared with all the $\text{CO}_2$ stored deeper in the Earth's interior.

Finally, in addition to be required by photosynthesis, $\text{CO}_2$ is part of the Earth's thermostat, that together with the silicate weathering have been key to keep the Earth's temperature in the range of liquid water. Arguably another very important need for the existence of life as we know it.

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  • $\begingroup$ Thank you very much for your reply. You mentioned: "CO2 is an important component of rocky planets, but it is mostly dissolved (in large quantities) in the magma that exist (or once existed) in the interior of such planets." Perhaps a silly question, but how is the CO2 than actually formed by itself on these rocky planets, just: C + O2 --> CO2 (with the right temperature and pressure?). This must be an inorganic proces. $\endgroup$
    – Marcel Plan B
    Commented Mar 27, 2018 at 20:40
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    $\begingroup$ I don't know what was exactly the process that lead to the production of CO2. But I undesrstand there was already some CO2 in the solar nebula before the formation of the sun and the planets. But most of the carbon was in CO and methane molecules. If you want to know more this could be a good source: lpi.usra.edu/books/MESSII/9033.pdf $\endgroup$
    – Camilo Rada
    Commented Mar 27, 2018 at 21:43
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    $\begingroup$ @Marcel Plan B: Sure, but reflect that there are a LOT of inorganic oxides in Earth's crust (and mantle, since per Google it's mostly silicates). Here's a link to the 10 most common compounds in the crust, all oxides: education.jlab.org/glossary/abund_com.html $\endgroup$
    – jamesqf
    Commented Mar 28, 2018 at 3:41
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    $\begingroup$ I found this interesting article on CO2 ice "A NEW SOURCE OF CO2 IN THE UNIVERSE: A PHOTOACTIVATED ELEY-RIDEAL SURFACE REACTION ON WATER ICES", URL: iopscience.iop.org/article/10.1088/2041-8205/791/2/L21 $\endgroup$
    – Marcel Plan B
    Commented Mar 28, 2018 at 13:02
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    $\begingroup$ Co2 is relatively easy to make , all you really need is carbon and oxygen, inorganic carbon and oxygen bearing minerals are fairly common. Co2 is basically the default atmosphere for warm rocky planets, consider Venus and Mars also have co2 atmospheres. $\endgroup$
    – John
    Commented Mar 28, 2018 at 17:59
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I thought they where always present. Methane has a higher effect on Earth? When the carbon dioxide is high enough it will eventually warm the Methane to melt at the poles which then would cause the bulk cloud cover triggering an ice age which then freezes the carbon dioxide and methane again. Methane takes for longer to be absorbed by the ocean and Earth like carbon dioxide. The Earth cannot get to hot or cold because of the balance of the 2 molecules and their role in weather. Plants absorb some carbon dioxide which helps balance this.

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