When would the earth run out of carbon dioxide without man made climate change?

Question

One of my friends told me quite seriously that Gaia had evolved humans to burn the fossil fuels and thus stop the earth running out of CO₂.

Leaving aside the obvious nonsense it did raise an interesting question, what stops us running out of CO₂? Apart from volcanism are there any processes that return carbon captured in fossil fuels and limestone to the atmosphere? Is it plausible that the planet could run so low on atmospheric carbon that plants couldn't photosynthesize?

Answer 1

The answer is The Carbon Cycle, which includes carbon cycling between atmosphere and oceans, vegetation and soils ensure the Earth does not run out. Volcanoes add very little in proportion to Plant Respiration or Soil Decomposition or Ocean Loss (or even fossil fuel burning) - but unlike those, volcanic CO₂ (and similarly, fossil fuel CO₂) is not accompanied by processes like Photosynthesis, Litterfall or Ocean Uptake that draw CO₂ out of the atmosphere. However there is some ongoing loss of carbon to Burial to Sediments.

Without the fossil fuel burning the fluxes (flows) were close to being in equilibrium, without ongoing consumption and enduring loss of atmospheric CO₂ -

Answer 2

Phanerozoic CO₂ atmospheric levels:

Biosphere has been consuming CO₂ and storing it in the lithosphere since the Cambrian explosion.

The cyclic pattern is related with the tectonic cycles, named the Wilson Cycle, wich has formed two super continents during Phanerozoic period: Rodinia and Pangea.

When a super continent breaks, a new ocean is formed and after Cambrian CO₂ begins to get stored in the carbonate platforms close to newly formed coastlines, reducing the levels in the atmosphere.

When the ocean formed in a Wilson Cycle is old enough, oceanic crust begins to subduct the continents and after Cambrian part of this carbon returns to the atmosphere from calc-alkaline magmas of volcanic arcs. This is what is starting to happen in the Atlantic Ocean.

Currently, the Atlantic has three isolated subduction zones: the Lesser Antilles in the Caribbean, the Scotia Arc in the South Atlantic, and the Gibraltar Arc in the Mediterranean.

source: earthmagazine.org

Part of the CO₂ gets stored in the Continental Crust, staying in the lithosphere reservoir in the form of CaCO₃. So the balance of Wilson Cycle since the Cambrian is a reduction of CO₂ in the atmosphere (shown in the first graph), and an increase of Continental Crust. The second consequence applies for the hole Earth History where the Wilson Cycle has been operating.

A teacher told us in University the Pacific ring is more powerfull than Atlantic dorsal, and Atlantic Ocean is going to disappear. If that is true, in the following millions of years, part of the CO₂ stored in the Atlantic Ocean will return to the atmosphere.

So answering your question, the Earth would not run out of CO₂ in the next million years with no human intervention. Some CO₂ stored in the Atlantic Ocean is going to be released to the atmosphere. Maybe in the next Wilson Cycle when the Atlantic Ocean closes forming a new supercontinent and breaking again forming a new ocean, but it is a nosense to talk about climate in said 100my as human technology for sure will modelate climate in the future.

Apart from volcanism are there any processes that return carbon captured in fossil fuels and limestone to the atmosphere?

Apart from humans burning fossil fuels and volcanism, CaCO3 starts to dissolve in the oceans at certain profundity/pressure. The level when CaCO₃ start to dissolve is known as Carbonate compensation depth (CCD). The ocean is in equillibrium with the atmosphere because of Henry's law so that affects atmospheric CO₂ levels.