Excess carbon dioxide in the atmosphere changes Earth’s climate to far less hospitable to us. However, Earth’s atmosphere is but a thin sliver of gas around a huge ball of rock. How large is our carbon dioxide problem, measured on geological scales?

Chalk is calcium carbonate or CaCO₃, a compound of calcium and carbon dioxide. For much of Earth’s history, marine creatures from microscopic free-floating algae, to oysters, to coral reefs used chalk to build their shells and houses. When these creatures died, their shells sank to the bottom of the sea, or were built upon by new generations of creatures. All CO₂ in them was safely stashed away from the atmosphere.

Over aeons thick layers of soft chalk formed around the globe. When we happen to live on land where there used to be oceans in earth’s history, we might be walking on layers of soft white matter tens or hundreds of meters deep. We may admire these layers when they are exposed, as in the white cliffs of Dover or in the mountains of Lebanon.

Even today, the ocean floor is being covered every day with more chalk. Assuming through some force we could encourage the creatures of the oceans to take up all the excess carbon dioxide from the atmosphere and store it away as chalk, how thick a layer would that form on the ocean floor?

Since 1850, humanity emitted about 4.4 * 10¹⁷ g carbon as CO₂. Chalk or CaCO₃ is a sink of carbon dioxide if chalk is produced from quicklime, CaO, according to the following formula:

CaO + CO₂ → CaCO₃

Humanity emitted enough carbon dioxide for forming 2.49 * 10¹⁸ g chalk this way:

M_CaCO3 = M_C/Mw_C * (Mw_C + Mw_Ca + 3Mw_O) = 4.4 * 10¹⁷ g / 12 g/mol * (12 + 40 + 16)g/mol = 2.49 * 10¹⁸ g

The volume of chalk would be 1400 cubic kilometres. This follows from the mean density of chalk, which is 1.79 * 10^6 g/m³:

M_CaCO3 / 𝝔_CaCO3 = 1.39 * 10¹² m³ = 1.39 * 10³ km³.

That seems much, but how thick would be this layer spread out evenly on the sea-bed which occupies 3.61 * 10⁸ km²? It would be 1.39 * 10³ km³ / 3.61 * 10⁸ km² = 3.9 * 10^-6 km. In more familiar units, it would be a layer of 3.9 mm chalk.

In consequence, what is the largest threat to humanity is a humblingly slim layer of 3.9 mm chalk over the ocean floor for geology. If only we knew how to entice sea animals and plants to use our emissions in the atmosphere for building shells fast enough.