4.9 The atmospheric bath tub
The natural changes in atmospheric CO2 are because of changes in the balance between the amount of CO2 that goes into the atmosphere (from volcanoes, changes in the oceans, changes in biological activity on large scales) and the amount that is removed.
You can think of the atmosphere as a bath tub, with the taps always on and the plug hole always open. The amount of CO2 is the water level in the tub. If water leaves through the plug hole at the same rate that it’s coming in from the taps, then the level doesn’t change. But if you turn the tap up or down, or make the plug hole bigger or smaller, then the level will not stay the same.
On a geological timescale, what humanity has done over the last 150 years or so is like taking a bucket of water and pouring it straight into the tub, so the water level (and so level of CO2) has gone up dramatically. Possibly faster than ever before in earth history, at least as far as we can tell.
So what controls the size of the plug hole?
In the Earth system, one way that CO2 is removed from the atmosphere is through the weathering of certain types of rocks which contain a lot of silicon – called silicate rocks. A lot of igneous rocks are silicates.
Silicate rocks are weathered when CO2 from the atmosphere combines with water to produce a very weak acid – carbonic acid (this is natural acid rain). There are lots of different sorts of silicate rocks, but we can simplify what’s going on by saying that they’re all essentially CaSiO3 (which is the mineral wollastonite [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ).
2 x carbon dioxide + 3 x water + silicate rock = calcium ions + 2 x bicarbonate ions + silicic acid.
The products of this reaction travel in rivers and through groundwater to the sea, where organisms use the calcium and the bicarbonate to make calcite shells (like chalk):
calcium ions + 2 x bicarbonate ions = calcium carbonate + carbon dioxide + water.
While other organisms (like diatoms) use the silicic acid to make shells of silica:
silicic acid = silica + 2 x water.
Then, when the organisms die, a lot of the material that made up the skeletons ends up being buried.
The important thing about the production of the calcium carbonate is that, although it takes two molecules of CO2 to dissolve the silicate rock, only one molecule of CO2 is released when the calcium carbonate is made. (No CO2 is involved in the silica part, but it’s nice to be neat and see where all the products go.)
So when the calcite skeletons are buried, they take CO2 with them.
In theory, therefore, if we can increase the amount of silicate weathering occurring on Earth, we can increase the amount of CO2 being removed from the atmosphere – increasing the size of the plug hole and perhaps going some way to draining that bucket of CO2 we’ve dumped in.