Is there Less CO2 in the Atmosphere than a Few Million Years Ago?

Question

Over time, doesn't the amount of carbon dioxide in the atmosphere tend to decrease because plants die and "pile up" after having taken so much CO2 out of the atmosphere?

If so, how long would it take for man burning fossil fuels to return to the atmosphere the amount of CO2 that has been taken away from plants dying and "piling up" as rock, soil, oil, coal, or whatever?

Answer 1

First question, no, the CO2 level of the atmosphere has been roughly where it is now (a few to several hundred ppm) over the past few tens of millions of years.

Carbon has a natural cycle, just like water and rock.  There is removal of carbon from the ocean-atmosphere system due to burial of organic matter and deposition of carbonate minerals.  However, this carbon loss is roughly balanced by the return of carbon from rock through erosion and volcanic degassing.

Normally, atmospheric carbon dioxide levels are controlled by the chemistry of the ocean-that is, the ocean has a lot more CO2 in it and amount of CO2 in the atmosphere is always essentially being "forced" to an equilibrium level.

That "forcing" takes time.  It isn't instantaneous, so there can be periods where extreme volcanic activity, or in the present case, excessive burning of carbon fuels, can cause the CO2 of the atmosphere to get way out of equilibrium with the CO2 activity (content) in the ocean.  One can expect the system to eventually adjust and move back into equilibrium, but it will likely take centuries.  Air-ocean mixing is slow.

So, in effect the burning isn't returning the CO2 content to some pre-existing condition, but has pushed it out of equilibrium with what would otherwise be the natural condition.

You should note that in the far past (hundreds of millions of years ago), in contrast to the low contents of today, CO2 levels in the atmposphere have been calculated as an order of magnitude greater (low thousands of ppms).

Long story how this is figured out and why this was the case.

In short, fossil fuel burning by man is causing a change in the chemistry of the atmosphere.  Some people believe that this has had a large impact on climate and will continue to do so until the system is once again back into equilibrium.

I am not a big fan of that particular viewpoint, but there is no doubt that man's activities have caused some serious changes in the atmosphere and hydrosphere, and CO2 contents may not even be among the most important of the changes.

Answer 2

Another factor besides fossil fuels that is usually overlooked is the carbon released by Calcium Carbonates (limestone) due to acid rain and industrial uses.

Going back to the first few billion years of Earth's history, the Banded Iron formations indicate a time when free oxygen was first readily available to oxidize iron. This free oxygen would have been from organisms metabolizing the carbon dioxide which was a very abundant component in the early atmosphere.

The first answerer was correct, and the comments about how we have affected the chemistry of the atmosphere and hydrosphere are particularly important. Especially the part about carbon possibly not being the most important change.