The title of this post is a purposeful play on the title of a 2010 paper by Lacis, Schmidt, Rind, and Ruedy, entitled Atmospheric CO2: Principal control knob governing Earth’s temperature.

In that paper, the authors claimed that the existence of CO2 is what provides enough warming to keep the Earth from becoming an ice planet. They also claim that, because CO2 is “non-condensing” (whereas water vapor, Earth’s most abundant greenhouse gas, does condense) this gives it special status as some sort of primary control knob governing Earth temperatures.

That latter argument has never quite convinced me of anything…both CO2 and water vapor have various sources and sinks, and just because water vapor goes through a phase change and CO2 doesn’t is, in my mind, irrelevant. Yes, the CO2 source/sink processes act more slowly than the water vapor ones (evaporation and precipitation) do, but on the long time scales of Earth’s history, who cares?

But the main point of the current post is to stimulate some thought regarding another possibility regarding how we view carbon dioxide: rather than the existence of CO2 being the primary warming mechanism of climate, maybe it is the existence of life on Earth which provides the primary cooling mechanism.

Think about this: What is the natural state of of the planets closest to Earth? Both Mars and Venus have atmospheres which are nearly 100% carbon dioxide. Presumably, without life on Earth, we too would have a planet with nearly 100% CO2.

So doesn’t it make more sense to use THAT as a starting point for discussion, rather than a hypothetical Earth with NO carbon dioxide?

The small amount of CO2 in Earth’s atmosphere (about 4 parts per 10,000) is presumably due to biological processes “sucking on” that “food” source as hard as possible (further evidenced by the fact that our production of CO2 through fossil fuel burning is 50% consumed by natural processes).

So, maybe it is life on Earth that should be viewed as the primary CO2 cooling mechanism for the Earth’s climate system, by reducing atmospheric CO2 concentrations from nearly 10,000 parts per 10,000, to only 4 parts per 10,000.

Just thinking out loud…