The Mystery of the Missing Carbon

Last week’s essay [1] introduced decarbonization as a supplement to greenhouse gas emissions reduction. This essay expands on decarbonization by providing an example. It also emphasizes the importance of appreciating how complex systems behave. — C.H.

Biosphere 2

Biosphere 2 sits in the Arizona desert, north of Tucson. It looks like a figment from a science fiction novel. At 3.14 acres, Biosphere 2 is the largest artificial closed ecological system ever constructed.  It encloses an artificial rainforest, wetland, grassland, fog desert and agricultural area, each with appropriate flora and fauna. Biosphere 2 was intended to resemble Biosphere 1 – the earth’s surface and atmosphere – as closely as possible.

Dreamstime - Biosphere 2In September 1991, eight people entered Biosphere 2, closed the airlock and remained within for two full years. They grew their own food and recycled their own waste. They exhaled carbon dioxide in exchange for photosynthetic oxygen from the plant life. Of course, Biosphere 2’s designers took care to provide enough plants to provide enough oxygen for the eight humans and other animals enclosed with them.

Or did they? The oxygen level in the air within Biosphere 2 began at 20.9%, then declined steadily to 14.5% sixteen months later. At 14.5%, air is so thin that human functioning begins to be impaired, as skiers and mountain climbers know. It was necessary to twice add oxygen from outside the enclosure. [2]

The Mystery

Respiration involves exhaling one molecule of carbon dioxide for each molecule of oxygen consumed. When the concentration of oxygen in a closed quantity of air decreases, the carbon dioxide concentration increases in a readily calculable manner. Except that it didn’t. Much of the carbon dioxide disappeared!

Much later, the mystery was solved. The missing carbon dioxide had reacted with exposed concrete to form calcium carbonate. Calcium carbonate is a common mineral, found as marble, chalk, limestone, egg shells, oyster shells and pearls.

So What?

There are two matters of interest here:

First, Biosphere 2’s designers overlooked the potential for reaction of carbon dioxide and oxygen in the air with unsealed concrete surfaces – a chemical reaction that was likely accelerated by carbon dioxide concentrations that were much higher in the closed Biosphere 2 environment than those normally experienced in the world at large.

The point here is that an ecosystem – be it Biosphere 2 or Biosphere 1 – is a really complex system. It is quite easy to overlook factors that can affect the performance of a complex system. Had Biosphere 2 been on the moon instead of in Arizona, eight people would have died.

Second, there is a lot of exposed concrete surface here in Biosphere 1. If a cheap and effective way can be found to promote the reaction of carbon dioxide from ambient air with that concrete where it is, quite a lot – even gigatons — of carbon dioxide would disappear from the atmosphere. That carbon dioxide would become part of the concrete — concrete to be buried or recycled in due course. The reaction which occurred spontaneously in Biosphere 2 was certainly both cheap (free) and effective at sequestering carbon dioxide.

Chuck - SedonaThoughtful comments and experience reports are always appreciated.

…  Chuck Harrington (

P.S: Contact me when your organization is serious about pursuing Sustainability — C.H.

This blog and associated website ( are intended as a resource for smaller manufacturers in the pursuit of Sustainability. While editorial focus is on smaller manufacturers, all interested readers are welcome. New blog posts are published weekly.

Image: © Woodkern | Dreamstime.comSpace Colony At Biosphere 2 Photo

[1] See Profiting from Decarbonization, this blog,

[2] The Wikipedia wiki on Biosphere 2 provided the specifics cited in this paragraph. It also summarizes the rest of Biosphere 2’s interesting story: