Eight people sealed inside a miniature Earth… whose oxygen is disappearing

Captured cockroaches become chicken feed. Inside Biosphere 2, the inhabitants protect their crops however they can and salvage anything still useful. They may live in a glass-and-steel laboratory, but animals need tending, crops need harvesting and meals need preparing.

On 26 September 1991, eight people moved into a structure near Oracle, Arizona, bringing together landscapes normally thousands of kilometres apart. A rainforest stands alongside a savannah, a desert and marshes. A small ocean contains a coral reef. Farmland and living quarters complete the place.

The name captures the project's ambition: biosphere number one is Earth. The second is designed to investigate what happens when living environments share their water and air, with humans among them. The knowledge might also help in designing habitats for long stays in space.

The participants are volunteers. The airlocks are not locked; they can leave if they decide to stop or if their health requires it. A doctor lives with them, and an outside team supports them. The challenge is to stay long enough to keep this little Earth running and understand what happens to it.

The farm takes almost half their time

The first job is practical: growing food. Rice, sweet potatoes, bananas and papayas. Not all the plants thrive as expected. Pests attack crops; diseases damage roots. The eight must choose alternative varieties, wash leaves and trap insects.

Agriculture and food-related work take around 45% of the crew's time, including animal care and cooking. The farm produces plenty, but it needs more light. The glass and frame block a significant share before it reaches the plants.

By the end of the two years, the crops will have supplied about 80% of their food. Stocks present at the start, including food already grown in the facility, supplement the harvests. Mark Nelson, one of the eight inhabitants, will remember persistent hunger and plates cleaned of every last morsel. The group loses weight, especially in the early months.

Water is recycled and waste treated so its nutrients can be used again. The crops deliver results people can weigh and eat. For the air, the inhabitants have instruments tracking what they cannot see.

One number starts to fall.

A different atmosphere under glass

Initially, the air contains about 21% oxygen, just like the air outside. A little under a year later, it contains only around 16%. The decline continues as the inhabitants keep working among their plants.

The building lets very little air through. It is not absolutely airtight, but the exchanges are too small for Arizona's atmosphere to refresh the air inside Biosphere 2. Changes occurring within can therefore accumulate over months.

Researchers also monitor carbon dioxide. It rises at night, when the respiration of living organisms dominates, and falls during the day, when plants use light to grow. A passing cloud shows up on the graphs. On this scale, a change in sunlight quickly alters the air everyone shares.

During periods of low light, the crew reduces composting, among other measures, to limit CO₂ production. But the missing oxygen poses another problem: the amount of carbon dioxide found in the air cannot account for where it has gone.

The decline is no longer just a laboratory result. By January 1993, after about sixteen months, oxygen is approaching 14%. Managers decide to bring it in from outside to prevent serious health problems.

The little Earth will receive oxygen its own plants cannot supply.

The building is part of the search

The oxygen investigation leads to the soils. They were chosen to be rich in organic matter to support lasting vegetation. Microbes decomposing that material consume oxygen and release CO₂. Plants do produce oxygen through photosynthesis, but not enough to offset all the consumption.

That leaves the missing gas. Some of the CO₂ produced reacts with exposed concrete inside the facility, becoming bound as calcium carbonate. Researchers could not reconstruct the story by looking only at the air: part of it was now in the materials.

The chamber door opens

In an account published much later, Nelson returns to a night in January 1993. The outside team has injected oxygen into an adjoining chamber. The inhabitants enter and quickly feel the difference. He remembers recovering energy he has not had in a long time.

The addition allows the stay to continue. The eight complete the planned two years in September 1993. The experiment has not demonstrated perfect self-sufficiency: electricity already came from outside, stocks supplemented the food supply and oxygen had to be added.

They could leave Biosphere 2 at any time. On Mars, they would have needed another solution.