An experiment born from ecology and the space age
In 1991, eight people entered Biosphere 2, a vast glass enclosure in the Arizona desert designed to test whether humans could live inside a materially closed ecosystem. Conceived by Space Biospheres Ventures as both an experiment in ecological engineering and a possible prototype for long-duration space habitation, the complex brought rainforest, savanna, desert, ocean, agriculture, laboratories, and living quarters under one envelope. What happened inside over the following two years was an unusually revealing account of how ecological, architectural, technological, and human systems become entangled when there is nowhere else to go.
exterior view of the University of Arizona’s 40-acre Biosphere 2 campus in Oracle, Arizona | all images courtesy of University of Arizona
Biosphere 2 used ‘lungs’ to regulate its enclosed atmosphere
Biosphere 2 emerged from an optimistic period in which ecological thinking, counterculture, systems theory, and space exploration increasingly overlapped. John P. Allen, working with Texas businessman and philanthropist Edward P. Bass, developed the project around the idea that a contained ecological system could be studied at a scale large enough to include people. The ambition was simultaneously terrestrial and extraterrestrial: to understand Earth’s cycles by reproducing them in miniature while exploring whether similar principles could eventually support human life on the Moon or Mars.
That proposition made the building itself part of the experiment. Constructed in Oracle, Arizona, between 1987 and 1991, Biosphere 2 was housed within a steel-and-glass envelope that looked like a greenhouse filled with plants, enclosing approximately 200,000 cubic meters of atmosphere, while a stainless-steel liner beneath the site was intended to prevent uncontrolled exchanges with the surrounding soil and water. Extensive piping, pumps, heat exchangers, air systems, and water-recycling infrastructure formed a hidden technosphere beneath the visible landscape.
Because the sealed structure heated and cooled under the Arizona sun, its internal atmosphere expanded and contracted. Engineers therefore installed two enormous variable-volume chambers known as the ‘lungs.’ Flexible diaphragms moved in response to changes in air volume, helping regulate pressure without simply venting the atmosphere to the outside. The system is an important reminder that Biosphere 2 was as much a machine for managing atmospheric physics as it was a miniature world.
The project attempted to compress ecological diversity into a relatively small footprint. A tropical rainforest occupied a high glass volume, while savanna, mangrove, ocean and reef, and fog desert environments formed a sequence of contrasting habitats. An agricultural zone was intended to provide much of the food for the crew, while residential spaces and laboratories were integrated into the same closed environment. The underlying strategy was to create enough biological complexity that the ecosystem could begin to organize itself. But the experiment quickly demonstrated that biodiversity cannot just be assembled from a list of species and expected to reproduce the relationships of a mature ecosystem.
When the atmosphere stopped behaving
After the first crew entered the enclosure on September 26th, 1991, oxygen concentrations began falling. By January 1993, atmospheric oxygen had dropped from approximately 20.9 percent to 14.5 percent. The conditions placed considerable physiological stress on the crew and eventually required oxygen to be introduced into the supposedly closed system. The problem was not simply that the plants were producing too little oxygen but that the artificial ecosystem contained a vast reservoir of organic matter in its soils. Microorganisms consumed this carbon through respiration, using oxygen and producing carbon dioxide. Photosynthesis was expected to compensate for that exchange, but the balance was disrupted by reduced light levels inside the structure and unusually cloudy conditions during the first winter.
The use of concrete for the building was another variable that had not been adequately accounted for. Carbon dioxide produced by microbial respiration reacted with calcium compounds in the structure’s concrete, effectively removing part of the gas from the atmosphere and resulting in a peculiar imbalance in which oxygen continued to disappear while much of the corresponding carbon dioxide failed to remain in the air.
Biosphere 2’s rainforest biome recreates a dense tropical environment beneath the glass enclosure
The ecosystem was not a miniature Earth
Other failures were biological. Pollinating insects and several introduced vertebrate species disappeared, while opportunistic species such as ants and cockroaches flourished. Aggressive vines spread through the rainforest and crop production became difficult as pollination, pests, disease, weather, and available light interacted in ways that were difficult to predict from individual ecological components.
The agricultural system nevertheless supplied a substantial proportion of the crew’s food. But the shortfall contributed to the human body adapting to scarcity, another experiment unfolding alongside the ecological one.
The crew lived under conditions of confinement, restricted food availability, altered atmospheric composition, intensive work, and continuous observation. Roy Walford, the mission physician, also used the enclosure as an opportunity to study caloric restriction, placing the crew on a nutritionally dense but low-calorie diet. Weight loss, fatigue, hunger, and physiological adaptation became part of the experiment’s lived reality.
This is where Biosphere 2 becomes more interesting than a simple story of an engineering project that did not work.
The experiment was trying to reproduce an ecosystem, but it also had to reproduce the conditions under which humans could function inside one. That meant that governance, psychology, nutrition, maintenance, labor, and interpersonal relationships became environmental variables.