Biosphere 2 experiment
- The record does not name which of my species, waters, soils or forests were harmed.
- Researchers Edinburgh archive and Mental Floss: Oxygen fell from 20.9% to about 14.5%. University of Edinburgh archive (J. Deacon)
- Standard-setters and registries Britannica: Gives the figure as 14.2%.
Eight people lived for two years in a sealed 1.27-hectare structure in Arizona. Oxygen fell from 20.9% to about 14.5% over about 16 months and oxygen was supplied from outside.
Equivalent to an altitude of about 4,100 metres; oxygen was supplied from outside.
Status As of October 2026: historical experiment (1991–93); oxygen and ant findings recorded; pollinator and cockroach accounts not verified.
What happened
Biosphere 2 was a sealed artificial ecosystem of about 1.27 hectares in Arizona. Eight people lived inside it from 26 September 1991 to 26 September 1993.
Over about 16 months the oxygen level inside fell from 20.9% to about 14.5%, equivalent to an altitude of about 4,100 metres. The publications cited in the sources give the cause as respiration by soil microbes exceeding photosynthesis, with carbon dioxide bound in the structure's exposed concrete; excess organic matter had been put into the soils. Oxygen was supplied from outside, by liquid-oxygen tankers.
One source gives the figure as 14.2% rather than about 14.5%. The primary publications for the oxygen figures could not be checked.
The structure's ant community changed: a species not found in 1990–91, Paratrechina longicornis, accounted for more than 99.9% of the ants coming to baits in 1996.
The structure's advisory board resigned and management changed in 1994. The cost is reported as between about USD 150 million and more than USD 200 million.
- 1991Eight people enter the sealed structure (26 September)
- 1993Two-year stay ends (26 September); oxygen had fallen to about 14.5% over about 16 months and oxygen had been supplied from outside
- 1994Advisory board resigns and management changes
- 1996Ant survey: Paratrechina longicornis accounts for more than 99.9% of ants at baits; Science publishes an assessment by Cohen and Tilman (15 November)
What was said
Where accounts differ
Each party's account, side by side, with its source. The record does not settle these points; it shows them.
Oxygen fell from 20.9% to about 14.5%.
University of Edinburgh archive (J. Deacon)Gives the figure as 14.2%.
How it looked
Shown
Recorded
- Eight people lived in the structure for two years, from 26 September 1991 to 26 September 1993.
- Oxygen fell from 20.9% to about 14.5% over about 16 months, and oxygen was supplied from outside.
- Oxygen fell from 20.9% to 14.5% (Mental Floss).
- In 1996 Paratrechina longicornis, absent in 1990–91, made up more than 99.9% of ants at baits.
In court and with insurers
Lawsuits and rulings
Insurers
What nature saw
Nature's report opens this page. Below: further outcomes measured on the ground.
What we don't know
Nature
- Not measured Harm to nature: No independent ecological evaluation is on record.
- Disputed What was the lowest oxygen level reached?
Record
- Insufficient Reported but not independently verified: that all pollinators in the structure died and that cockroaches multiplied. The sources reviewed could not confirm this from a primary source.
- None on record The primary publications on the oxygen decline (Severinghaus and colleagues, 1994) and the Science assessment by Cohen and Tilman (1996) could not be opened in full.
Did the harm stop?
The two-year stay ended on 26 September 1993.
Who paid?
The cost is reported as between about USD 150 million and more than USD 200 million; the sources reviewed do not say who paid.
Sources
Press and investigations
Researchers
- Biosphere 2University of Edinburgh archive (J. Deacon) ·