In the middle of the vast Arizona desert, in the United States, there’s a structure that seems taken straight out of the pages of…

       

        In the middle of the vast Arizona desert, in the United States, there’s a structure that seems taken straight out of the pages of science fiction. Inside a massive complex of glass pyramids and towers, spread across 1.2 hectares, stands a tropical rainforest topped by a 7.6-meter-high waterfall, a savannah and a fog desert. It’s seemingly a little capsule of Earth, which is why the structure is called Biosphere 2 — named after our own planet, Biosphere 1.

        The scenery forms the perfect background for the futuristic experiment that once took place here. In the early 1990s, eight people locked themselves inside, sealed off from the outside world for two years, to explore the challenges of living in a self-contained system — a prerequisite for building colonies in outer space. They fed themselves from the crops they grew, they recycled their own wastewater and they cared for the plants that produced their oxygen.

        In terms of sustaining human life, the experiment did not go well. Oxygen levels fell significantly, making the inhabitants sick, while carbon dioxide (CO2) levels increased. Countless animals died, including the pollinators the plants needed to reproduce. And although the “biospherians” did survive on their homegrown food, they lost weight to the point where they became a case study for calorie restriction. When supplementary oxygen needed to be brought in, commentators blamed the project as a failure, calling it a “new-age silliness masquerading as science”. In recent years, however, many experts have come to see the Biosphere 2 experiment in a new light, with valuable lessons about ecology, atmospheric science and importantly, the irreplaceability of our own planet.

(Katarina Zimmer. www.bbc.com, 05.07.2025. Adapted.)

According to the third paragraph, one major ecological imbalance that compromised human habitability in Biosphere 2 was the