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Inside the concrete dam, machines hum, running on empty. Three out of ten turbines are silent. The water level in Southeast Brazil’s largest reservoir has dropped so low that the blades are about to gulp air. The country’s 160 hydropower plants, which supply more than half its electricity, are linked into a single water network where every drop counts.

Every month, satellites measure temperature and humidity over different parts of the planet. Over 24 years, thousands of such paired measurements have accumulated. Normally, temperature and humidity change in concert: when it warms, the air holds more water; when it cools, it releases it. But before a sharp climate shift, this coordination breaks down.