Soil breathes. After rain, it soaks up water, then slowly releases it into the air. This process happens across the planet, but in each place it is different. The SMOS satellite measured soil moisture from an altitude of 750 kilometers for eight years, recording how it changes every three days. The result is a vast recording, like the sound of breathing, but too complex to immediately discern its rhythms.

Scientists devised a method that works like a doctor listening to the lungs. Previously, they heard only one loud sound: the annual cycle, when moisture rises in spring and falls in autumn. But within that sound were other, quieter rhythms. The new method separates them, as if decomposing music into individual notes. It found four main rhythms. The first is the annual one, the strongest. The second and fourth are half a year and four months, linked to how rains move from the tropics to temperate latitudes. The third rhythm is slow, with a period of about 4.5 years, and it does not depend on the seasons.

This slow rhythm turned out to be linked to El Niño, the warming of water in the Pacific Ocean that alters weather worldwide. The connection is very strong: when El Niño intensifies, soil moisture increases in Eastern Europe and the southern United States, while decreasing in Australia and northern South America. Now, by looking at satellite data, we can anticipate where droughts and floods will occur. Soil breathes, and we have learned to distinguish its inhalations and exhalations.