In spring 2020, our planet seemed to hold its breath. Cars stopped, factories fell silent, and smoke from chimneys nearly vanished. The carbon dioxide we release when burning fuel stopped entering the air in the usual amounts. Scientists wondered: can you spot this change in the atmosphere when there is so much natural noise around, with forests breathing, the ocean exchanging gases, and winds mixing everything up?

To find out, they built a computer model that mimics air movement across the entire planet. They fed it data on how much carbon dioxide factories and cars emitted before the pandemic, then sharply reduced those emissions as happened in reality. The model showed that from February to April 2020, carbon dioxide in the atmospheric column decreased by 0.13 parts per million. That is like removing one teaspoon of salt from an entire swimming pool, but instruments can detect such a difference.

It turned out that more than half of that reduction, 54%, was directly linked to the shutdown of factories and transport. The rest came from plants, which absorb carbon dioxide in spring, and from random weather quirks. In some cities, the signal was especially clear: in Chengdu, China, carbon dioxide concentrations dropped by 10–20 parts per million exactly on the day of the lockdown, as if someone had flipped a switch. Previously, it was unclear whether such a short-lived event could be detected on a global scale, but now we know it can. This experience will help monitor whether countries are meeting their emission reduction promises, because you cannot fool the air.