Two people are talking at once, and by the loudness of their voices you can't tell who is answering whom. That's how climatologists have for decades searched for a link between warm water off the coast of Peru and droughts in Australia: they compared how synchronously ocean temperature and rainfall change in different parts of the planet. Synchrony, known as correlation, hints that a connection exists, but it doesn't say what influences what, or with what delay. Meanwhile, the ocean warms the atmosphere, not the other way around, and the response may come months later.

The interpreter is distracted by every rustle in the hall, and a one-hour meeting stretches into two. Correlation works similarly: it catches any coincident noise, whether it's a common warming trend or seasonal variations, and is easily fooled. Granger causality works differently. It asks: if you know the past temperature in one place, can you more accurately predict future rainfall in another? If yes, then the signal is indeed transmitted, like a reply from one speaker to another.

Comparing the two methods on the same data from 1980 to 2018 gave an unexpected result. Both methods found the classic El Niño influence zones: arid Australia, rainy Amazon, the Horn of Africa. But Granger causality saw what correlation missed: a link to rainfall in central India and southern California. There the signal is weak, like a quiet voice in a noisy room, and only an attentive listener who follows the order of replies can discern it.

The method proved robust: conclusions did not change when the data were smoothed in different ways, from a week to a month. It's like an interpreter who understands meaning whether speech is fast or slow. However, sensitivity to memory length remained: if you allow looking too far into the past, the test starts seeing connections where there are none. So the authors checked the result under several settings and kept only what was repeated.

Now climatologists have a tool that listens not only to loudness but also to the order of words. It is ready to search for teleconnections between any climate fields: ocean temperature and winds, pressure and soil moisture. And the newly found regions, such as central India, require physical explanation, which could lead to the discovery of unknown mechanisms. The conversation about climate becomes clearer when you can hear who is answering whom.