If two distant cities experience warming and cooling at the same time, an invisible thread can be drawn between them. Across the whole Earth, such threads number in the hundreds of thousands, forming a web. Each year, the web looks different. Scientists tracked how it changed from 1948 to 2009 and noticed: when El Niño intensifies in the Pacific, the web sharply contracts, but when El Niño is weak or of a different type, it remains nearly unchanged.
Volcanoes also influence the web, but not immediately. After the eruption of Mount Pinatubo in 1991, the threads only rearranged several months later, because the ash and gases from the volcano spread slowly through the atmosphere, altering temperatures unevenly in different places. It turns out that the shape of the web can distinguish El Niño from a volcanic signature and even reveal which type of El Niño has arrived.
The edge density in the network is only 0.5%, meaning that out of more than 52 million possible node pairs, only about 262,000 are connected. Yet the network still captures global climate events.