Every spring in Alaska, residents of the town of Nenana gather by the frozen Tanana River. They've bought lottery tickets, each marked with the day and minute they think the ice will break up. On the ice sits a tripod, connected by a rope to a clock on the shore. When the ice goes out, the tripod falls and stops the clock. That moment is the winner. The lottery has been running since 1917, and its tickets have turned into a climate thermometer.

El Niño is an abnormal warming of water in the tropical Pacific Ocean. It alters atmospheric circulation across the entire planet. But in Alaska, thousands of kilometers from the tropics, its signal is usually weak and gets lost in local weather. To see it, you need a very long observational record. The Nenana lottery provided 90 years of daily data.

Scientists compared river breakup dates in El Niño years versus other years. In El Niño years, the ice moved on average 3.4 days earlier. The difference is small but statistically significant. A Monte Carlo test (randomly selecting 14 years out of 90) showed that the probability of getting such a deviation by chance is only 1.1%. This means the connection is real.

The reason is clear from meteorological data. In El Niño years, the air over Alaska is warmer and less snow falls. A thin snowpack melts faster, exposing the ground earlier. And the river breakup date is closely tied to the day the snow disappears: the correlation between them is about 0.5. Warm weather accelerates the melting of river ice.

The lottery ticket turned out to be a precise instrument. It showed that the tropical ocean reaches all the way to subarctic rivers. This is a rare example of a long-distance teleconnection, a link between weather in distant regions. And it confirms that the observed trend toward earlier river breakups (about 5.5 days per century) has a climatological nature, not just randomness.