Watch the Alaska Ice festival footage

It may not be the world’s most exciting spectator sport, but watching the ice melt in Alaska has become an unmissable rite of spring for two very different but related groups — gamblers and climatologists.

An annual contest to guess the exact moment the ice breaks on the River Tanana, 300 miles north of Anchorage, is attracting global interest, both as a chance to win a $300,000 (£151,000) prize and as one of the world’s most precise scientific indicators of the effects of global warming.

For 91 years the 500 villagers of Nenana, a frigid hunting and mining outpost, have been keeping records of the minute when the Tanana ice cracks. Their unusual history of climate change has provided researchers with insights into the shrinking of America’s frozen north.

Earlier this month a group of Nenana volunteers erected a 26ft wooden tripod on ice in the middle of the river, connected by cable to a siren and a clock on shore. The minute it topples over as the ice begins to shift down-river, a tripwire triggers the siren and stops the clock. Anyone who has guessed the right minute is entitled to a share of the prize.

Tens of thousands of Alaskans and a number of foreigners bet on the Nenana Ice Classic. The sale of $2.50 tickets last year generated a $303,272 jackpot, shared among 22 winners who had predicted that the ice would break up at 3:47pm, Alaskan standard time, on April 28.

Betting for this year’s contest will close at midnight on April 5. “People tend to wait until the last minute to buy their tickets to get a bearing on what’s happening to the ice,” said Cherrie Forness, manager of the contest. The organisers measure the ice cover twice a week from mid-March and publish the results on their website — last week it was 44.5in thick, down from 51in the previous week.

The contest was started in 1917 by bored railway builders, wondering how long it would be before they could get back to work. Since then the records show a marked shift in the thaw averages. Spring now arrives 10 days earlier than in the 1950s.

“The Nenana classic is a pretty good proxy for climate change in the 20th century,” noted Dr Martin Jeffries, a geophysicist at the University of Alaska.

The Nenana records have been used by scientists studying everything from river currents and solar cycles to greenhouse emissions and local land use.

However, perhaps the most important lesson learnt is that no amount of scientific knowledge — or educated guesses about climate patterns — can assure a share of the prize. While almost a century of data yielded long-term trends, the data also revealed striking departures from the average. One of the earliest thaws on record was on April 26, 1926; as recently as 2001 the ice did not break until May 8.

This year threatens to be especially problematic — many parts of the world have experienced record spells of either warmth in January or cold in February. “Sometimes I just pull numbers out of a hat,” said Forness.

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