The ability is a boon for shipping companies hoping to exploit Arctic routes, which are becoming more accessible due to declining sea ice in the region.

In principle, predictions of the opening and closing of the Arctic shipping season can be made as early as January, the researchers found, although there is a step-change to much higher predictability after May. By July, two months before the usual sea-ice minimum, it should be possible to predict the fastest open-water route through the Arctic to within 200 kilometres, the scientists believe.

“Skilful predictions at seasonal timescales are rare in the climate and meteorology world,” said Nathanael Melia of the University of Reading. “This study establishes that skilful seasonal predictions for the availability of Arctic shipping routes are potentially possible.”

The decline in sea ice is likely to bring more opportunities for shipping through the Arctic. Historically, cargo ships have been able to sail only during the summer when the sea-ice has either receded or thinned, and even then only with the path cleared by special icebreaker ships.

Arctic routes are attractive commercially, because they are quicker than taking southern routes, such as those through the Suez Canal, and sailing through Siberian waters does not incur costs. But sea ice poses risks. Between December 2016 and January this year, a flotilla became stuck in Arctic sea ice on Russia’s Northern Sea Route.

On the other hand, in August 2017 a commercial tanker sailed through the Arctic from Europe to Asia without an icebreaker in a record six and a half days, although this was partly thanks to its uniquely reinforced hull.

Systems for predicting the opening and closing of Arctic shipping routes already exist, but Melia and his colleagues wanted to answer a more fundamental question: how early can these routes be predicted? The earlier that is, the more that can be scheduled.

To find out, the researchers simulated possible shipping routes on a model prediction of Arctic sea ice. They ran the model several times with slightly different starting conditions, and discovered how many interactions it took for the outcomes to look significantly different from one another, which they took as a horizon of predictability. Finally, to understand how far in advance the shipping season could be predicted, they repeated the simulations starting in earlier months.

Melia was surprised by the jump in predictability between May and July. “You might imagine that the closer to the event you get, the better you get at predicting the outcome,” he said. “For example, you’re watching a football match, you might imagine that at half time you have a better idea of who is going to win than at the start. What we effectively find is that you have no more idea at half time than at the start, and you only really can have a reasonable idea from 60 minutes in.”

Melia believes Arctic routes will become more common, although he stresses that they will not replace conventional global routes via canals because the sea ice will return every winter. In 2013, he says, there were 71 transits through Russia's Northern Sea Route, yet more than 17,000 through the Suez Canal.

“Now that we have established that predictive skill for Arctic shipping routes is available in climate models on seasonal timescales, the next step would be to investigate the performance of operational prediction systems for this task,” he said.

Melia and colleagues reported the study in Environmental Research Letters (ERL) .

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