ARCTIC sea-ice is melting. For many that is a source of alarm. But for others, the ice is still not melting fast enough. They would like to give it a helping hand. Clear lanes through the Arctic ocean would permit commercial and naval shipping to travel quickly between the Atlantic and the Pacific. These lanes might also assist the search for oil and gas.

The Russian authorities seem particularly keen on the idea. Last year they launched Arktika, the first of three giant, new nuclear-powered icebreakers intended to help open such routes. But some people think this approach—bludgeoning through the ice with what is, in essence, an armour-plated knife—is old-fashioned. They believe the job could be done faster and more elegantly using a piece of physics called flexural gravity-wave resonance. If they are right, the icebreakers of tomorrow might be submarines.

Resonance icebreaking was discovered in 1974 by Canada’s coast guard, when it began using icebreaking hovercraft able to operate in waters too shallow for conventional icebreakers. At low speeds, these craft work much as icebreaking ships do, by forcing sections of pack-ice in front of their bows to rise up and detach themselves from the main sheet. When travelling above 20kph, though, they cause oscillations, known as flexural gravity waves, in the ice sheet they have passed over. At the correct speed of passage these waves hit a resonant frequency—increasing in amplitude as the critical speed is maintained until, at an amplitude dependent on the thickness of the sheet, that sheet will crack up and disintegrate, leaving a navigable passage behind.

For things like freeing river mouths of ice, this approach can work well. But hovercraft skirts are easily damaged by ridged ice (the sort that forms when previously broken ice refreezes), so the vehicles cannot be used in places that require frequent clearance. Also, resonance-breaking by hovercraft does not work for ice sheets more than about a metre thick.

That limit is, however, no constraint on the thinking of researchers led by Viktor Kozin of the Komsomolsk-on-Amur State Technical University, in Russia. Dr Kozin and his team have been investigating resonance icebreaking since the 1990s. But, instead of hovercraft, they use submarines.

Dr Kozin’s original research was on ways to permit naval submarines to surface safely and quickly through ice, the previous method having been simply to rise until contact was made with the ice sheet and then increase buoyancy until the ice cracked (as an American vessel is pictured doing above). That, though, is slow and can damage the boat. Dr Kozin found that the bow wave from a submarine travelling close to the surface pushes the ice sheet upwards, making flexural gravity waves in it, which cause it to break up.

Follow-up studies by Dr Kozin and his pupil, Vitaliy Zemlyak, who is now at the Sholem-Aleichem Priamursky State University in Birobidzhan, indicate that a submarine travelling 30 metres below the ice can break a sheet one metre thick. At 20 metres it could break ice two metres thick. And it can do it quickly. Comparable data are not available for Arktika, but America’s heavy icebreaker, Polar Star, can break a channel through two-metre ice at a rate of three knots. A submarine could force such a passage ten times as fast.

Dr Kozin and Dr Zemlyak have also found that the area of ice broken can be increased greatly by using two submarines moving together on parallel courses, and they are now looking at increasing the pressure exerted on the ice still further, by adding wedges, spoilers and vortex generators to a submarine, or even installing an impeller, a giant propeller mounted horizontally. Experiments on these ideas will start next winter.

Building new submarines to act as icebreakers would be a huge investment. But here, Russia may have a short cut. It tends to retire its naval submarines faster than America does. At the moment such boats are normally consigned to the scrapyard. Turning them into icebreakers to open up the Arctic might give them a second life.