But little do they know that every winter the celebrated ice drift that washes onto Japan's northern coast from Siberia is getting thinner.

"Without dispute, global warming is reducing it. The sea ice is diminishing," said Masaaki Wakatsuchi, an expert on winter weather at Hokkaido University.

Every year the Amur, Siberia's longest river, dumps tonnes of ice into the Sea of Okhotsk, creating the world's largest ice drift which, depending on the winds, bastes Japan's Abashiri port in late January or early February.

But in 2006, the volume of ice was the smallest since statistics were first kept four decades ago.

This year, even though Japan has experienced an exceptionally mild winter, the ice floe arrived nine days earlier than in 2006. But it was still later than normal.

"The winds weren't good. For the past two years the ice has landed late on the Japanese coast. A year ago, we couldn't even see it from Abashiri port," lamented a sailor on the 350-seat Aurora.

The ice floe generally stays until early April, allowing ferries of tourists to explore it for around 50 days. Last year, the ice barely approached Abashiri, meaning the season lasted only around a dozen days.

On a recent trip, it took a quarter of an hour on sea to notice the massive white sheet on the horizon. Temperatures needed to be one degree Celsius colder for the sea to freeze, making it impossible to get off and walk on the ice floe.

As seagulls seem to fly close enough to touch, the Aurora slithers its way slowly through the pristine ice, which cracks open before quickly closing back up.

A Japanese tourist claps. But there are no cheers from scientists.

When the Sea of Okhotsk takes in the waters of the Amur, it is replenished with an enormous number of nutrients, such as iron, which in turn feed into the Pacific Ocean and beyond, said Wakatsuchi, the professor.

"The Sea of Okhotsk and the Pacific Ocean will be deprived of their richness, such as fauna, flora and the phytoplanktons which are the nutrients for other fauna and flora. So the impact will be very, very big," he warned.

A recent UN study blamed human activity for global warming and warned that Earth's average surface temperatures could rise between 1.1 and 6.4 degrees Celsius by 2100.

For Wakatsuchi, even if "it's difficult to prove completely," the rising temperatures on the Sea of Okhotsk and the decline of the ice floe are proof the Earth is getting warmer.

"Global warming is weakening nature's ability to create the sea ice that is the origin of the richness of the sea," he said. "The great environment of the sea is being destroyed." -- A global impact -- The decline of the ice floe may disappoint tourists, but the most serious impact is invisible to most people underneath the Sea of Okhotsk.

"There are 500 million planktons which are the food for the plants that grow below the sea-ice. And these plants are food for the animals in the sea," said Yoshiyuki Fujii, head of the National Polar Research Institute in Tokyo.

"So when sea ice melts, this will affect the whole eco-system," he said.

Back on dry land and another ferry ride away, the effects are already being felt on the northernmost point of Japan, Rebun Island, where fishermen say that herrings -- their lifeblood -- are becoming scarcer.

Stung by the harsh winds, the tiny island facing Russia's Sakhalin looks all but deserted in the winter, although it is only half covered by snow. Tourists mostly come in the summer to the so-called "Flower Island".

One of the few signs of life is at the port, where a twice-daily but mostly empty ferry connects to Wakkanai port on Japan's main northern island of Hokkaido.

In the 20 years to 2005, the number of islanders in Rebun has fallen to 3,410 from 5,724 and the number of fishermen has dropped to 495 from 857.

In the picturesque volcanic island of Rishiri, the population has dropped to 2,951 from 5,352 and the number of fishermen has gone down to only 369 from 796 in 1985.

Along the coast dotted by sites of the neolithic Jomons, the first culture to have produced pottery, the fishermen dry herrings outside their modest wooden houses, nets guarding the catch from hungry birds.

"We've seen a long decline in herrings over the past 30 or 40 years," said Mikami Hideshige, leader of the fishermen's association in Kafuka, Rebun's capital.

But in recent years other fish from the deep blue, including highly prized sea urchins and a local variety of mackerel, have also been in decline.

"We are worried. Our stocks have gone down by 20 to 30 percent in the past two years," Hideshige said.

"We had a bad catch of codfish in November and December because the water wasn't cold enough. For the last two seasons, the cold northwesterly winds didn't blow in August. The sea isn't getting cold," he said.

From the sight of bare white mountains and frozen lakes, it is hard to believe that Rebun's climate has become forgiving.

But nonetheless, this winter the snow has been cruelly deficient, in what Japanese meteorologists link to the El Nino effect of temperature fluctuations in the Pacific Ocean.

"In December, there was only half the snow as usual. When I was a kid, it used to reach the second floor of my house," recalled one fisherman as he cleaned one of the impeccably lined-up trawlers off the shore.

Less snow in the sea means fewer nutrients, disrupting the entire cycle.

"We live on what we earn in the summertime as we lose money fishing in the winter. But now we don't have anything left to save," Hideshige said.

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