Drift ice, the annual winter spectacle that draws tour boats in Japan’s northern seas, will soon be few and far between due to global warming, scientists warn.

A study by Hokkaido University researchers has shown that floating ice in the Sea of Okhotsk is not only dwindling in terms of coverage area, but has also lost 30 percent of its thickness across a span of 30 years.

If things continue on the same course, the researchers predict there will be times in the future when no ice floes will reach Hokkaido’s coast.

BIGGER IMPACT THAN EXPECTED

This past winter, as in all earlier years, drift ice (also called ice floes) descended on various sections of Hokkaido’s northeastern coast facing the Sea of Okhotsk.

The city of Nemuro saw the first drift ice of the winter along its coast on Feb. 10.

The sea’s surface there was closely packed with big blocks of white ice as far as the eye could see.

Although the view is overwhelmingly magnificent, the ice that makes it up is thinner than it used to be, scientists said.

Kay Ohshima, a Hokkaido University professor of physical oceanography, analyzed with his colleagues how climate change has affected drift ice in the southern part of the Sea of Okhotsk, south of 48 degrees latitude.

They released their latest data at an international scientific conference in the United States in February.

Ohshima and his coworkers showed that ice floes in the southern Sea of Okhotsk have been thinning at a rate of 7.4 centimeters per decade.

They estimated the average drift ice thickness fell 30 percent over three decades from 73 cm in 1990 to only 51 cm in 2020.

It had been reported earlier that the sea ice coverage in the Sea of Okhotsk is dwindling due to observations using artificial satellites.

The latest study also showed that the thickness of ice is also continuing to decrease.

“We learned that drift ice is decreasing faster than we had ever thought,” Ohshima said.

Humio Mitsudera, another Hokkaido University professor of physical oceanography, has been using a supercomputer to predict what will become of the drift ice should global warming continue.

Mitsudera used state-of-the-art scenarios presented by the U.N. Intergovernmental Panel on Climate Change to calculate how the coverage area of drift ice will change in the portion of the Sea of Okhotsk that lies south of the 46th parallel north and is close to Hokkaido.

His study showed that drift ice coverage in 2050 would be only one-third of the corresponding area averaged over 1994 through 2017, even in a scenario where there are relatively low greenhouse gas emissions.

“I was astonished by such results, realizing how much less drift ice there will be in the future,” Mitsudera said. “The impact of global warming on floating ice turned out to be bigger than I had ever expected.”

He said the drift ice overage in 2050 is expected to be one-quarter of the reference value in a scenario of mid-level greenhouse gas emissions and only one-fifth in a high-emissions scenario.

“There could be years in the future, if things go on like this, when no drift ice will be spotted at all along Hokkaido’s coast,” Mitsudera said.

CHAIN REACTION FEARED

There are concerns that the lack of such ice will have a negative impact on fisheries and the marine ecosystem.

“There would be an insufficient supply of iron, which is essential for the growth of phytoplankton,” said Jun Nishioka, a professor of chemical oceanography with Hokkaido University.

Ice floes contain abundant iron that originates, among other places, along the Amur River on the Siberian continent.

Iron is discharged into the ocean when the drifting ice thaws in the spring. Phytoplankton, as a result, proliferates in the Sea of Okhotsk during that season.

It is then preyed on by zooplankton and other creatures and thereby props up an entire marine ecosystem, including fish.

The Sea of Okhotsk waters off Hokkaido are called a “sea of fertility,” as the area is home to scallops, horsehair crabs and walleye pollock, and it boasts one of the world’s highest fishing outputs.

The iron transported by drift ice is one factor that accounts for the richness of the sea there.

The large volumes of ice floes formed along the Siberian coast also generate a circulation of iron-rich seawater in the mid-depth range.

Iron that is transported by that circulation and wells up into the surface layer prompts the proliferation of phytoplankton and generates abundant fishery resources as far as in areas covered by the Oyashio current in the Pacific, Nishioka said.

“Less drift ice means less iron supply and less phytoplankton,” he said. “That could change the ecosystem and diminish the fertility of marine resources not only in the Sea of Okhotsk, but also in the Oyashio areas, including the waters off the Sanriku coast (of the Tohoku region).”

Drift ice, or ice floes, refers to a category of sea ice or frozen seawater that is not anchored along the coast but floats in the ocean.

Large masses of sea ice are formed in the Sea of Okhotsk, where cold seasonal winds arrive from the Siberian continent and chill the sea’s surface in the winter.

Sea ice blocks that are formed along the coast of Siberia and Sakhalin are sent south by winds and ocean currents and descend on Hokkaido in the form of drift ice.