Snow atop Arctic sea ice has thinned dramatically since the mid-20th century, declining by more than a third in the western Arctic and by more than half in the Chukchi and Beaufort seas, reports a new study led by researchers at the University of Washington and NASA.

The study crunched numbers collected in a variety of ways over the decades -- from simple handheld meter sticks poked into snow at Soviet ice stations as early as 1937; from modern, depth-recording poles thrust into the snow by researchers traveling to the ice pack in recent years; from measurements taken from instruments on buoys mobilized by the Cold Regions Research and Engineering Laboratory; and from a sophisticated aerial radar system used by NASA's Operation IceBridge program.

The results? Spring snowpack on sea ice in the western Arctic went from average depths of about 14 inches in the 1954-1991 period to about 9 inches in the 2009-2013 period. On the Chukchi and Beaufort seas, the decrease was bigger, from 13 inches to 6 inches.

The study has been accepted for publication in the Journal of Geophysical Research and is available online in a draft form.

Implications of sparser snow are mixed, said one of the lead authors.

"It has a lot of effects, depending on the time of year," said Melinda Webster of the University of Washington.

Thick snow layers in the fall and early winter would inhibit sea-ice growth because snow is an insulator trapping heat, Webster said. That means thin snow cover in the fall would aid in freeze-up, she said.

But sparse accumulations early in the snow season lead to thin layers and quicker melt-out in the spring, she said. Unlike past times, the snow on top of ice now disappears each year, she said.

"It's pretty common to say that it melts out every summer," she said. That leaves bare ice exposed to the sun's heat and thawing powers, she said.

Changes in sea ice and snow appear interrelated, and they feed on each other, according to the study.

Ice pack starts to form later now than in the past, in mid-September or even later, compared to the late-August or early-September timing of past decades, Webster said.

That leaves less ice on the water to catch the precipitation that falls in autumn, generally the heaviest snow period of the year in the Arctic, she said.

"If there's no platform for the snow to accumulate on, it's just going to go in the water and melt," she said.

Ice melt is also starting earlier in the spring than in past decades -- on average, seven days earlier than in the late 1970s, or about two days earlier per decade, according to the National Snow and Ice Data Center in Boulder, Colorado.

The long-term changes in snow have implications beyond ice coverage.

They could harm Arctic animals that use the snow, such as ringed seals, which burrow into the snow atop the sea ice to create caves for their pups. Ringed seals were listed in 2012 as threatened, in part because of the scarcity of spring snow needed for pupping.