NEWBURY, N.H. — Helena Williams had a great day of skiing here at Mount Sunapee shortly after the resort opened at the end of November, but when she came back the next day, the temperatures had warmed and turned patches of the trails from white to brown.

“It’s worrisome for the start of the season,” said Ms. Williams, 18, a member of the ski team at nearby Colby-Sawyer College. “The winter is obviously having issues deciding whether it wants to be cold or warm.”

Her angst is well founded. Memories linger of last winter, when meager snowfall and unseasonably warm weather kept many skiers off the slopes. It was the fourth-warmest winter on record since 1896, forcing half the nation’s ski areas to open late and almost half to close early.

Whether this winter turns out to be warm or cold, scientists say that climate change means the long-term outlook for skiers everywhere is bleak. The threat of global warming hangs over almost every resort, from Sugarloaf in Maine to Squaw Valley in California. As temperatures rise, analysts predict that scores of the nation’s ski centers, especially those at lower elevations and latitudes, will eventually vanish.

Under certain warming forecasts, more than half of the 103 ski resorts in the Northeast will not be able to maintain a 100-day season by 2039, according to a study to be published next year by Daniel Scott, director of the Interdisciplinary Center on Climate Change at the University of Waterloo in Ontario.

By then, no ski area in Connecticut or Massachusetts is likely to be economically viable, Mr. Scott said. Only 7 of 18 resorts in New Hampshire and 8 of 14 in Maine will be. New York’s 36 ski areas, most of them in the western part of the state, will have shrunk to 9.

In the Rockies, where early conditions have also been spotty, average winter temperatures are expected to rise as much as 7 degrees by the end of the century. Park City, Utah, could lose all of its snowpack by then. In Aspen, Colo., the snowpack could be confined to the top quarter of the mountain. So far this season, several ski resorts in Colorado have been forced to push back their opening dates.

“We need another six or eight inches to get open,” said Ross Terry, the assistant general manager of Sunlight Mountain, near Aspen, which has delayed its opening a week, until Friday.

The warming trend “spells economic devastation for a winter sports industry deeply dependent upon predictable, heavy snowfall,” said another report, released last week by the Natural Resources Defense Council and Protect Our Winters, an organization founded to spur action against climate change.

Between 2000 and 2010, the report said, the $10.7 billion ski and snowboarding industry, with centers in 38 states and employing 187,000 people directly or indirectly, lost $1.07 billion in revenue when comparing each state’s best snowfall years with its worst snowfall years.

Even in the face of such dire long-range predictions, many in the industry remain optimistic. Karl Stone, the marketing director for Ski New Hampshire, a trade group, said that good winters tended to come after bad ones — the winter of 2010-11 was one of the snowiest in recent memory — and that a blizzard could balance out a warm spell. The basic dynamic he lives with is unpredictability; some areas that were warm last week have snow this week and vice versa.

“Things can change quickly, thanks to one storm, and that’s usually how it works this time of year,” he said, noting the current on-again, off-again snow pattern.

On a warm day last week, when the thermometer reached 51, Bruce McCloy, director of marketing and sales here at Mount Sunapee, was generally upbeat about the coming season, but he could not ignore the brown slopes outside his office window.

“The real problem with a day like this is that you can’t make more snow,” he said. “There are only so many days until Christmas, and we need so many days at certain temperatures to get the whole mountain done.”

Even in the Rockies, it is difficult to find enough water to make snow. After last year’s dry winter and a parched, sweltering summer, reservoirs are depleted, streams are low, and snowpack levels stand at 41 percent of their historical average.

At Sunlight in Colorado, the creek that supplies the pond that, in turn, provides water for snow guns has slowed to a near-trickle.