Jessie Diggins fell in love with snow as a child.

Diggins is a member of the United States women’s cross-country skiing team, which is favored to win a medal at the Pyeongchang Olympics. It would be only the second medal in the sport for the United States and the first for the women’s team.

Before Diggins was an Olympic skier, before she had even taken her first steps, her parents strapped her into a baby carrier and took her cross-country skiing near her hometown, Afton, Minn. But she worries that future generations of American children might miss that opportunity.

The risk is real. In the United States, the average time between the last frost of the spring and the first of the winter has expanded by 10 days since the first half of the 20th century.

Winter is shrinking in places like Hayward, Wis., a town of 2,300 that swells to more than 12,000 people in late February during the American Birkebeiner, North America’s largest cross-country ski race. Last year, organizers canceled the event for lack of snow.