TALKEETNA, Alaska — At 16,400 feet on Mount McKinley, Eric Roche looked toward the summit, still nearly 4,000 feet distant through deep and perilous snow. Then he looked at the picture of his wife and son, mounted on his ice ax and carried through two weeks of climbing. And he changed his mind. He would go home.

“The avalanche risk seemed too great,” he said last week as he unpacked his kit in this small town at the foot of McKinley in south-central Alaska. “I feel good about the decision.”

In climbing lore, coming back down the mountain safely is the ultimate measure of a climber’s success, not the number of summits achieved. And around the world this year, it has been a bad season in that respect. A climbing disaster in the French Alps last week, with nine climbers killed by an avalanche, was only the most recent example.

Scientists, mountaineers and parks managers say it is a pincerlike motion of forces: more people seeking adventure even as the risks involved are becoming more variable.