On the wall of my hotel room here at the Jackson Lake Lodge in Moran, Wyo., there is a framed photograph of the spectacular Teton Range taken during the 1950s.

In it, a handful of tourists in short-sleeve shirts admire the jagged, snow-capped peaks. While it is clearly summer in the picture, the mountains are blanketed in snow, with huge glaciers covering much of the granite.

Looking out my window at the same vista this morning, there is not a trace of fresh snowfall to be seen on the peaks forming one of America’s most beautiful mountain ranges. And the glaciers are markedly smaller than they were just 70 years ago.

The signs of a warming planet are everywhere here in Grand Teton National Park. With retreating glaciers, lakes depleted by drought and forests parched by hotter temperatures, climate change is upending one of the great intact ecosystems in North America.

It’s a story playing out around the country. After more than a century of trying to preserve some of the world’s most wondrous natural landscapes, the National Park Service in recent years has shifted its mission away from absolute conservation. It is now making tough decisions about which plants and animals to save, and which they may need to let go. In Washington State, the glaciers on Mount Rainier are melting. In Maine, ecologists are working to save vulnerable trees in Acadia National Park.

“These things are happening so quickly, and they’re being sped up by all the human activity,” said Kirk Ryder, a longtime nature guide in the area who showed me and my family around the park yesterday morning. “Fire activity is increasing, and heat is stressing the wildlife.”