Secretary of the Interior Sally Jewell will make climate change a focus of the celebration of the National Park Service's 100th birthday on Thursday.

Jewell is celebrating the centennial of the park service by spending the morning in Montana's Glacier National Park. She will spend much of the morning hiking around the peaks of the park, learning about how climate change is affecting the mountain glaciers that give the park its name.

Many of the park's glaciers have shrunk over the last few decades. Scientists predict the park will be glacier-free by mid-century if the current rate of global warming persists. Many scientists blame the burning of fossil fuels for causing climate change and the subsequent warming of the globe.

According to the National Park Service, Glacier National Park had about 150 glaciers in the mid-1800s. By 1968, the number had shrunk to 50 and now there are currently about 25 left.

The service says models show all active glaciers, which are those that are large enough to be moving, will be gone from the park by 2030.

Jewell also will visit Yellowstone National Park in southern Montana on Thursday to deliver remarks commemorating the 100th anniversary of the National Park Service. Yellowstone was the first national park in the United States and the world.

The National Park Service is celebrating its 100th anniversary this weekend by allowing free entry to all NPS lands.