By Marguerite Holloway
Yellowstone has an unusual mix of snow and steam due to its volcanic nature. A tourist photographing Castle Geyser. (Josh Haner/The New York Times)
On a recent autumn afternoon in the Lamar Valley, visitors watched a wolf pack lope along a thinly forested riverbank, 10 or so black and gray figures shadowy against the snow. A little farther along the road, a herd of bison rooted for food in the sagebrush steppe, their deep rumbles clear in the quiet, cold air.
In the United States, Yellowstone National Park is the only place bison and wolves can be seen in great numbers. Because of the park, these animals survive. Yellowstone was crucial to bringing back bison, reintroducing gray wolves, and restoring trumpeter swans, elk, and grizzly bears — all five species driven toward extinction found refuge here.
But the Yellowstone of charismatic megafauna and of stunning geysers that four million visitors a year travel to see is changing before the eyes of those who know it best. Researchers who have spent years studying, managing, and exploring its roughly 8,800 square kilometers say that soon the landscape may look dramatically different. Over the next few decades of climate change, America’s first national park will quite likely see increased fire, less forest, expanding grasslands, more invasive plants, and shallower, warmer waterways — all of which may alter how, and how many, animals move through the landscape. Ecosystems are always in flux, but the changes are happening so quickly that many species may not be able to adapt.
Yellowstone National Park, established in 1872, is one of the Unesco World Heritage sites threatened by climate change. It is home to some of America’s oldest weather stations. Data from the area has helped scientists track climate change in the Western United States.
Since 1948, the average annual temperature in the Greater Yellowstone Ecosystem — an area of 89,000 square kilometers that includes the park, national forests and Grand Teton National Park — has risen about 1 degree Celsius. Researchers report that winter is, on average, 10 days shorter and less cold.
“For the Northern Rockies, snowpack has fallen to its lowest level in eight centuries,” said Patrick Gonzalez, a forest and climate change scientist at the University of California, Berkeley. Because snow is a cornerstone of the park’s ecology, the decline alarms some ecologists.
Summers in the park have become warmer, drier and increasingly prone to fire. Even if rainfall increases in the future, it will evaporate more quickly, said Michael Tercek, an ecologist who has worked in Yellowstone for 28 years. He said, “By the time my daughter is an old woman, the climate will be as different for her as the last ice age seems to us.”
Yellowstone’s unusual landscape — of snow and steam, of cold streams and hot springs — is volcanic. Magma gives rise to boiling water and multihued thermophiles, bacteria that thrive at high temperatures. In 1883, The New York Times described the park as an “almost mystical wonderland.”
For many, Yellowstone represents American wilderness: a place with open skies where antelope and bison roam. “You run into visitors and they thank you for the place,” said Ann Rodman, a park scientist. “They are seeing elk and antelope for the first time in their lives.”
Ms. Rodman, who has been working in Yellowstone for 30 years, has pored over weather data. “When I first started doing it, I really thought climate change was something that was going to happen to us in the future,” she said. “But it is one of those things where the more you study it, the more you realize how much is changing and how fast.”
Ms. Rodman added, “All of a sudden it hits you that this is a really, really big deal and we aren’t really talking about it and we aren’t really thinking about it.”
She has seen vast changes near the town of Gardiner, Montana, at the north entrance to Yellowstone. Nonnutritious invasive plants like cheatgrass and desert madwort have replaced nutritious native plants. Cheatgrass has already spread into the Lamar Valley. “This is what we don’t want — to turn into what it looks like in Gardiner,” Ms. Rodman said. “The seeds come in on people’s cars and on people’s boots.”
Cheatgrass can ignite “like tissue paper,” she said. It takes hold after fires, preventing native plants from regrowing.
If cheatgrass and its ilk spread, bison and elk could be affected. Cheatgrass, for instance, grows quickly in the spring. “It can suck the moisture out of the ground early,” Ms. Rodman said. “Then it is gone, so it doesn’t sustain animals throughout the summer the way native grasses would.”
In recent years, elk have lost forage when drier, hotter summers have shortened what ecologists call the green wave, in which plants become green at different times at different elevations, said Andrew J. Hansen of Montana State University. Some elk now stay in valleys outside the park, nibbling lawns and alfalfa fields, Dr. Hansen said. And where they go, wolves follow. “It is a very interesting mix of land-use change and climate change, possibly leading to quite dramatic shifts in migration and to thousands of elk on private land,” he said.
Drier summers also mean that fires are a greater threat. The conditions that gave rise to the fires of 1988, when a third of the park burned, could become common. By the end of the century, “the weather like the summer of ’88 will likely be there all the time rather than being the very rare exception,” said Monica G. Turner of the University of Wisconsin-Madison. “As the climate is warming, we are getting fires that are happening more often. We are starting to have the young forests burn again before they have had a chance to recover.”
Repeated fires could lead to more grassland. “The structure of the forests is going to change,” Dr. Turner said. “They might become sparse or not recover if we keep doing a double and triple whammy.”
Forests shade waterways. “We can very definitely see warming trends during the summer and fall,” said Daniel J. Isaak of the United States Forest Service. “Stream and river flows are declining as snowpack declines.”
As fish become concentrated in smaller areas, Dr. Isaak said, disease transmission is easier. In 2016, the Yellowstone River — famous for its fly fishing and cutthroat trout — was closed to anglers after a kidney disease killed thousands of fish.
Torrents of water from quickly melting snow in early spring have affected the nesting of water birds like common loons, American white pelicans, double-crested cormorants and trumpeter swans.
On the shores of Yellowstone Lake, dozens of late-season visitors watched two grizzly bears eating a carcass, while a coyote and some ravens circled, just a hundred meters from the road. “If they run this way,” a ranger called out, “get in your cars.”
Grizzlies are omnivores, eating whatever is available, including the fat- and protein-packed nuts of the whitebark pine. That pine is perhaps the species most visibly affected by climate change throughout the Western United States. Warmer temperatures have allowed a native pest, the mountain pine beetle, to better survive winter, move into high elevations and have a longer reproductive season. In the last 30 years, an estimated 80 percent of the whitebark pines in the park have died by fire, beetle or fungal infection.
The trees play a central role in the structure of the ecosystem. They colonize exposed mountain sites, allowing other plants to get a root-hold. Their wide canopies protect snowpack from the sun. They provide food for birds like the Clark’s nutcracker, which, in turn, create whitebark pine nurseries by caching nuts. And they are an important food source for squirrels, foxes and grizzlies. When pine nuts are not plentiful, bears consume other foods, including the elk or deer innards left by hunters outside the park. And that can bring the grizzlies, relisted as threatened this past September, into conflict with people.
The loss of the pines “has far-reaching implications for the entire ecosystem,” said Jesse A. Logan, a retired Forest Service researcher. “The rest of the landscape, even in the mountainous West, has been so altered that Yellowstone becomes even more important.”
Yellowstone offers a landscape unlike any other: a largely intact ecosystem rich in wildlife and geothermal features. Its beauty was forged by volcanic heat; heat from humanity could be its undoing.