Thawing permafrost prompts Denali National Park to reimagine its future

Much of the ground in Alaska's Denali is losing its top layer of permafrost, resulting in landslides and a sometimes-blocked park road.

A single, mostly gravel road connects 600,000 visitors a year with the 6 million acres of Alaska’s Denali National Park and Preserve. Most visitors ride park buses to marvel at grizzly and black bears, wolves, caribou, moose, and Dall sheep—the “Big Five” wildlife species—and hope for a peek at North America’s tallest mountain—20,310-foot Denali—which is often shrouded by clouds.

But in recent years, the 92-mile-long Denali Park Road that provides park access has experienced slumps and slides—which require considerable maintenance and occasionally block the road—and scientists believe climate change is likely to blame. Road maintenance challenges are just one of the many effects that warming temperatures are having on Alaska’s national parks.

Like much of Alaska, many parts of Denali National Park are underlain with permafrost, meaning ground that remains at or below freezing for two or more years in a row. As climate change warms the planet, much of the shallow permafrost is thawing. In the 1950s, 75 percent of Denali had near-surface permafrost, which is located just below the active layer that freezes and thaws seasonally. The figure dropped to around 50 percent in the 2000s and is projected to drop to 6 percent by the 2050s. In areas where the near-surface permafrost is thawing, deeper permafrost may still exist, since permafrost can be hundreds of feet thick, and even over 2,000 feet thick in some northern locations in Alaska.

Recently, three National Park Service researchers, including David Swanson, an ecologist, and Pam Sousanes and Ken Hill, both physical scientists, published a study in the journal Arctic, Antarctic, and Alpine Research analyzing air and ground temperatures in Alaska’s eight northernmost national parks. They found the mean annual air temperature increased by at least 1.8 degrees Fahrenheit (1 degree Celsius) from 2014 to 2019 in the study area when compared with the previous 30-year period, with a near 3.6°F (2°C) increase in Denali and most Arctic parks. The increase rose to around 5.4°F (3°C) in certain western coastal park areas. Mean annual ground temperatures were also on the rise.

“If the warm temperatures observed during 2014 to 2019 persist, there will be widespread degradation of permafrost in portions of these national parks and similar environments across Alaska,” the authors wrote. Projections show Denali temperatures will likely continue to rise, according to a 2018 study by Patrick Gonzalez and colleagues published in Environmental Research Letters.

Alaska has eight national parks and 16 other sites overseen by the National Park Service, including national monuments, preserves, and historic areas. Thawing permafrost is affecting those lands in many ways, from infrastructure challenges to ecosystem impacts.

In Denali, slides occasionally force closure of the Denali Park Road. In 2013, blocks of permafrost the size of small houses slid down a steep hillside above Igloo Creek and blocked the road for four days, and in 2014 part of the road washed out so severely that private planes and park helicopters were enlisted to extract visitors from a private lodge. In July 2016, the Eagle’s Nest landslide caused a closure that lasted for four days, with several additional days of limited access. Then, in 2019, 17 buses with 300 visitors were trapped on the far side of the park, uninjured but delayed for hours.

Thawing permafrost affects parks across Alaska

Scientists wanted to learn more about how these changes might further impact the park road, so from 2003 to 2019 they drilled boreholes in area soil. They found “significant amounts of thaw-sensitive (relatively warm) ice-rich permafrost directly beneath the road,” according to the park’s website, which also notes a likely explanation for the park road’s challenges: “the most plausible answer is that the permafrost in the area is thawing, consistent with regional trends, resulting in loss of slope stability.”

Permafrost thaw isn’t just an issue affecting Denali. The National Park Service is responsible for more than 85 million acres of land, and Alaska contains approximately 54 million of those acres Around 40 million of them lie within a zone that is underlain with more than 50 percent permafrost.

National parks are also a vital economic driver in Alaska. In 2018, parks there admitted 2.9 million visitors with a “cumulative benefit to the state economy of over $1.98 billion,” according to the park service.

The effects of thawing permafrost differ from park to park. In northern Alaska’s Arctic national parks—which are remote and difficult to access—there are few visitors and almost no infrastructure. But permafrost thaw can cause hydrology changes and allow water to seep into the ground, drain lakes, or create new ponds. It also can cause soil erosion, add sediment and silt to streams and lakes, impact nutrient cycles, and release greenhouse gases. Such changes can alter vegetation and wildlife habitat.

“As the climate warms, and brush gets taller and thicker, that will affect backpackers, and it will certainly affect the wildlife,” says Swanson. He says the changes may not be detectable to the average visitor to the Arctic parks at first. “Things grow slow in the Arctic and that includes what we think of as relatively rapid climate change related [things],” Swanson says.

Park Road problems

Change is more visible along Denali’s Park Road, which was built between 1922 and 1938, with a “High Line” route along Polychrome Mountain highlighting stunning panoramic views. But now, at mile 45.5 along that stretch, the Pretty Rocks Landslide is causing headaches, according to the Denali Geology Road Guide. First observed in the 1960s, the landslide was a minor maintenance issue, requiring a bit of upkeep every few years. But, as Alaska warmed, it became a larger issue.

In 2016 and 2017, 300 feet of the road began to slump more significantly—up to six inches a month. By 2018 and 2019, it was slumping up to two inches per day, and some days in August 2020 it moved 3.5 inches. This spring, the park used 6,700 cubic yards of gravel (the equivalent of 550 truck loads) to repair the road.

More than 140 “mapped unstable slopes” exist along the park's road. “We do our best to parse our attention between the things that are really well known and in our face like Pretty Rocks, but to not get so focused that we lose perspective on other challenges,” says Denali National Park and Preserve geologist Denny Capps.

Denali participates in the Unstable Slope Management Program for Federal Land Management Agencies and is working with the U.S. Geological Survey on high-resolution geologic mapping to learn more about areas along the Denali Park Road that are vulnerable to geohazards, including landslides, slumping, and earthquakes. An automated monitoring system notifies Capps of any significant movement. When he receives a notification, he assesses the data, even in the middle of the night.

“I can get up, log in, and look and see, well actually, it was just one of the [sensors], maybe it was just a bear scratching its back on it—that happens,” Capps says. But if he finds a concerning pattern of movement, he will head to the site to inspect it.

The park is investigating a number of options to address the park road dilemma, including engineered solutions—such as a bridge—as well as a possible reroute. But there are no easy answers. One potential reroute option would go around Polychrome Mountain, sacrificing the iconic view and exchanging one set of geotechnical challenges for another on a route with known landslides.

Another reroute possibility would bring the road to the valley floor where networks of channels known as braided rivers and floodplains could create additional issues. Both potential reroute options traverse thawing, unstable permafrost and would require building approximately six miles of road in federally designated wilderness—a complicated process involving extensive environmental review and public participation.

As parks work to address current issues, scientists are also collecting vital data that will help plan for the future. Sousanes, the Park Service physical scientist, studies climate and works with both the Central Alaska and Arctic Inventory and Monitoring Networks. She focuses on keeping the networks’ data stations running 365 days a year.

“My piece of this whole puzzle in this long-term monitoring effort is to have these really key baseline data from these national parks that we haven’t had over the past hundred years,” Sousanes says. “We’re starting to get a fairly good record now, and it can help us understand the changes that we’re going to be seeing on the landscape.”

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