Crossing 0°C: How Warming Is Thawing Alaska’s National Parks

Alaska is among the fastest-warming regions on Earth: studies show the Arctic has warmed almost four times faster than the globe over the last four decades. Alaska's national parks are defined by ice and snow, and as their mean annual air temperature rises above the freezing point, 0°C, glaciers, permafrost and snow melt and thaw, reshaping the land. Eight Park Service scientists reviewed what even a small rise in temperature is doing to these parks.

A retrogressive thaw slump in the upper Noatak National Preserve, where a bank of ice-rich permafrost thaws, collapses and is carried away. Image from the National Park Service's Alaska Park Science article "Crossing the Zero-Degree Threshold."

The Arctic parks

For tens of thousands of years, the average air temperature of the Arctic parks has stayed below 0°C, keeping the ground frozen as permafrost. Now:

• Average annual temperatures in communities near the Arctic parks rose by more than 2.6°C from 1950 to 2021. Statewide warming began in the late 1970s, and an upward jump in 2014 brought Alaska's two warmest years on record.
• Comparing 2014–2019 with the previous 30 years, most park weather stations warmed at least 1°C, and the northwest Arctic parks up to 3°C — two to three times more than other Alaska parks in the past decade.
• At some warmer sites, the mean annual ground temperature rose above 0°C for the first time since measurements began.
• By century's end, the Arctic parks are projected to warm by more than 7°C on average, with sea ice and seasonal snow becoming less defining features.

Average annual temperatures from seven long-term stations in and around northern Alaska parks, 1952–2019; note the jump in 2014, which brought temperatures near 0°C. NPS.

What thaw is doing:

Modeled permafrost in the Arctic Network parks, 2000s versus 2090s; red marks unfrozen ground. Image from the National Park Service's Alaska Park Science article "Crossing the Zero-Degree Threshold."

Arctic rivers, shaped by permafrost thaw, provide nutrients and habitat for fish. NPS/Ken Hill.

A voice from northwest Alaska

Jeanette Koelsch, superintendent of Bering Land Bridge National Preserve and a member of the Nome Eskimo Community, the largest tribe in the Bering Strait region, sees the changes firsthand. People who live in and around Alaska's parks, she stresses, hold practical knowledge of the land and are on the front line of climate change. In September 2022, the remnants of Typhoon Merbok, fed by unusually warm Pacific water and arriving before any sea ice could buffer the coast, destroyed fish camps, swept away boats and thawed freezers full of winter food. With warmer oceans and longer ice-free seasons, coastal communities will grow more exposed to such storms.

The Interior parks

Many Interior parks average just below 0°C. After the 2014 jump, several places in and near Denali, Wrangell-St. Elias and Yukon-Charley Rivers averaged at or above freezing.

• Around 2000, about half of Denali and three-quarters of Wrangell-St. Elias had permafrost, much of it at or just below 0°C; models predict most will begin thawing by the end of the century.
• At Gosling Lake in Denali, permafrost has degraded in recent decades from warming, wet summers and fire — yet cold winters with little snow let it temporarily recover, showing how sensitive ground near 0°C is.
• Thaw-driven landslides along the Denali Park Road have closed it and required costly repairs; disturbed ground thaws faster, a self-reinforcing loop.

Snow is changing too:

• Albedo — the share of sunlight reflected — falls 79% when dry snow melts, and 25% as spring snow thaws and refreezes daily, speeding melt and warming.
• From 1988 to 2018, spring snow-off came earlier across Alaska by an average of 0.39 days a year (12 days over the period), ranging from 0.11 to 1.31 days a year by place.
• Rain-on-snow events leave an icy crust that keeps caribou from lichens beneath; they are most common in fall and spring along the Bering Sea coast and in the interior, and are expected to increase, especially in southwest and interior Alaska.
• As more places cross 0°C, the months of reliable snow cover shrink, mainly at lower elevations and latitudes.

Rain-on-snow days per year across Alaska; southwest and central Alaska see the most, mostly in November and December. Image from the National Park Service's Alaska Park Science article "Crossing the Zero-Degree Threshold."

The coastal mountain parks

Alaska's southern coastal parks hold the country's largest share of glaciers and icefields.

New land, lakes and habitats open as the ice retreats — at Bear Glacier in Kenai Fjords, visits to its growing lake have risen. At Glacier Bay, thinning glaciers may send the Alsek River away from its Dry Bay outlet to a steeper outlet through Grand Plateau Lake, more than 17 miles (28 km) southeast, disrupting commercial fishing, subsistence and sport hunting and fishing, and famous wilderness rafting.

The phase shift

As Alaska's parks cross from frozen to unfrozen, thawing ground raises landslide and erosion risks along roads and rivers and damages infrastructure built for frozen soil; snow seasons shorten and rain-on-snow increases; and glaciers thin and retreat. Because Alaska's parks are large, naturally functioning and changing fast, the Park Service's long-term data from them feed global climate, permafrost, carbon and methane research.

Sources

Based on Sousanes PJ, Hill K, Swanson D, O'Donnell J, Kirchner P, Kurtz D, Loso M, Bliss A, "Crossing the Zero-Degree Threshold," Alaska Park Science 22(1), National Park Service (2023); a work of the United States government in the public domain.