Thawing permafrost in Alaska: sinking houses, vanishing pingos, draining lakes

Many parts of Alaska have frozen ground just below the surface, called permafrost. Its depth and extent vary with average air temperature, soil type, topography and surface vegetation. This National Park Service article, part of the series Observing Change in Alaska's National Parks, collects what people in the state say they see as it thaws. Speakers are paraphrased here.

Rising temperatures are melting the frozen ground and changing the landscape. Roads and buildings sink into softer ground, sinkholes and slumps form when frozen layers beneath thaw, and lakes that permafrost once held have begun to drain.

Melting ground

Shifting sands of coastal barrier islands (NPS).

Skagway is so far south that it has little permafrost, and its residents were not much concerned. People in Nome were. One Nome resident said the average temperature had passed above freezing, so the ground is thawing: houses shift, wallboard cracks and sewer and water lines break. Another said the ice cellars dug into the permafrost are melting and can no longer keep traditional foods, so people use freezers. The same person said thaw causes more slumping along riverbanks, bigger sandbars and more sediment, which makes river travel harder and more dangerous.

A pingo on otherwise flat tundra (NPS).

Thaw and slumping change the shape of the land. Pingos, the conical ice-filled hills of regions with continuous permafrost, collapse as their frozen cores melt. A Nome resident recalled about 20 pingos in the 1980s and only one today. An Anchorage observer described collapsed permafrost whose meltwater runs off and causes thermal erosion on top of ordinary wind and wave erosion, and said some tundra appears to be sinking and drowning. The observer thought this is changing near-shore habitat, and that more is going on than is fully understood.

Draining lakes

A drained lake now colonized by the mastodon flower, a small flowering plant (NPS).

When permafrost thaws, surface water seeps deeper and drains lakes and wetlands. Many lakes in northwest Alaska have drained this way. The tundra around them gets drier, which changes the landscape, the plants and animals that can live there and, in some places, raises the risk of wildfire.

A Fairbanks observer explained why lakes can drain suddenly: frozen ground forms a barrier that holds the water in, and when it thaws a passage opens. Flowing water then speeds the thaw, which the observer called a perfect storm. The observer said this happens naturally but now at an unprecedented rate.

Draining also leaves more sediment in waterways from bank erosion, and falling lake levels and drying wetlands affect the fish and birds that depend on them. The Anchorage observer said a drained lake fills with vegetation, so it is a change of habitat type: fish may lose and some birds may gain. The Fairbanks observer added that these shallow lakes usually hold few fish, but their loss is a problem for waterfowl, which use them for nesting and food.

Sources

• National Park Service, Observing Change in Alaska's National Parks, "Permafrost" (https://www.nps.gov/articles/000/akprjjukepermafrost.htm); the page quotes named residents and observers, who are paraphrased and not named here; rewritten in hubnx's own words.