Akshay DineshOn a summer day, Arctic ground can look solid enough to carry a road, a house, or a herd of caribou. Yet some of that ground is held together by ice. When the ice thaws, the surface may sink, tilt, split, or slump into a river. The result is not simply softer soil: an entire landscape can be reorganized as water finds new routes and frozen organic matter becomes available to microbes.
Permafrost is ground that remains at or below 0 degrees Celsius for at least two consecutive years. It can be soil, sediment, or bedrock, and it does not have to be covered in snow or visibly icy. Some permafrost contains little ground ice; other deposits contain thick ice wedges or more ice than mineral soil by volume. That difference helps explain why thaw is gradual in one place and abruptly destructive in another.
Permafrost is not the same as permanently frozen surface soil
The top of a permafrost landscape usually thaws every summer and freezes again in winter. Scientists call this seasonally changing zone the active layer. Beneath it lies permafrost, which can extend from a few meters to hundreds of meters deep. A cold winter does not automatically restore permafrost that has warmed or thawed at depth, because summer heat, snow insulation, vegetation, soil moisture, and underground heat all shape the ground’s energy balance.
Permafrost also varies across the map. In continuous-permafrost regions, frozen ground underlies nearly the entire landscape. Farther south, discontinuous and sporadic permafrost occurs in patches controlled by slope, vegetation, snow, soil, and local climate. A shaded peatland and a nearby gravel ridge can therefore respond differently to the same air temperature.
Ground ice is the structural piece that matters most for collapse. Water can freeze in pores between soil particles, build layers, or fill winter contraction cracks that reopen over many years. Repeated freezing can grow wedge-shaped bodies of ice several meters deep. While frozen, ice-rich sediment behaves like a firm foundation. Once that ice melts and drains away, the remaining soil cannot occupy the same volume.
How thaw turns level ground into thermokarst
Imagine a bucket filled with soil and large ice cubes. If the ice melts and the water escapes, the contents settle below their original level. Ice-rich permafrost behaves in a similar way, although the real ground contains uneven ice lenses, wedges, roots, and sediment. The sinking and irregular terrain produced by thawing ground ice is called thermokarst. The name reflects its resemblance to the pits and hollows that water carves in soluble limestone, even though the process is different.
Thermokarst can begin with a small change at the surface. A wildfire may remove insulating vegetation, a road may alter drainage, heavy equipment may disturb the organic layer, or a pond may absorb more summer heat than the land around it. Thaw deepens below the disturbed patch. The surface then subsides, water collects in the depression, and that water transfers still more heat into the ground. A shallow hollow can grow into a wet trough, a thaw pond, or a lake.
The process does not always move in one direction. A thermokarst lake may expand as its warm water thaws the banks and lake bed, but it can later drain if erosion opens a channel or the ground beneath it becomes more permeable. The exposed lake bed may refreeze in part, dry out, or grow new vegetation. Permafrost landscapes are mosaics of thaw, drainage, erosion, and occasional stabilization rather than a uniform sheet melting from north to south.
That unevenness makes local ground-ice content crucial. A 2025 U.S. Geological Survey study in Point Lay, Alaska, combined about 60 boreholes with ground-temperature records, remote sensing, and community interviews. The studied deposits averaged roughly 40 percent excess ice, with some ice wedges extending more than 12 meters deep. In developed areas, mapped ice-wedge thermokarst expanded from less than 5 percent of the surface in 1949 to more than 60 percent by 2019, showing how climate warming and surface disturbance can reinforce each other.
Thaw changes where water flows
Frozen ground often acts like a barrier beneath the active layer. Rain and snowmelt cannot easily seep downward, so water remains near the surface in ponds, wetlands, and shallow flow paths. When permafrost thaws, some places become wetter because the land sinks and holds water. Others become drier because new underground routes let lakes and wetlands drain. The same region can experience both changes within a short distance.
Rivers respond as collapsing banks deliver sediment, dissolved organic matter, and nutrients to the channel. Water may become cloudier, channels may shift, and erosion may threaten travel routes or places used for fishing and hunting. NOAA’s 2025 Arctic Report Card highlighted another visible consequence: some Arctic streams are turning orange as thaw exposes minerals and mobilizes iron and other metals. The color is striking, but the larger concern is how changing water chemistry affects habitat and drinking-water sources.
NASA’s Frontlines of Rapidly Transforming Ecosystems mission, known as FORTE, is tracking these connections on Alaska’s North Slope. The campaign combines satellites and aircraft with drones, boats, buoys, river instruments, and observations from local partners. Its first long-term station in a channel of the Colville River records flow, temperature, dissolved oxygen, turbidity, dissolved organic matter, and algae. When a 2025 flood altered the river, the station captured changes that a satellite image alone could not explain.
Frozen carbon and built foundations both become vulnerable
Permafrost has stored plant and animal remains for centuries or millennia because cold, waterlogged conditions slow decomposition. Thaw does not release all of that carbon at once. It makes some previously frozen material available to microbes, which can produce carbon dioxide in oxygen-rich soil and methane in waterlogged, oxygen-poor conditions. Plants may absorb more carbon as growing seasons lengthen, while fire, erosion, drainage, and soil type change the balance from place to place.
This is a climate feedback: warming promotes thaw, and some gases released after thaw add to future warming. It should not be reduced to a single countdown or a claim that every thawing patch emits the same amount. Researchers must determine how much carbon becomes available, whether it moves into rivers or the atmosphere, which gas is produced, and how quickly new vegetation takes up carbon. Those are measurement questions, not details that can be filled in from air temperature alone.
For northern communities, the ground itself is the immediate issue. Subsidence can twist roads, pull apart water and sewer lines, destabilize fuel tanks, and make doors or floors shift as different parts of a building settle at different rates. Engineers respond with site-specific methods such as piles anchored below the active layer, ventilated foundations that keep heat away from the ground, aboveground utilities, better drainage, and snow management. A design that works on cold, ice-poor bedrock may fail on warm sediment filled with wedges of ice.
The Point Lay study documented damage to homes and utilities alongside the physical measurements. It also showed that development is not merely a passive victim: dark surfaces, altered drainage, snow accumulation, and disturbed vegetation can warm the ground locally. Adaptation therefore begins with detailed ground-ice maps and community knowledge, not just a regional projection of average temperature.
How scientists observe a process hidden underground
Permafrost cannot be measured well by one instrument. Boreholes reveal temperature with depth and show whether the ground is approaching the thaw point. Probes and mechanical measurements track the active layer at the end of summer. Cores expose the amount and arrangement of ice, sediment, and organic matter. On the surface, repeat surveys can measure subsidence by centimeters, while water sensors record the consequences downstream.
Satellites extend that view across vast and difficult terrain. Radar can detect small changes in surface elevation, optical imagery can map expanding ponds and eroding banks, and thermal sensors can follow surface conditions. Yet satellites generally do not see the depth and shape of ground ice directly. Researchers test remote observations against boreholes and field surveys, then use models to connect air temperature, snow, vegetation, water, and soil properties.
Long records from residents add another kind of resolution. People who travel a river or tundra route year after year notice when a bank disappears, a channel becomes unsafe, or water changes color. NASA’s FORTE team built its North Slope work with the Native Village of Nuiqsut because instruments need that place-based history to be interpreted well. The partnership also helps researchers measure changes that matter to daily life rather than treating the landscape as an empty laboratory.
Thawing permafrost is often described as frozen ground becoming unfrozen, but the more useful picture is structural. Ice holds up soil, blocks drainage, preserves carbon, and shapes where plants, water, roads, and buildings can exist. When that ice disappears, the landscape does not simply warm; it rearranges itself. Understanding where the ice lies and how water responds is what turns a broad climate trend into practical knowledge for the people and ecosystems living above it.
