Permafrost is ground that stays frozen at 0°C (32°F) or colder for at least two consecutive years. It covers roughly 15% of the Northern Hemisphere’s land surface, an area of about 14 million square kilometers, and stores enormous quantities of carbon, mercury, and even ancient microbes. While it sounds like a permanent fixture, permafrost is thawing at accelerating rates, with consequences that reach far beyond the Arctic.
How Permafrost Is Structured
Permafrost isn’t a single slab of ice. It’s a layered system. The top portion, called the active layer, thaws each summer and refreezes in winter. In northern Alaska, this active layer is typically only about 40 to 50 centimeters deep. Below it sits the permafrost itself, which can extend hundreds of meters into the earth and remain frozen for thousands of years.
The frozen ground is a mix of soil, rock, ice, and organic material. Some permafrost contains massive wedges of pure ice, while other patches are mostly frozen sediment with relatively little ice content. This variation matters because ice-rich permafrost is far more destructive when it thaws. The ground collapses unevenly, creating pits, sinkholes, and tilted landscapes in a process called thermokarst.
Where Permafrost Exists
Permafrost is classified into four zones based on how much of the land it covers. In continuous permafrost zones, more than 90% of the ground is frozen. Discontinuous zones have 50% to 90% coverage, sporadic zones 10% to 50%, and isolated patches less than 10%. Continuous permafrost dominates the far north of Alaska, Canada, and Siberia, while the southern edges of the permafrost region tend to be discontinuous or sporadic.
There’s also permafrost beneath the ocean floor. Subsea permafrost formed on land during ice ages and was later flooded as sea levels rose roughly 18,000 years ago. On the U.S. Beaufort Sea margin, most remaining subsea permafrost sits close to the present-day shoreline in water less than 20 meters deep, according to a USGS and BOEM analysis of over 100,000 line kilometers of seismic data. This underwater permafrost is slowly degrading as warmer ocean water eats into it from above.
A Massive Carbon Reservoir
Permafrost regions in the Northern Hemisphere hold an estimated 1,460 to 1,600 billion metric tons of organic carbon. That’s roughly twice the amount of carbon currently in the entire atmosphere. This carbon comes from ancient plant and animal material that accumulated over tens of thousands of years but never fully decomposed because the ground stayed frozen.
When permafrost thaws, microbes wake up and begin breaking down that organic material, releasing carbon dioxide and methane. Methane is especially concerning because it traps about 80 times more heat than carbon dioxide over a 20-year period. In subsea permafrost areas, thawing can also destabilize gas hydrates, icelike deposits that lock methane into the sediment at low temperatures. When those hydrates break down, methane can bubble up through the water column and, on shallow continental shelves, reach the atmosphere. Even methane that doesn’t make it to the surface gets converted to carbon dioxide by ocean microbes, making the water more acidic.
Mercury Locked in Frozen Ground
Carbon isn’t the only thing stored in permafrost. Northern Hemisphere permafrost regions contain an estimated 1,656 gigagrams of mercury, with about 793 gigagrams frozen directly in permafrost. That’s nearly twice as much mercury as all other soils, the ocean, and the atmosphere combined. The mercury binds naturally to organic matter in soil and has accumulated over millennia. As permafrost thaws, this mercury becomes vulnerable to release into waterways and ecosystems, where it can enter the food chain and concentrate in fish and wildlife.
Ancient Microbes and Disease Risk
Permafrost also preserves biological material with remarkable fidelity. In 2016, an unusually hot summer in Siberia thawed ground containing anthrax spores from long-dead reindeer carcasses. The resulting outbreak killed 2,649 reindeer and sickened 36 people, including a 12-year-old boy who died. The bacterium responsible, Bacillus anthracis, had been locked in frozen soil for decades.
Researchers at Aix-Marseille University have isolated viable viruses from Siberian permafrost dating back as far as 48,500 years. The 13 viruses they identified infect amoebae and pose no direct threat to humans, but their survival demonstrates that permafrost can preserve infectious agents for tens of thousands of years. Other studies have found Clostridium bacteria in thawing permafrost, including strains responsible for food poisoning, toxic shock, and botulism. The deeper concern is that pathogens humans have never encountered, or haven’t encountered in centuries, could re-emerge as warming accelerates.
Infrastructure Damage and Costs
For the roughly four million people who live in permafrost regions, thawing creates immediate, tangible problems. When ice-rich ground melts, it loses volume and structural integrity. Buildings sink unevenly, roads buckle and crack, runways warp, and pipelines shift. In places like Fairbanks, Alaska, paved roads are visibly deformed by differential thaw settlement. In the village of Point Lay, homes have sustained serious structural damage.
The financial toll is substantial. A 2025 study in Nature estimated that building and road losses from permafrost thaw could cost Alaska between $37 billion and $51 billion depending on the emissions scenario, after mapping 53 million square meters of building footprint and over 50,000 kilometers of road across the state. Previous studies that only counted publicly owned infrastructure had placed the estimate at $2.1 to $11.8 billion. The higher figure reflects what happens when private homes, businesses, and local roads are included in the count.
How Fast Permafrost Is Changing
The Arctic is warming more than twice as fast as the rest of the planet. Surface air temperatures across the Arctic from October 2024 through September 2025 were the warmest recorded since 1900, with the last ten years ranking as the ten warmest on record. Autumn 2024 and winter 2025 were especially extreme, ranking first and second warmest respectively.
The effects are already visible on the landscape. In over 200 watersheds across Arctic Alaska, iron and other elements released by thawing permafrost have turned once-clear rivers and streams orange over the past decade. The “greening of the Arctic,” first detected in the late 1990s, continues to expand as shrubs and vegetation colonize ground that was previously too cold to support them. This greening creates a feedback loop: darker vegetation absorbs more heat than snow or bare ground, which accelerates warming and further thaw.
Permafrost that took thousands of years to form is disappearing in decades. The carbon, mercury, and microbes it contains are entering active cycles for the first time in millennia, with consequences that extend well beyond the Arctic itself.