Permafrost Thaw Explained: Why Frozen Ground Matters for the Climate

Across much of the far north, the ground beneath your feet is frozen solid, and has been for thousands of years. This frozen ground, called permafrost, rarely makes headlines until it starts to thaw. When it does, roads buckle, buildings tilt, coastlines crumble and long-stored carbon can begin to escape into the air. This explainer covers what permafrost is, why it is thawing, and why scientists pay such close attention to it.

Key points

Permafrost is ground that stays at or below 0°C (32°F) for at least two years in a row.
It underlies a large share of land in the Northern Hemisphere, mostly in Siberia, Alaska, northern Canada and the Tibetan Plateau.
Frozen soils hold a very large store of carbon from ancient plants and animals, roughly twice as much as is currently in the atmosphere, by common scientific estimates.
As the Arctic warms, permafrost is warming and thawing in many places, damaging infrastructure and releasing carbon dioxide and methane.
Thaw is a slow-moving feedback that adds to warming, but its size and timing are still uncertain.

What is permafrost?

Permafrost is not a type of soil. It is a condition: any ground (soil, sediment or rock) that stays frozen for two or more consecutive years. Some permafrost is only a few meters thick; in parts of Siberia it extends hundreds of meters down. Much of it formed during past ice ages and has stayed frozen ever since.

On top of the permafrost sits the active layer, a layer of ground that thaws every summer and refreezes every winter. Plants grow in the active layer, and its depth varies from place to place and year to year. When scientists say permafrost is thawing, one of the main things they measure is the active layer getting deeper, reaching into ground that used to stay frozen.

The National Snow and Ice Data Center estimates that permafrost or seasonally frozen ground covers a substantial fraction of the Northern Hemisphere's land surface, with permafrost itself underlying roughly a quarter of exposed land there.

Ice in the ground

Permafrost often contains a lot of ice: in pore spaces between soil grains, in lenses and wedges, and sometimes in massive buried ice bodies. That ice acts like glue. When it melts, the ground can lose its strength and slump or sink, which is why thaw causes so much physical damage.

Why permafrost is thawing

The Arctic is warming much faster than the planet as a whole, a pattern called Arctic amplification. Studies put the rate at roughly two to four times the global average over recent decades, depending on the time period and region analyzed. One reason is that as bright snow and sea ice disappear, darker land and ocean absorb more sunlight, which drives further warming.

Warmer air temperatures gradually warm the ground below. Long-term monitoring boreholes across the Arctic show permafrost temperatures rising in many locations. NOAA's annual Arctic Report Card tracks these changes alongside sea ice, snow cover and other indicators.

Other factors matter too. Deeper winter snow can insulate the ground and keep it warmer. Wildfires burn away the insulating layer of moss and organic soil, exposing the permafrost to summer heat. Changes in rain and drainage can speed thaw locally.

The carbon question

This is the part that makes permafrost a global issue rather than just a regional one. Over tens of thousands of years, dead plants and animals in cold northern soils froze before they could fully decompose. That organic matter has been locked away ever since.

Scientific estimates suggest northern permafrost regions hold somewhere around 1,400 to 1,600 billion tonnes of carbon, though estimates vary. That is roughly twice the amount of carbon currently in the atmosphere. Not all of it is at risk of thawing soon, but even a fraction matters.

When permafrost thaws, microbes wake up and start breaking down the organic material. Depending on conditions, they release:

Carbon dioxide, mostly where soils are dry and oxygen is available.
Methane, mostly in waterlogged soils, wetlands and thaw lakes where oxygen is scarce. Methane is a much more powerful heat-trapping gas than CO2 over the short term, though it breaks down faster.

Those gases add to the greenhouse effect, causing more warming, which causes more thaw. This loop is called the permafrost carbon feedback. For background on why these gases trap heat, see What is the greenhouse effect?

Is it a "methane bomb"? Headlines sometimes describe permafrost as a ticking time bomb. The IPCC Special Report on the Ocean and Cryosphere assesses that permafrost carbon will be released mostly gradually over this century and beyond, adding meaningfully to warming, rather than in a single sudden burst. Scientists stress that the feedback is real and important, but that cutting human emissions remains by far the biggest lever on future warming.

Gradual and abrupt thaw

Most models focus on gradual, top-down thaw as the active layer deepens. But in ice-rich ground, thaw can happen abruptly. The ground collapses into pits and hummocky terrain called thermokarst, lakes form and expand, and hillsides slump. Abrupt thaw can expose deep carbon much faster than gradual warming would. Because it is patchy and hard to model, it is one of the main sources of uncertainty in permafrost projections.

Effects on people and infrastructure

Millions of people live on permafrost, and much of the Arctic's infrastructure was built on the assumption that the ground would stay frozen. Thaw is already causing problems:

Buildings and roads crack, tilt or sink as the ground beneath them settles unevenly.
Pipelines, airstrips and railways need costly repairs and redesign.
Coastal erosion speeds up where thawing cliffs meet waves that are no longer held back by sea ice for as much of the year. Some Alaska Native villages are planning or considering relocation.
Water and sanitation systems can be disrupted, and traditional food storage in ice cellars can fail.

Engineers use techniques such as raising buildings on piles, installing cooling devices in foundations, and choosing routes that avoid ice-rich ground. These help, but they add cost and do not stop the underlying thaw.

How scientists study permafrost

Researchers combine several approaches:

Boreholes with temperature sensors that record how the ground is warming at depth over many years.
Active layer measurements, often by pushing a metal probe into the ground at the end of summer to see where it hits frozen soil.
Gas flux measurements from towers and chambers that track how much CO2 and methane the land releases or absorbs.
Satellites and aircraft that map land surface changes, lakes, ground subsidence and wildfire scars across huge areas.

This is part of the broader body of evidence described in How do we know the climate is warming?

What can be done

There is no practical way to refreeze permafrost across the Arctic. The amount of future thaw depends mainly on how much the planet warms, which depends on global greenhouse gas emissions. Assessments consistently find that lower-emission pathways mean much less permafrost loss and less carbon released than high-emission ones.

Locally, communities and governments are adapting through better monitoring, updated building codes, and planning for relocation where necessary. Protecting northern forests and peatlands from fire also helps keep carbon in the ground.

The bottom line

Permafrost is a vast frozen archive of ancient carbon and a foundation for life and infrastructure across the north. It is warming and thawing in many places, causing real damage today and adding a slow but meaningful feedback to global warming. How much more thaws depends largely on choices made about emissions in the coming decades.