Permafrost is permanently frozen ground that consists of soil, gravel and sand, usually bound together by ice. Permafrost typically remains at or below 0°C (32°F) for at least two years. Permafrost can be found on land and below the ocean floor. It is found in areas where temperatures rarely rise above freezing. This means permafrost is often found in Arctic regions such as Greenland, the U.S. state of Alaska, Russia, Mongolia, China and Eastern Europe. Permafrost thickness can range from less than 1 meter (3 feet) to more than 1,500 meters (4,921 feet). Permafrost covers approximately a quarter of Earth's Northern Hemisphere or about 23 million square kilometers (8.9 million square miles).

Not all frozen ground is considered permafrost. A layer of soil that freezes for more than 15 days per year is called seasonally frozen ground. A layer of soil that freezes between one and 15 days a year is called intermittently frozen ground. To be considered permafrost, the ground must be frozen for two or more consecutive years.Permafrost does not always form in one solid sheet. There are three major ways to describe its distribution: continuous, discontinuous and sporadic. Continuous permafrost is just what it sounds like: a continuous sheet of frozen material that covers 90% or more of the landscape. Continuous permafrost extends under all surfaces in an area except large bodies of water. The part of Russia known as Siberia is the largest continuous permafrost region.

Discontinuous permafrost and sporadic permafrost are broken up into separate areas. Discontinuous permafrost covers between 50% and 90% of the landscape, while sporadic permafrost covers less than 50%. Some ice—in the shadow of a mountain or thick vegetation—stays all year. In other areas of discontinuous permafrost, the summer sun melts the ice for several weeks or months. The land near the southern shore of Hudson Bay, Canada, has discontinuous permafrost.

Most of today’s permafrost is from glaciation periods that took place over the past 10,000 years. However, some permafrost is much older. The oldest continuous permafrost developed about 650,000 years ago in what is now Siberia. Conditions in the Arctic were quite different in the past. Scientists believe that over 400,000 years ago, ice was less stable and more likely to thaw compared to ice today.

Scientists who study permafrost are able to understand changes in Earth's climate by observing changes in permafrost. The Arctic is warming about four times faster than the rest of the world. Studies show the Arctic has warmed by 3°C (5.5°F) since the Industrial Revolution, when humans began using fossil fuels at large scales, emitting warming greenhouse gases into the atmosphere. Scientists predict widespread melting of permafrost by 2100.

Melting permafrost and warming global temperatures have a positive feedback loop. Permafrost stores massive amounts of greenhouse gases like carbon dioxide and methane. As climate change from burning fossil fuels causes permafrost to thaw, these gases are released into the atmosphere. This heats the planet further, causing more permafrost to melt, continuing the cycle. 

When permafrost melts, it raises water levels in Earth's oceans. It also increases erosion, because soil and sediment are easily washed away without the ice binding them together. This makes the soil less stable and can result in new lakes and cause landslides.

Greenhouse gases are not the only thing trapped in the ice. Melting permafrost also releases heavy metals, such as mercury, and other toxins. As it melts, the contaminated permafrost can pollute water sources for local people. It is even possible that thawing permafrost may release dormant, disease-causing microbes that are harmful to animals and humans.

Melting permafrost threatens the way of life for the 35 million people who live in regions with permafrost. This is especially true for Indigenous peoples of the Arctic, who have fewer options for adaptation, given a long history of inequality. Most Arctic infrastructure was built on permafrost with the assumption that the ground would remain frozen. Melting causes the buildings and roads to buckle and collapse. In some Russian cities, up to 80% of the buildings have been damaged due to permafrost melting. Buildings can be fortified against melting, but the process is expensive.

The plants and animals that inhabit areas covered in permafrost are also at risk. When the permafrost melts, they either face the consequences of a warming environment or must migrate farther north.

Local and Indigenous-led groups across the Arctic are using their expertise to address permafrost melting. Permafrost Pathways has created a network across the Arctic to monitor carbon emissions from melting permafrost. This data collection effort allows for a more comprehensive view of carbon emissions and can be used for broader monitoring efforts.

Including local perspectives in this research is critical, according to Corina Qaaġraq Kramer, an Iñupiaq leader from Kotzebue, Alaska, and co-founder and CEO of Respectful Research, a collaborating organization of Permafrost Pathways. She told Permafrost Pathways, “Researchers come from outside into our region, and there is just this chasm that isn’t able to be bridged because of the communication—or the lack of it. There’s a lack of understanding on both sides, really, because of the multiple decades—centuries—of colonialism and the damage that was done.”

The best way to stop the permafrost from melting is to prevent further warming by curtailing the use of fossil fuels. While melting permafrost mainly impacts the Arctic, the effects will be widespread—and the solution should be too. People from across the globe must work together through policies and behavioral changes to reduce dependence on fossil fuels.