Permafrost

Permafrost is defined as soil, rock, or underwater sediment that remains continuously frozen for at least two years.

This frozen ground serves as a massive storage system for the planet. Some of the oldest permafrost has stayed frozen for approximately 700,000 years. The vertical depth of permafrost varies greatly depending on the location. Some areas have a depth of less than one meter. Other regions contain permafrost that is deeper than 1,500 meters.

It is a critical component of the Earth's cryosphere and climate systems.

On land, permafrost typically exists beneath a seasonal layer called the active layer.

The active layer is the top portion of soil that freezes and thaws every year. This seasonal cycle allows plant roots to take hold during the warmer months. The boundary between this seasonal layer and the permanently frozen ground is known as the permafrost table. Below this table, the ground stays at or below 0°C. In the deepest sections, the temperature profile eventually meets geothermal heat from the Earth's crust. This heat eventually warms the ground back above the freezing point.

Scientists classify permafrost into three main types based on how much land it covers. The first type is continuous permafrost, which covers 90% to 100% of a specific area. This is often found in the coldest Arctic regions. The second type is discontinuous permafrost, which covers between 50% and 90% of the landscape. This is often called the extensive discontinuous permafrost zone (DPZ). The third type is sporadic permafrost, which covers less than 50% of the area. This is often called the sporadic permafrost zone (SPZ).

These zones depend heavily on the mean annual air temperature.

Permafrost is found across many different parts of the globe. In the Northern Hemisphere, it covers about 15% of the land area. This includes massive regions in Alaska, Canada, Greenland, and Siberia. It also exists on the Tibetan Plateau. In the Southern Hemisphere, permafrost is less common and mostly found on mountain slopes. Examples include the Andes in Patagonia and the Southern Alps of New Zealand. Some permafrost is also located in Antarctica.

Additionally, subsea permafrost exists beneath the seabed on continental shelves. This was formed when sea levels were lower during the last Ice Age.

Research into permafrost has evolved significantly over time. Systematic studies of alpine permafrost did not begin until the 1970s. High mountain regions are particularly difficult for scientists to study. In 2009, researchers discovered permafrost on Mount Kilimanjaro in Africa. This was a surprising find because the mountain is near the equator. By 2014, regional estimates suggested that global alpine permafrost covered about 680,000 square kilometers. These discoveries help scientists map how much frozen water is stored in mountains.

Thawing permafrost creates a significant climate change feedback loop.

The ground contains large amounts of dead biomass that has accumulated over millennia. Because the ground was frozen, this organic matter could not fully decompose. As global warming heats the ecosystem, this soil thaws. This allows decomposition to start again. This process releases greenhouse gases like carbon dioxide or methane. These emissions can impact global carbon budgets. Annual emissions are estimated to be comparable to those from large countries like Russia or the United States.

Beyond climate impacts, thawing permafrost poses many physical and environmental risks. When ice-rich ground thaws, it can become saturated with water. This causes the ground to shift or collapse entirely. This instability threatens human infrastructure. It is estimated that nearly 70% of buildings on permafrost are at risk by 2050.

There are also concerns about pollution. Between 13,000 and 20,000 sites containing toxic waste are located in permafrost. Thawing could also release natural mercury deposits. While some worry about ancient pathogens, scientific reviews currently describe that risk as generally low.