The natural cycle of carbon dioxide in the atmosphere has long been influenced by the alternation between glacial and interglacial periods, but a recent discovery could upend our understanding of these mechanisms.
For decades, scientists attributed the rise in CO₂ levels during interglacial periods mainly to the oceans. When oceans warm and mix more, they release stored carbon, explaining an increase of about 100 parts per million. This view was widely accepted until new research came to challenge it.
Cavity caused by permafrost thaw in Siberia.
Vladimir Pushkarev
A team from the University of Gothenburg used pollen analysis and climate models to reconstruct vegetation and carbon stocks over the last 21,000 years. By examining snapshots every thousand years, they were able to estimate how carbon was exchanged between the soil and the atmosphere. This approach revealed that permafrost thaw played a much larger role than expected in CO₂ emissions after the last ice age.
The researchers explain that during ice ages, large amounts of organic carbon were trapped in frozen soil, forming thick deposits called 'loess'. With warming, this permafrost thawed, gradually releasing CO₂ into the atmosphere. They estimate that nearly half of the increase in atmospheric CO₂ during the transition to the current interglacial period came from these terrestrial emissions in the northern hemisphere.
However, this massive release was partially offset by the development of peatlands, which stored carbon over time. This natural balance helped stabilize CO₂ levels around 270 parts per million for millennia, until modern human activity disrupted this delicate equilibrium by burning fossil fuels.
Today, with anthropogenic climate warming, permafrost is thawing again, but without the natural compensation mechanisms that existed in the past.
Permafrost and its role in the carbon cycle
Permafrost is soil that remains permanently frozen for at least two consecutive years, often found in Arctic and subarctic regions. It acts as a vast reservoir of organic carbon, trapping plant and animal matter that does not decompose due to low temperatures.
When permafrost thaws, microorganisms become active and begin to decompose this organic matter, releasing carbon dioxide and methane into the atmosphere. This process can accelerate climate warming, creating a positive feedback loop where more heat leads to more thaw and thus more emissions.
The stability of permafrost is therefore crucial for maintaining the balance of greenhouse gases. Modern climate change threatens to release colossal amounts of carbon stored for millennia, potentially much larger than during past natural transitions.
Understanding these dynamics helps predict future impacts and develop strategies to mitigate the effects of warming, particularly by protecting permafrost areas and promoting natural carbon sinks.
Glacial and interglacial cycles
Glacial and interglacial cycles are natural periods of Earth's cooling and warming, influenced by orbital variations such as eccentricity, obliquity, and precession. These cycles typically last about 100,000 years, with cold glacial periods and warmer interglacials.
During glaciations, vast ice sheets cover parts of the continents, trapping water and lowering sea levels. Atmospheric carbon then decreases, partly due to its absorption by cold oceans and storage in frozen soils.
During transitions to interglacial periods, ice melt and warming gradually release this carbon, contributing to rising temperatures. These natural variations have shaped ecosystems and Earth's climates for millions of years.
Today, human activity is superimposing a rapid increase in greenhouse gases on these natural cycles, disrupting established balances and accelerating changes that could have irreversible consequences.