If you've ever defrosted a freezer, you know what happens to forgotten food left too long in the back. It thaws, softens, and starts to decompose. Something similar is happening across the Arctic right now, but instead of last year's leftovers, the ground itself is thawing—and it contains the remains of plants and animals frozen since mammoths roamed the Earth.

This frozen ground, called permafrost, has kept vast amounts of carbon locked away for thousands of years. As global temperatures rise, this ancient freezer is losing its grip. What happens when all that organic matter finally thaws? The answer is transforming how scientists think about the future of our climate.

Permafrost is any ground that stays frozen for at least two consecutive years, though most Arctic permafrost has remained frozen for millennia. Beneath the thin layer of soil that thaws each summer lies a deep freeze extending hundreds of meters down in some regions. This frozen earth covers about 25 percent of the Northern Hemisphere's land surface—an area larger than North America.

Within this frozen ground lies an extraordinary carbon vault. When plants died thousands of years ago in these cold regions, they didn't fully decompose. Instead, the freezing temperatures stopped microbial activity cold, literally preserving organic matter like a giant planetary freezer. Leaves, roots, and even entire animal carcasses remained locked in suspended animation. Scientists estimate permafrost holds around 1,500 billion tonnes of carbon—roughly twice what's currently in our entire atmosphere.

This carbon accumulated slowly over tens of thousands of years, layer by frozen layer. Each summer, new plant matter fell to the ground. Each winter, it froze before decomposers could break it down completely. The result is a time capsule of ancient carbon, some dating back 50,000 years or more, preserved in the frozen darkness beneath the Arctic tundra.

Takeaway

Permafrost acts like a planetary freezer, storing twice the carbon currently in our atmosphere—organic matter that accumulated over thousands of years because cold temperatures stopped natural decomposition.

When permafrost thaws, something awakens. Soil microbes that have been dormant for millennia spring back to life and immediately begin doing what microbes do—eating organic matter. As they feast on this ancient buffet of preserved plants, they release carbon dioxide and methane as waste products. It's the same process that happens in your compost bin, just on a massive scale with very old ingredients.

The type of gas released depends on oxygen availability. In well-drained soils where air circulates freely, bacteria produce carbon dioxide. But in waterlogged conditions—common when ice-rich permafrost melts and creates pools—different microbes take over. These anaerobic bacteria produce methane, a greenhouse gas roughly 80 times more potent than CO2 over a 20-year period. This makes waterlogged thaw particularly concerning for climate scientists.

Here's what makes this process so troubling: it creates a feedback loop. As permafrost releases greenhouse gases, the atmosphere warms further, which thaws more permafrost, which releases more gases. Unlike burning fossil fuels, which humans can choose to stop, this process becomes increasingly self-sustaining once it starts. Scientists call this a positive feedback—not because it's good, but because it amplifies itself.

Takeaway

Thawing permafrost triggers dormant microbes to decompose ancient organic matter, releasing CO2 and methane—and the warming this causes thaws more permafrost, creating a self-reinforcing cycle that's difficult to stop.

The Arctic landscape itself reveals how fast permafrost is changing. When ice-rich permafrost melts, the ground literally collapses. This creates a distinctive terrain called thermokarst—a pockmarked landscape of sinkholes, slumping hillsides, and new lakes that form in the depressions. Satellite images show thousands of these lakes appearing across Siberia and Alaska, each one a scar marking where frozen ground has given way.

These thermokarst lakes are particularly effective at accelerating permafrost loss. Water absorbs and retains heat much better than soil, so lakes essentially bore downward into the remaining permafrost like warm drill bits. The dark water also absorbs more sunlight than reflective snow and ice, heating up and transferring that warmth to surrounding frozen ground. Some lakes have been observed thawing the permafrost beneath them at rates five times faster than typical surface thaw.

Local communities are witnessing dramatic changes. Buildings tilt as their foundations shift. Roads buckle and crack. In some Alaskan villages, entire coastlines are eroding as the frozen ground that once held them together turns to mud. These visible transformations are early warning signals of the massive carbon release happening underground—physical proof that the ancient freezer is failing.

Takeaway

Thermokarst lakes and collapsing ground across the Arctic are visible evidence of accelerating permafrost loss—each new lake creates a warm spot that drills deeper into remaining frozen ground, speeding the thaw.

Permafrost thaw represents one of climate science's most concerning feedback mechanisms—a process that could accelerate warming regardless of human emission reductions. The carbon frozen in these soils took thousands of years to accumulate. Its release could happen over mere decades.

Understanding this process helps explain why climate scientists emphasize urgency. Every fraction of a degree matters because it determines how much of this ancient carbon stays locked away versus entering our atmosphere, amplifying warming for generations to come.