Beneath the frozen soils of Siberia, Alaska, and northern Canada lies an estimated 1.5 trillion tons of carbon — roughly twice the amount currently in the entire atmosphere. Permafrost kept organic material frozen for thousands of years, but rising temperatures are thawing it at accelerating rates.

As permafrost melts, microbes decompose the newly exposed organic matter and release CO2 and methane. This creates a self-reinforcing cycle: warming thaws permafrost, releasing greenhouse gases, causing more warming, thawing more permafrost.

Some Arctic regions already release more carbon than they absorb, and that shift is expected to grow as thaw spreads.