The fastest-warming biome on Earth — and, as of 2024, no longer a net absorber of carbon but a net emitter of it.
The Arctic tundra is a treeless biome across the northernmost reaches of North America, Europe and Asia, defined by permafrost — ground that stays frozen year-round — beneath a thin active layer that thaws each summer. For thousands of years it functioned as a carbon sink, with plants and frozen soil locking away more carbon than the system released.
The Arctic has warmed nearly four times faster than the global average over the past four decades — 0.73°C per decade in the Arctic versus 0.19°C per decade globally — a phenomenon known as Arctic amplification, driven largely by the loss of reflective snow and ice cover.
Source: Communications Earth & Environment, 2022; Finnish Meteorological InstituteNOAA's 2024 Arctic Report Card — compiled by 97 scientists from 11 countries — concluded that the Arctic tundra has become a net source of carbon dioxide to the atmosphere, ending its multi-millennial role as a net carbon sink.
Source: NOAA, 2024 Arctic Report CardThe shift is driven by two compounding processes: microbial decomposition in a warming active layer (which releases CO₂ even when the soil stays below freezing), and increasingly frequent wildfires, which 2024 ranked as the second-highest year on record for emissions north of the Arctic Circle.
This sink-to-source flip is arguably the single most consequential Arctic finding of the past several years, because it represents a self-reinforcing feedback loop rather than a one-time loss: warming thaws permafrost, thawed permafrost releases stored carbon, released carbon drives further warming. Unlike deforestation, where the carbon release is a direct, visible consequence of a human action, permafrost carbon release happens gradually and largely out of sight — which makes it easy to underweight in public attention relative to its long-term significance.
The warming trend and the sink-to-source carbon shift are both well-supported by long-term monitoring networks and are not seriously contested. Greater uncertainty surrounds the pace of future permafrost carbon release under different emissions scenarios, and how much of it will be gradual (microbial decomposition) versus abrupt (thermokarst collapse, large-scale erosion) — the latter is harder to model and could release carbon faster than current projections assume.