Warming degrades carbon stores in boreal forests and Arctic tundra. Warmer soils accelerate microbial decomposition, releasing stored carbon as CO2. This is one of the most robust findings in climate science and one of the most dangerous feedback loops — warming releases carbon, which causes more warming.
Boreal Sphagnum peatlands do the opposite. Long-term warming experiments in Finnish peatlands show that warming enhances soil carbon accumulation, not loss (Nature Ecology & Evolution, 2026). The response is not merely a slowed decline. It is an increase. The peatlands store more carbon under warming than they did before.
Three mechanisms explain the reversal, and all three are driven by Sphagnum itself. First, warming stimulates Sphagnum growth. More moss means more photosynthesis, more carbon fixed from the atmosphere. Second, the additional Sphagnum produces more antimicrobial secondary metabolites — compounds that suppress the soil microbes responsible for decomposition. The moss grows faster and simultaneously poisons the organisms that would break down what it builds. Third, Sphagnum promotes the formation of reactive iron hydroxides in the soil — iron “rust” that physically binds to organic carbon, creating mineral-associated organic carbon resistant to microbial attack. The researchers call Sphagnum a “rust engineer”: it creates the chemical conditions for iron oxidation, and the iron oxide then preserves the carbon the moss produced.
The structural observation: the organism converts each consequence of warming into a component of its defense. Warming increases growth — the growth produces antimicrobials. Warming increases microbial activity — the antimicrobials suppress it. Warming destabilizes organic matter — the iron oxides stabilize it. Every link in the causal chain from “warming” to “carbon loss” is intercepted and reversed by a biochemical response that uses the warming signal as a construction cue. The same forcing that degrades carbon in forests becomes a building material for carbon protection in peatlands.
The scale matters. The carbon gain in boreal Sphagnum peatlands may offset nearly half of the decline in the boreal forest carbon sink under warming. This is not a minor curiosity in a marginal ecosystem. Boreal peatlands cover millions of square kilometers and store roughly twice as much carbon as all the world's forests combined. A system that responds to warming by storing more carbon — because its dominant organism treats warming as a growth and defense signal — is a genuine countercurrent in a feedback loop that most models assume runs in only one direction.