Harvard Forest soil warming experiment shows microbes release stored carbon after 37 years

A 37-year soil warming experiment at Harvard Forest reveals that microbes began releasing carbon once thought safely locked away after the fourth decade of sustained 5°C heating. The finding challenges assumptions about forest soils as permanent carbon sinks and suggests climate feedback loops may be stronger than models predict. Researchers installed underground cables to continuously warm soil 5°C above ambient temperature, mimicking projected climate conditions. For the first three decades, the soil held its carbon. But in the fourth decade, microbial activity shifted and started releasing stored carbon dioxide into the atmosphere. This suggests a tipping point where soils switch from carbon sinks to carbon sources under sustained warming.