37-Year Soil Experiment Reveals a Hidden Climate Feedback Loop

For nearly four decades, scientists have been heating a patch of forest floor in Massachusetts — and what they found challenges a fundamental assumption about how soils respond to global warming.

The world's longest-running soil warming experiment, led by Jerry Melillo of the Marine Biological Laboratory, has been warming plots of soil at the Harvard Forest in central Massachusetts by 5 °C since 1989. The results, published in July 2026, show that even the stable, mineral-bound carbon that scientists believed was largely protected from decomposition can break down as temperatures rise.

For the first three decades of the experiment, only the younger, more accessible carbon pools showed significant losses. But during the fourth decade, researchers observed something unexpected: deeper, older carbon — carbon that had been bound to soil minerals and considered safe — began disappearing too. Microbes in the warmed plots shifted their behavior, unlocking carbon stores that had remained intact for centuries or longer.

The implications are significant for climate modeling. If forest soils release stored carbon faster than expected under warming conditions, this creates a positive feedback loop: higher temperatures cause more soil carbon release, which adds more CO₂ to the atmosphere, which drives further warming. Current climate models do not fully account for this process, meaning projections of future warming may be underestimates.

The experiment's duration was key to the discovery. Short-term studies — lasting a few years — would never have caught the slow shift in microbial community composition and enzyme activity that eventually broke down the stable carbon. This highlights the importance of long-term ecological research in understanding climate change.

Soil organic matter holds roughly three times as much carbon as the entire atmosphere. Even a small percentage change in soil carbon storage translates into a massive shift in atmospheric CO₂ concentrations.