A decades-long experiment in a Massachusetts forest is uncovering unexpected behavior in soil carbon.
After nearly 40 years, the longest-running soil warming experiment in the world is producing an unexpected finding. What was once considered long-term, “stable” carbon in forest soils is proving far less permanent. As temperatures rise, even these stubborn carbon reserves can break down and release CO2 into the air.
“Microbes are critical components of soil ecosystems because they break down organic matter and recycle elements essential for plant growth,” explains Jerry Melillo, a Distinguished Scientist at the Marine Biological Laboratory, who has moved the needle on the world’s understanding of climate change over the course of his career. “As warming reshapes these microbial communities, it can speed the loss of carbon from soils.”
A Long-Term Experiment in Harvard Forest
Deep in Harvard Forest in central Massachusetts, researchers have been running a controlled warming experiment for 37 years. Electric cables buried underground keep sections of soil at a steady 5 °C (9 °F) above natural conditions through every season, from frozen winters to humid summers.
When the study began in the late 1980s, that temperature increase reflected the upper range of climate projections at the time. Today, it no longer seems extreme. Global temperatures have already climbed by about 1.1 to 1.4 °C since the Industrial Revolution, and future warming will depend heavily on how quickly greenhouse gas emissions are reduced.
“If we dramatically cut CO2 emissions from fossil fuel burning, or reduce deforestation, the projected increase would be lower,” Melillo says.
A Hidden Shift Underground
Soil holds more carbon globally than the atmosphere and all plant life combined, making it a critical piece of the climate system. Scientists have long assumed that a portion of this carbon is effectively locked away for centuries. The new findings challenge that assumption.
After decades of warming, researchers observed that these supposedly resistant forms of soil organic matter are beginning to decompose. This shift did not appear immediately. Instead, it emerged slowly, underscoring the importance of long-term experiments that capture changes unfolding over decades rather than years.
A Feedback Loop With Global Stakes
As these materials decompose, they release additional CO2 into the atmosphere.
This process could strengthen a feedback loop in Earth’s climate system: rising temperatures lead to more carbon release from soils, which in turn drives further warming. Including these results in climate models may improve predictions of future climate change.
Reference: “Three decades of continuous warming in temperate forests destabilizes persistent forms of soil organic matter” by Atzín X. San Román, Serita D. Frey, Melissa A. Knorr, Huan Tong, Jerry M. Melillo and Myrna J. Simpson, 7 April 2026, Science of The Total Environment.
DOI: 10.1016/j.scitotenv.2026.181777
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