Jerry Melillo, a Distinguished Scientist at the Marine Biological Laboratory, has spent the past 37 years studying heated plots in the Harvard Forest in central Massachusetts. Throughout the experiment, researchers kept the soil at 5 °C above the surrounding ground year round.

According to Melillo, the team selected a five degree increase because it represented the upper range of global warming projections when the experiment began decades ago.

"Microbes are critical components of soil ecosystems because they break down organic matter and recycle elements essential for plant growth," explains Melillo. "As warming reshapes these microbial communities, it can speed the loss of carbon from soils."

Rising Temperatures Change How Forest Soils Store Carbon

The decades long experiment allowed researchers to watch how the soil responded over an unusually long period. During the fourth decade of warming, they observed that stable portions of soil organic matter, once believed to resist warming mediated decomposition, also began to break down.

As these long lasting carbon stores decompose, they release additional CO₂ into the atmosphere. The finding suggests that forest soils may contribute more carbon to the atmosphere under continued warming than scientists previously expected.

Global average temperatures have already increased by about 1.1 to 1.4 °C since the Industrial Revolution. Melillo notes that future warming will depend largely on actions taken to reduce greenhouse gas emissions.

"If we dramatically cut CO₂ emissions from fossil fuel burning, or reduce deforestation, the projected increase would be lower," Melillo explains.

Better Climate Models Through Long Term Research

The newly observed breakdown of stable soil carbon points to a potentially stronger climate feedback loop. As the planet warms, soils can release more carbon, which in turn adds more CO₂ to the atmosphere and may contribute to additional warming.

Researchers say incorporating this newly identified process into climate models should improve projections of future climate change and provide a more complete picture of how Earth's carbon cycle responds to rising temperatures.