A two-decade study of a New Hampshire forest suggests warmer winters may reduce the ability of temperate forests to absorb carbon dioxide, raising new questions about how reliably forests will continue offsetting greenhouse-gas emissions as the climate changes.

Researchers analysed an unusually long record of measurements collected over approximately 20 years from a New England forest ecosystem.

The study focused on how changing winter conditions influenced the forest's annual carbon balance.

Forests play an essential role in the global climate system because trees remove carbon dioxide from the atmosphere during photosynthesis and store carbon in trunks, branches, roots and soil.

Climate models generally assume forests will continue absorbing a meaningful portion of human-generated carbon emissions.

The new research suggests that assumption may become less reliable in some regions.

Winter might appear less important than the growing season because trees conduct most photosynthesis during warmer months.

But cold conditions influence soil, roots, microbes and nutrient cycling for the entire year.

A reliable snowpack can act as insulation.

Although snow feels cold to humans, a thick layer protects soil from much colder air temperatures above it.

When winters become warmer and snow cover becomes thinner or less consistent, soil can experience repeated freezing and thawing.

Those cycles potentially damage roots and alter microbial activity.

Changes below ground can then affect how trees grow during spring and summer.

Warmer winters also influence respiration.

Plants and soil organisms release carbon dioxide while breaking down stored organic material.

If temperatures remain higher, biological activity can continue longer into winter, potentially increasing the amount of carbon returned to the atmosphere.

The balance between photosynthesis and respiration determines whether a forest acts as a strong carbon sink.

If trees absorb less carbon or soils release more, the forest's net climate benefit can weaken.

The researchers used long-term observations because forest responses can be difficult to identify from only a few years of data.

Weather varies dramatically from year to year.

A 20-year record makes it easier to separate short-term fluctuations from persistent trends.

The study does not mean forests suddenly will become sources of carbon everywhere.

Different species, soils, climates and management practices create substantial regional differences.

Many forests continue absorbing large quantities of carbon.

The concern is that climate change itself could weaken some of the natural systems currently helping slow climate change.

That creates what scientists describe as a potential feedback effect.

Human emissions warm the climate.

Warmer conditions weaken carbon absorption.

More carbon remains in the atmosphere, contributing to additional warming.

Understanding whether that feedback becomes significant is important for climate policy because governments frequently count forest protection and reforestation toward emissions targets.

Planting trees remains valuable for biodiversity, water management and carbon storage.

But climate plans cannot assume every forest will absorb carbon at the same rate indefinitely.

The New Hampshire study demonstrates why winter conditions deserve much more attention.

Climate change is often discussed through summer heatwaves, drought and wildfire.

The quieter transformation occurring during winter may also determine how much carbon forests can continue removing from the atmosphere.

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