The Problem With a Simple Assumption

Many climate plans assume something straightforward: more carbon dioxide in the air makes trees photosynthesize faster, which means they grow bigger and store more carbon. A study published in January 2026 in the journal New Phytologist found that assumption is wrong in an important way. Trees can photosynthesize — absorb CO₂ from the air — without necessarily growing more wood.

This matters because governments and companies are betting on trees to help them meet climate targets. If trees are not actually storing as much carbon as we think, those plans may not work as intended.

Where the Carbon Goes

When a tree takes in CO₂ through its leaves, that carbon does not go straight into building wood. Some is used for the tree to breathe and survive. Some goes into making flowers and seeds. Some goes into growing roots or making chemicals to fight disease. Only what is left over becomes wood — the hard part that actually locks carbon away for decades.

Scientists have suspected this for a while, but the new research confirms it: the amount of carbon a tree captures and the amount of wood it grows are not as tightly connected as models assume.

A previous study calculated that rising CO₂ has increased how much carbon trees capture worldwide by about 13.5% since factories and cars started burning fossil fuels. But if most of that extra carbon is not becoming wood, then projections about how much carbon forests will remove from the air could be too high.

What This Means for Carbon Offsets

Companies and governments buy forest carbon credits — essentially paying others to plant or protect trees, under the assumption that those trees will absorb a specific amount of CO₂. If we cannot reliably calculate how much carbon a tree will store, it becomes hard to stand by those numbers.

Research by scientists at Harvard Forest found another piece of this puzzle: forests do not absorb the same amount of carbon everywhere. How much carbon a forest stores depends heavily on how it is managed and what happens to it — not just on the fact that trees are there and CO₂ is rising.

Are We Planting Too Many Trees and Ignoring Other Solutions?

Another concern: tree planting has become such a popular climate solution that it may be overshadowing other ways to remove carbon from the air. Research from April 2025 suggests this "flagship effect" — where one solution becomes so famous it crowds out others — could be a problem. We probably need multiple approaches, not just more trees.

That said, urban trees are a different story. Trees planted along streets or in parks absorb CO₂ and also cool neighbourhoods, clean the air, and help manage stormwater. Those benefits matter beyond carbon counting, so planting urban trees still makes sense for multiple reasons.

What Comes Next

Forests are still important for fighting climate change and will remain so. But the new findings mean we should be more cautious about how much carbon forests will store in the future. The estimates governments and carbon credit companies rely on may be too optimistic.

For policymakers trying to meet net-zero targets and for the organizations that approve carbon credit projects, this is a significant adjustment to make.