Study: Forest Carbon Models May Overestimate
Get the Energy newsletter
Daily energy & climate — solar, EVs, oil, the policy fights and tech bets shaping the transition. Free.
- Mukund Palat Rao and colleagues at the Lamont-Doherty Earth Observatory studied 137 U.S. forest sites and found that photosynthesis does not always translate into wood growth.
- At eastern U.S. sites, roughly 36% of yearly carbon uptake occurred after tree growth stopped in late summer; at California sites the figure was about 26%.
- More detailed measurements at four sites showed wood growth was restricted to periods of low aridity and temperature — conditions becoming rarer as heatwaves and droughts intensify.
- Rao said current models "assume that if you have photosynthesis, you have growth," and the new data shows that coupling breaks down almost instantly once hot, dry conditions hit.
- The researchers warned that earth system models assuming tight photosynthesis-growth coupling "may therefore overestimate future forest carbon sequestration under rising atmospheric moisture demand."
- The finding matters because land-based carbon removal — overwhelmingly tree planting — accounts for 99.9% of the 2.2 billion tonnes of CO2 humans remove from the atmosphere each year, with engineered methods making up just 0.1%.
- The team is now testing whether the photosynthesis-growth decoupling they documented appears in other tree species and regions beyond the U.S.
Why it matters: If 26-36% of forest carbon uptake is going to non-wood uses that don't store it long-term, and climate models treat photosynthesis as a proxy for sequestration, then the projections that underpin national carbon budgets and tree-planting strategies may be systematically optimistic — a problem given that land-based removal provides essentially all 2.2bn tonnes of human CO2 removal each year, with machines and chemicals contributing just 0.1%.
