Yes, Forest Carbon Models Likely Overestimate How Much Trees Will Save Us — Here's What the Science Shows

The claim is that Earth system models — the sophisticated computer simulations scientists use to project future climate — may be consistently overestimating how much carbon forests will absorb this century. Based on the available evidence, this is well-supported. It is not fringe skepticism; it is a concern raised by mainstream climate science, including the IPCC itself.

The biggest culprit is something called nutrient limitation. Plants do not just need CO2 to grow — they also need nitrogen and phosphorus from the soil. Many models assume that as CO2 rises, forests will simply grow faster and lock up more carbon. But real-world experiments say otherwise. Terrer et al. in Nature Plants analyzed dozens of Free-Air CO2 Enrichment experiments, where forests are exposed to elevated CO2 in the field, and found that biomass gains were strongly held back by nutrient availability. Trees cannot take full advantage of extra CO2 if the soil cannot supply enough nitrogen and phosphorus to build new tissue.

This plays out most sharply in tropical forests, which store enormous amounts of carbon. Fleischer et al. in Nature Geoscience ran a FACE experiment in an Australian tropical forest and found that phosphorus limitation cut the expected CO2 fertilization effect dramatically — biomass gains were far below what models predicted. Wieder et al. quantified the overall problem: models without nutrient cycling project roughly 340 billion tonnes of extra carbon absorbed by 2100, but add nitrogen limits and that drops to 140 billion tonnes. Add phosphorus limits too and it falls to just 40 billion tonnes. That gap is enormous.

Nutrients are not the only blind spot. Bugmann and Bigler in Global Change Biology showed that disturbances — drought, wildfire, insect outbreaks, windstorms — are poorly captured in most models. These events can flip forests from carbon sinks to carbon sources. A multi-model comparison by Huntzinger et al. in Environmental Research Letters confirmed the pattern: models that skip nutrient cycling consistently project higher carbon uptake than those that include it, pointing to a systematic high bias. The IPCC AR6 report explicitly flags all of these gaps as reasons land carbon projections carry large uncertainty.

To be fair, scientists are actively working to fix these models, and the exact size of the overestimate is still debated. The direction of the bias, however, is not seriously in dispute. This matters enormously for climate policy, because many net-zero pledges from governments and corporations rely heavily on forests absorbing carbon to offset ongoing emissions. If the models are too rosy, those pledges may be built on shaky ground.

This idea spreads because it cuts against reassuring narratives about nature-based climate solutions. When scientists and advocates point out that planting trees might not save us as much as promised, it feels alarming — and it should prompt scrutiny. Watch out for carbon offset schemes that cite model projections without acknowledging nutrient limits or disturbance risks. The science does not say forests are useless; it says we should not count on them to do more than they physically can.