Earth’s future carbon sink is weaker than climate models assume

Earth’s green engine runs on carbon and nitrogen. Elevated CO2 can turbocharge plant growth and pull more carbon from the air, but only if ecosystems can access enough usable nitrogen.

A new study argues that many climate models have been assuming too much natural nitrogen input. This will likely overestimate how much future plant growth will buffer warming.

How nitrogen limits CO2 fertilization

Plants can’t use the vast pool of atmospheric nitrogen gas directly. They depend on microorganisms to “fix” molecular nitrogen (N2) into biologically available forms. 

That microbial nitrogen fixation happens in natural ecosystems and on farms, but its true magnitude has been notoriously hard to pin down at global scales.

Because the CO2 fertilization effect depends on nitrogen availability, even modest errors in global nitrogen fixation propagate into big uncertainties about future land carbon uptake.

A recent reassessment led by researchers with ties to Columbia University concluded that natural biological nitrogen fixation is significantly lower than many inventories suggested. 

Building on that finding, the new study asks a simple but consequential question: if models are feeding in too much fixed nitrogen, how does that bias their projections of plant growth and carbon storage?

An overestimated carbon sink

The authors compared a suite of Earth system models against updated estimates of natural nitrogen fixation. 

Across models, they found a tight correlation: the higher a model’s prescribed or emergent fixation, the stronger its projected boost in plant productivity under rising CO2.

When the team replaced legacy fixation values with the newer, lower estimates, the implied “fertilization” was trimmed as well.

Their headline numbers are stark. The analysis indicates models have overestimated natural land nitrogen fixation by roughly 50 percent. In turn, that inflates the CO2 fertilization effect by about 11 percent. 

In practical terms, if you expect plants to soak up more carbon than they can actually manage given real-world nutrient constraints, you’ll paint too-rosy a picture of how much the land biosphere can offset emissions in coming decades.

Climate projections and policy

Earth system models underpin assessments by groups like the IPCC, informing everything from carbon budgets to national planning. 

If land carbon sinks are weaker than projected because nutrient limits bite harder, then the remaining “space” for fossil fuel emissions narrows.

That doesn’t render past assessments useless since uncertainty ranges already acknowledge nutrient constraints. However, it does argue for recalibrating models so the global nitrogen bookkeeping reflects current best evidence.

It also reframes the narrative about nature-based climate solutions. Forest restoration, reforestation, and improved ecosystem management are still vital. 

But their carbon payoffs depend on more than tree seedlings and CO2 levels. Nutrients, water, and disturbance regimes all set the ceiling.

Realistic nitrogen dynamics help keep expectations grounded and strategies better targeted.

The biology behind the carbon sink

At the root of the revision is basic biogeochemistry. Most nitrogen in the atmosphere exists as N2, a triple-bonded molecule that’s energetically expensive to break. 

Specialized microbes do that job, turning N2 into ammonia and related compounds plants can actually use.

The previous global tallies appear to have overstated how much of this microbial work is happening across unmanaged landscapes.

Because agricultural nitrogen inputs (from industrial fertilizers and legumes) are tracked separately, the paper focuses on natural lands – the forests, grasslands, and wetlands that anchor the terrestrial carbon sink.

If those ecosystems fix less nitrogen than once thought, their capacity to accelerate growth under elevated CO2 is throttled accordingly.

Implications for Earth system models

The authors recommend updating model parameterizations and constraints so that natural nitrogen fixation aligns with the newer estimates. That doesn’t just mean turning a single dial. 

In models that couple the carbon and nitrogen cycles, fixation interacts with plant physiology, soil organic matter turnover, and hydrology.

Bringing the nitrogen side closer to observed reality will cascade through those processes, likely moderating projections of net primary productivity and long-term carbon storage on land.

Crucially, the paper doesn’t argue that CO2 fertilization disappears, only that it’s smaller than many models currently simulate. That nuance matters. Nature keeps helping, but not enough to substitute for rapid emissions cuts.

The future of Earth’s carbon sink

Lead author Sian Kou-Giesbrecht (Simon Fraser University) and co-authors – including Columbia’s Duncan Menge and former lab members Benton Taylor and Anika Staccone – frame the work as a refinement, not a repudiation, of model-based climate science. 

“We’re proud that we can continue to build on these important findings on nitrogen fixation, and assess their full implications for the future climate,” said Menge.

If we want accurate projections of how much carbon the land can bank under rising CO2, we have to get the nutrient math right – especially nitrogen. 

With tighter constraints on natural fixation, Earth system models will better capture the limits of the biosphere’s free climate service.

That, in turn, will yield smarter policies that don’t overpromise on what plants can do while we tackle the main driver of warming: fossil fuel emissions.

The research is published in the journal Proceedings of the National Academy of Sciences.

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