A new study reveals that major climate models have overestimated natural nitrogen fixation by about 50 percent, leading to inflated projections of how much CO2 plants can absorb to mitigate warming. This error reduces the expected climate-cooling effect from plant growth under rising CO2 levels by around 11 percent. Researchers urge updates to these models for more accurate future climate predictions.
High atmospheric CO2 drives climate change but also boosts plant growth, potentially absorbing more carbon if sufficient nitrogen is available. However, a recent analysis shows that the key process of nitrogen fixation—where soil microorganisms convert nitrogen into a usable form—has been significantly misjudged in Earth system models.
The study, published in the Proceedings of the National Academy of Sciences, was led by Sian Kou-Giesbrecht from Simon Fraser University in Canada, with contributions from an international team including Bettina Weber from the University of Graz in Austria. The group, supported by the U.S. Geological Survey's John Wesley Powell Centre, compared model estimates to current measurements and found an overestimation of nitrogen fixation on natural surfaces by roughly 50 percent.
"We compared different Earth System models with current nitrogen fixation values and found that they overestimate the nitrogen fixation rate on natural surfaces by about 50 percent," Weber explained. This discrepancy matters because plants rely on fixed nitrogen for growth; without accurate figures, the CO2 fertilization effect—where elevated CO2 spurs biomass increase—is exaggerated.
The findings indicate an 11 percent downward adjustment in projected CO2 benefits. While natural fixation has been overstated, agricultural practices have boosted it by 75 percent over the past two decades. Weber highlighted broader implications: "This is because gases such as nitrogen oxides and nitrous oxide are produced as part of the nitrogen cycle. These can be released into the atmosphere through conversion processes and alter or disrupt climate processes."
These models underpin reports like the World Climate Report, so revisions are crucial for reliable ecosystem and climate forecasts. The work builds on earlier research reassessing nitrogen availability in nature.