Plants can’t absorb as much CO2 as climate models predicted
CO2 can stimulate plant growth, but only when enough nitrogen is available—and that key ingredient has been seriously miscalculated. A new study finds that natural nitrogen fixation has been overestimated by about 50 percent in major climate models. This means the climate-cooling benefits of plant growth under high CO2 are smaller than expected. The result: a reduced buffer against climate change and more uncertainty in future projections.
High levels of carbon dioxide in the atmosphere are a major driver of climate change. At the same time, increased CO2 can encourage plants to grow faster, allowing them to absorb more carbon and potentially slow warming. That benefit, however, depends on whether plants have access to enough nitrogen, a nutrient that is essential for growth. Scientists have only recently taken a closer look at how much nitrogen is actually available in nature. New research involving the University of Graz shows that the so-called CO2 fertilization effect has been significantly overstated.
Plants cannot use nitrogen on their own. The nutrient must first be converted into a usable form through a process called nitrogen fixation, which relies on microorganisms in the soil. This process takes place in natural ecosystems as well as on farmland. "While this process has been significantly overestimated in nature, it has increased by 75 percent over the past 20 years due to agriculture," says Bettina Weber, a biologist at the University of Graz, summarizing findings from a study published earlier this year.
Building on those results, a new analysis shows that the way nitrogen fixation is calculated in some Earth System models has now been reassessed. These models are widely used to project climate trends and inform major assessments, including the World Climate Report. The updated findings were published in the scientific journal PNAS.
New Findings Prompt Climate Model Revisions
The study was led by Sian Kou-Giesbrecht of Simon Fraser University in Burnaby, Canada. The work was carried out by an international research group focused on biological nitrogen fixation, which includes Bettina Weber. This working group receives support from the U.S. Geological Survey (USGS) John Wesley Powell Centre for Analysis and Synthesis.
"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 explains. Because plants depend on this process to access nitrogen, the overestimate has meaningful consequences. According to the study, it results in an overall reduction of about 11 percent in the projected CO2 fertilization effect.
Why Updating Models Is Critical
Weber emphasizes the importance of adjusting climate models to reflect these updated measurements. "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." Accurately accounting for nitrogen dynamics, she says, is essential for making reliable predictions about how ecosystems and the climate will respond in the future.