Plants absorb less carbon dioxide than models show, study shows

Former UM doctoral student Bill Smith and current UM Professors Cory Cleveland, Ashley Ballantyne and Steve Running studied the relationship between atmospheric carbon dioxide from human emissions and a corresponding growth in plant life, and they compared their results with existing models. The study was published this month in the journal Nature Climate Change.

Carbon dioxide enhances plant growth, and plants absorb atmospheric carbon dioxide. Smith and the study co-authors compared measurements of plant productivity estimated by models with those measured by satellites. They concluded that current models unrealistically overpredict the ability of plants to offset growing greenhouse gas emissions, suggesting that Earth's capacity to take up future carbon dioxide emissions may be less than previously thought.

"Current earth-system models assume that global plant growth will provide the tremendous benefit of offsetting a significant portion of humanity's CO₂ emissions, thus buying us much-needed time to curb emissions," Smith said. "Unfortunately, our observation-based estimates of global vegetation growth indicate that plant growth may not buy us as much time as expected, [so] action to curb emissions is all the more urgent."

The authors identify two important factors that could drive the divergence between satellite-based results and model-based results: availability of water and availability of nutrients. Satellite data indicate warmer climate conditions resulting from rising atmospheric carbon dioxide may increase stress in plant water, counteracting any positive effect of carbon dioxide. Additionally, limited availability of nitrogen and phosphorus in the environment also could limit the ability of plants to soak up additional carbon dioxide.

These findings indicate that current climate models do not accurately predict future plant growth and suggest that allowable emissions targets based on these models may need re-evaluation. The authors recommend better integration among model, satellite and on-the-ground measurement approaches to improve our understanding of the effects of rising atmospheric carbon dioxide on plant growth.

The work provides an important step toward understanding how plants may respond (or not respond) to rising atmospheric carbon dioxide, as well as highlights ways scientists from different specialties can work together to reach a deeper understanding of how ecosystems will respond to global change, says Sasha Reed, a U.S. Geological Survey scientist and a co-author of the paper.

"We have many scientific tools in our toolbox," Reed said, "and bringing them together is a powerful approach to asking questions and to solving problems."

Running said the study could help researchers understand how Montana forests and crops will respond to the changing climate.

"While increasing CO₂ does aid photosynthesis, the changing climate is reducing growing season water supplies," he said. "The result will help form policies for forest, range and water management."

Running suggests that satellite-derived measurements might, for now, be a more accurate way to measure how carbon dioxide fertilization impacts global terrestrial ecosystems.