Rising CO₂ first impacts ecosystem productivity before hitting the water cycle

A new study reveals that surprisingly small increases in atmospheric CO₂ lead to detectable effects on ecosystem functioning. Using simulations of the land surface model developed at the Max Planck Institute for Biogeochemistry, an international team of scientists finds that enhanced CO₂ first affects entities of the carbon cycle such as vegetation productivity and the extension of leaf area. Changes in the plant-controlled entities of the water cycle can only be detected if CO₂ rises to higher levels. These CO₂ effects in the carbon cycle emerge first in tropical ecosystems and then in the ecosystems of the higher latitudes. These results provide important guidance for studying well-defined CO₂ effects on ecosystems with real-world measurements.

Both climate changes and rising atmospheric CO₂ concentrations are challenging global ecosystems. Disentangling the effects of climate and CO₂ changes on ecosystems is difficult as they occur simultaneously. However, understanding and isolating their separate roles is crucial in order to anticipate the development of ecosystems under global change, and to more accurately project our future climate.

In a new study published in Global Change Biology, a group of researchers from China, Germany and Italy scrutinized land surface model simulations which were derived with different CO₂ evolutions but identical climate. In their analyses, the authors focused on the exclusive effects of CO₂ on ecosystems and their impact on related carbon and water fluxes. In contrast to real-world measurements, these simulations offer the opportunity to disentangle the CO₂ and climate effects. „Our results show that the productivity change of ecosystems is already clearly evident even at small increases in atmospheric CO₂, as low as about 20 ppm, which occurs within 5-6 years at the current rate of carbon emissions. Changes in the amount of water transpired by plants, however, are only detectable at higher CO₂ levels “ says Chunhui Zhan, doctoral candidate at the Max Planck Institute for Biogeochemistry and the Technical University of Munich in Germany and first author of the study.

Detecting the significant change of plant productivity due to the ongoing rise of atmospheric CO₂ is a crucial step towards climate change mitigation. Changes in CO₂ and climate can cause contrasting effects on ecosystems, leading to a small net effect in CO₂ uptake by ecosystems - currently the land sink of anthropogenic carbon. For the future, it is unclear how this net effect will evolve under the tugging of the CO₂ and climate effects. “Our study provides an important step towards measuring and understanding these contrasting effects”, says Dr. Alexander J. Winkler, another lead author of the study. “We now know better when and where we expect to detect the CO₂ effects in in-situ flux measurements, and how to detect them. Our results encourage future efforts to improve the understanding and quantification of these effects in observations of terrestrial carbon and water dynamics.” Dr. Winkler adds.