ORNL researches global warming, establishes new projection models

Researchers from Oak Ridge National Laboratory and the National Center for Atmospheric Research have recently developed improved global warming projection models.

The new simulations include the limitation of growth of plants by nutrients, specifically nitrogen, and the models show that plants grow less in higher carbon dioxide conditions than previous models have assumed. The results of their experiments are published in the current issue of Biogeosciences.

Researchers often keep climate models as simple as possible, only adding new factors one at a time, in order to more fully explore and understand the interactions in a complex system.

“While we know that nitrogen is an important nutrient, we do not know exactly the role it plays in suppressing the response to higher CO2 levels,” Natalie Mahowald, professor in the Department of Earth and Atmospheric Sciences at Cornell University and co-author of the paper on the study, said. “So previous model studies ignored this process because it is simpler to ignore and because it is controversial how important nitrogen limitation is to carbon dioxide fertilization of plants.”

Peter Thornton, who works in climate and ecosystem processes in the Environmental Sciences Division at ORNL and co-author of the paper as well, said that we are currently in the midst of a transition from “climate system models” to “Earth system models,” with two major distinctions between them.

“One: robust and mechanistic treatment of the interactions of biological and biogeochemical systems with the physical climate system,” Thornton said. “And two: inclusion of humans and their actions as integrated components of the Earth system. For example, the socio-economic drivers of land use and land cover change and associated impacts on carbon cycle and carbon-climate feedbacks.”

There still exist many challenges for future models. A better understanding of how plants will respond to future climate change and changes in carbon dioxide is essential, as well as the synthesis of models and data.

“This is the first generation of these models, so there are many ways that they need to be improved,” Mahowald said. “The largest uncertainties in the carbon cycle models are probably in the carbon dioxide fertilization response and in the response of respiration (return of carbon dioxide to the atmosphere) to temperature and moisture in the future.”

Researchers must now also consider how other nutrients, such as phosphorus, might influence the interactions, as different nutrients play important roles in different ecosystems.

“We also know that there are important interactions between nutrient dynamics and disturbances like fire, forest harvest and conversion to agriculture,” Thornton said. “That is a very active area of research at the moment.”

The research was funded by the DOE Office of Science, and additional contributors include the Terrestrial Ecology Program of NASA Earth Science Enterprise, as well as the NCAR Community Climate System Model and the NCAR Biogeosciences programs of the National Center for Atmospheric Research.