Scientists Integrate Nitrogen Cycle into Climate Model; Results Suggests Atmospheric CO2 Concentrations May End Up Higher Than Expected
A team of climate scientists from eight US national labs and academic institutions have successfully incorporated the nitrogen cycle into global simulations for climate change for the first time, questioning previous assumptions regarding carbon feedback and potentially helping to refine model forecasts about global warming.
The results illustrate the complexity of climate modeling by demonstrating how natural processes still have a strong effect on the carbon cycle and climate simulations. In this case, scientists found that the rate of climate change over the next century could be higher than previously anticipated when the requirement of plant nutrients are included in the climate model.
The study, by scientists from institutions including the US Department of Energy’s (DOE) Oak Ridge National Laboratory (ORNL); the National Center for Atmospheric Research (NCAR); the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL); Woods Hole Oceanographic Institution; UC Irvine; Cornell University; UC Berkeley; and the University of Kansas, was published 8 October in Biogeosciences, an interactive open access journal of the European Geosciences Union.
Inclusion of fundamental ecological interactions between carbon and nitrogen cycles in the land component of an atmosphere-ocean general circulation model (AOGCM) leads to decreased carbon uptake associated with CO2 fertilization, and increased carbon uptake associated with warming of the climate system. The balance of these two opposing effects is to reduce the fraction of anthropogenic CO2 predicted to be sequestered in land ecosystems.
—Thornton et al. (2009)
To date, climate models have ignored the nutrient requirements for new vegetation growth, assuming that all plants on earth had access to as much nutrient flow as needed. By taking the natural demand for nutrients into account, the authors have shown that the stimulation of plant growth over the coming century may be two to three times smaller than previously predicted.
However, this reduction in growth is partially offset by another effect on the nitrogen cycle: an increase in the availability of nutrients resulting from an accelerated rate of decomposition of dead plants and other organic matter that occurs with a rise in temperature.
Combining these two effects, the authors discovered that the increased availability of nutrients from more rapid decomposition did not counterbalance the reduced level of plant growth calculated by natural nutrient limitations; therefore less new growth and higher atmospheric CO2 concentrations are expected.
...we note that the present simulations have not included the influence of disturbance history and land use. These factors have been shown to interact strongly with C-N dynamics. We are currently exploring these interactions in the context of the fully-coupled climate system model, and we expect that these interactions will result in larger values of atmospheric CO2 concentration than predicted here.
—Thornton et al. (2009)
ORNL’s Peter Thornton, lead author of the paper, describes the inclusion of these processes as a necessary step to improve the accuracy of climate change assessments.
We’ve shown that if all of the global modeling groups were to include some kind of nutrient dynamics, the range of model predictions would shrink because of the constraining effects of the carbon nutrient limitations, even though it’s a more complex model.
The inclusion of the nitrogen cycle marks one more step toward a more realistic prediction for the future of the earth’s climate. Nevertheless, potentially significant processes and dynamics are still missing from the simulations. Thornton also stresses the importance of long-term observation so scientists can better understand and model these processes.
A 15-year study of the role nitrogen plays in plant nutrition at Harvard Forest was an important observational source used to test their mathematical representation of the nitrogen cycle—a long experiment by any standards, but still an experiment that, according to Thornton, could improve the accuracy of the simulation if conducted even longer.
Other shortcomings of climate simulations include the disregard of changing vegetation patterns due to human land use and potential shifts in types of vegetation that might occur under a changing climate, although both topics are the focus of ongoing studies.
The research was funded by the DOE Office of Science. Additional resources were contributed by NASA Earth Science Enterprise, Terrestrial Ecology Program; National Center for Atmospheric Research through the NCAR Community Climate System Modeling program and the NCAR Biogeosciences program.
P. E. Thornton, S. C. Doney, K. Lindsay, J. K. Moore, N. Mahowald, J. T. Randerson, I. Fung, J.-F. Lamarque, J. J. Feddema, and Y.-H. Lee (2009) Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model. Biogeosciences, 6, 2099-2120