Scientists integrate nitrogen cycle into climate model; Results suggests atmospheric CO2 concentrations may end up higher than expected

Green Car Congress: 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.

…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.

…[Oak Ridge National Laboratory’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….

Schematic illustrating feedback pathways coupling terrestrial carbon and nitrogen cycles in the integrated model. Blue arrows show, in general, the processes represented in previous carbon-only land model components. Orange arrows show the additional processes represented in the coupled carbon-nitrogen land model, differentiated here between rapid internal cycling (solid arrows), and slower fluxes between land pools, the atmosphere, and ground water (dashed arrows). Source: Thornton et al. (2009)