Nitrate concentrations in Greenland ice have almost doubled since the onset of the Industrial Revolution

Nature Reports Climate Change, published online 11 June 2009; doi: 10.1038/climate.2009.55

Abnormal nitrogen

by Alicia Newton, Science 324, 5932 (2009)

MEREDITH HASTINGS

Nitrate concentrations in Greenland ice have almost doubled since the onset of the Industrial Revolution, according to scientists. The rise in nitrate is accompanied by a sharp drop in the isotopic signature of the nitrogen, beginning just as humans started pumping nitrogen oxides into the atmosphere.

Meredith Hastings of Brown University and colleagues used a 100-metre-long ice core from Summit, Greenland, to track changes in nitrogen composition over the past three centuries. Beginning in 1850, the isotopic ratio of the nitrogen, which in part reflects the source of the nitrate, began to decline, just as greenhouse gas concentrations were starting to rise in response to the widespread burning of fossil fuels. The sharpest jump in the isotopic ratio came between 1950 and 1980, when emissions also soared. This overall trend would be difficult to explain through changing chemical processes in the snow or atmosphere alone, leaving fossil fuel combustion as the most likely driver.

Nitrogen oxides are among the six greenhouse gases regulated under the Kyoto Protocol. The team hopes that further work will allow them to determine how changes in climate influence natural nitrogen oxide sources.

Link:  http://www.nature.com/climate/2009/0907/full/climate.2009.55.html

P.E. Thorton et al., Biogeosciences, Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: Results from an atmosphere-ocean general circulation model

Biogeosciences, 6(10) (2009) 2099-2120.

Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: Results from an atmosphere-ocean general circulation model

P. E. Thornton (Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6335, U.S.A.), S. C. Doney (Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1543, U.S.A.), K. Lindsay (Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307-3000, U.S.A.), J. K. Moore (Department of Earth System Science, University of California, Irvine, CA 92697-3100, U.S.A.), N. Mahowald (Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14850, U.S.A.), J. T. Randerson (Department of Earth System Science, University of California, Irvine, CA 92697-3100, U.S.A.), I. Fung (Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, U.S.A.), J.-F. Lamarque (NOAA Earth System Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80305-3337, and Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307-3000, U.S.A.), J. J. Feddema (Department of Geography, University of Kansas, Lawrence, KS 66045-7613, U.S.A.), and Y.-H. Lee (Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307-3000, U.S.A.)

Abstract

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. The primary mechanism responsible for increased land carbon storage under radiatively forced climate change is shown to be fertilization of plant growth by increased mineralization of nitrogen directly associated with increased decomposition of soil organic matter under a warming climate, which in this particular model results in a negative gain for the climate-carbon feedback. Estimates for the land and ocean sink fractions of recent anthropogenic emissions are individually within the range of observational estimates, but the combined land plus ocean sink fractions produce an airborne fraction which is too high compared to observations. This bias is likely due in part to an underestimation of the ocean sink fraction. Our results show a significant growth in the airborne fraction of anthropogenic CO₂ emissions over the coming century, attributable in part to a steady decline in the ocean sink fraction. Comparison to experimental studies on the fate of radio-labeled nitrogen tracers in temperate forests indicates that the model representation of competition between plants and microbes for new mineral nitrogen resources is reasonable. Our results suggest a weaker dependence of net land carbon flux on soil moisture changes in tropical regions, and a stronger positive growth response to warming in those regions, than predicted by a similar AOGCM implemented without land carbon-nitrogen interactions. We expect that the between-model uncertainty in predictions of future atmospheric CO₂ concentration and associated anthropogenic climate change will be reduced as additional climate models introduce carbon-nitrogen cycle interactions in their land components.

Final Revised Paper (PDF, 2280 KB)   Discussion Paper (BGD)

www.biogeosciences.net/6/2099/2009/

Thornton, P. E., Doney, S. C., Lindsay, K., Moore, J. K., Mahowald, N., Randerson, J. T., Fung, I., Lamarque, J.-F., Feddema, J. J., & Lee, Y.-H. (2009). Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model, Biogeosciences, 6, 2099-2120.

© Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License.

Link to abstract:   http://www.biogeosciences.net/6/2099/2009/bg-6-2099-2009.html

Inclusion of nitrogen cycle, a key variable in climate models, refines predictions of atmospheric CO2

Key new ingredient in climate model refines global predictions

	ORNL's Peter Thornton is helping climate scientists incorporate the nitrogen cycle into global simulations for climate change.

OAK RIDGE, Tenn., Oct. 9, 2009 — For the first time, climate scientists from across the country have successfully incorporated the nitrogen cycle into global simulations for climate change, questioning previous assumptions regarding carbon feedback and potentially helping to refine model forecasts about global warming. The results of the experiment at the Department of Energy's Oak Ridge National Laboratory (ORNL) and at the National Center for Atmospheric Research (NCAR) are published in the current issue of Biogeosciences. They 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.

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."

To date, climate models ignored the nutrient requirements for new vegetation growth, assuming that all plants on earth had access to as much "plant food" as they needed. But 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. Since less growth implies less CO2 absorbed by vegetation, the CO2 concentrations in the atmosphere are expected to increase.

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 -- the rotting 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.

The study's author list, which consists of scientists from eight different institutions around the U.S. including ORNL, the National Center for Atmospheric Research, the National Oceanic and Atmospheric Administration Earth System Research Laboratory, and several research universities, exemplifies the broad expertise required to engage in the multidisciplinary field that is global climate modeling.

"In order to do these experiments in the climate system model, expertise is needed in the nitrogen cycle, but there is also a need for climate modeling expertise, the ocean has to be involved properly, the atmospheric chemistry . . . and then there are a lot of observations that have been used to parameterize the model," said Thornton, who works in ORNL's Environmental Sciences Division.

"The biggest challenge has been bridging this multidisciplinary gap and demonstrating to the very broad range of climate scientists who range everywhere from cloud dynamicists to deep ocean circulation specialists that [incorporating the nitrogen cycle] is a worthwhile and useful approach."

The ability to handle the increase in complexities of these models was facilitated by the capabilities of ORNL's Leadership Computing Facility, which currently houses the world's fastest supercomputer for civilian research. Jim Hack, director of the National Center for Computational Sciences, emphasizes that Thornton and his team were not limited by computational resources in the construction of his model. "It's one of the laboratory competencies, so we want to make sure we enable leadership science," he said.

This breakthrough is 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.
UT-Battelle manages Oak Ridge National Laboratory for the Department of Energy.

Link:  http://www.ornl.gov/info/press_releases/get_press_release.cfm?ReleaseNumber=mr20091009-00