Incorporating nitrogen cycle leads to predictions of more atmospheric CO2
PETER THORNTON, ORNL
New climate research that acanced results than those from less Thornton notes that the 2007 complex models. Accounting for counts for the nutrient requireassessment by the Intergovernnitrogen limitation in some of the ments of vegetation and other mental Panel on Climate Change 17 types of plant communities inaspects of the nitrogen cycle sugwas the first to incorporate intercluded in the simulations lowered gests that over the coming cenactions between climate and the the estimated quantities of carbon tury ecosystems could accumulate carbon cycle. “The resulting range 2-3 times less carbon of model predictions was than previously estiso large that policy makmated. The likely conseers protested [that] the quence is that levels of models are getting more atmospheric CO2 will be complex, but the uncersignificantly higher than tainty is growing,” he predicted, researchers says. “Our results show conclude in a recent rethat adding the nitrogen port in Biogeosciences. cycle narrows the range Previous climate modof possibilities; so in this els that focused on links case, added complexity between planetary carbon reduces uncertainty.” balances and climate Chris Field of the Carhave largely overlooked negie Institution for Scithe nitrogen cycle, assumence says that “without ing that nitrogen is biogeochemical readily abundant as a feedbackssespecially plant nutrient worldwide. carbon-nitrogen However, ecologists have Schematic illustrating feedback pathways included in the interactionsswe know researchers’ model. Blue arrows designate processes represented the models can’t be long recognized that niin previous carbon-only land model components; orange artrogen is limited in some right, and this paper rows show the additional processes represented in the coupled ecosystems, including makes an impressive efcarbon-nitrogen land model. temperate and boreal forfort at addressing that ests, which are considered deficit. While it’s difficult taken up by photosynthesis. Howcritical carbon reservoirs. A previto assess the quantitative accuever, this reduction in plant growth ous study that assessed the impact racy of the findings at this point, was partially offset by the freeingof terrestrial carbon-nitrogen inthey make up of plant nutrients as a result of teractions on global carbon balsense, and the conclusion that CO2 accelerated decomposition of orfertilization is constrained by nitroances concluded that if models ganic matter under rising temperagen fits with a wide range of evineglect this factor, they will subtures. Nonetheless, nitrogen dence that the fertilization effect is stantially underestimate the reduclimitation trumped this compensatnot likely to produce a huge intions in carbon emissions required ing factor. “This suggests that nitrocrease [in] carbon storage.” to stabilize atmospheric CO2. gen limitation will produce a much In the new study, led by Peter However, Pierre Friedlingstein weaker fertilization effect than previously predicted, so overall the Thornton of Oak Ridge National of the University of Bristol (U.K.) land will take up less CO2,” ThornLaboratory, the researchers modiraises the issue of model assesston says. The results also showed fied a state-of-the-art climate ment. He contends that reducing that a steady decline in the ocean’s model to reflect interactions beuncertainty in model predictions capacity to store carbon also will tween changing levels of carbon requires “better evaluation against contribute to higher CO2 levels. and nitrogen under different scecritical sets of data.” Many surThe researchers found good narios. Although the model incorface, atmospheric, and remotely agreement between their model’s porated some ocean data, the sensed data are now being used representation of the nitrogen simulations primarily addressed for this task, he notes, as well as cycle and experimental data from two opposing climate responses in studies that manipulate atmoa study on nitrogen and plant nuterrestrial ecosystems: the fertilizaspheric and soil conditions. trition carried out at the Harvard tion of plant growth by mounting “Models should be tested against Forest. Nonetheless, the incorpolevels of atmospheric CO2; and the such data to see if we can simuration of effects such as changing release of carbon, driven by warmlate the ecosystem response to vegetation patterns due to human ing and shifts in precipitation CO2 and warmingsparticularly in land use and climate change the context of carbon-nitrogen patterns. could improve the accuracy of interaction.” Including nitrogen-cycle effects simulations, Thornton says. in the model produced more nu—NOREEN PARKS 10.1021/es903219c
2009 American Chemical Society
Published on Web 11/04/2009
December 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8713
