B21M-07
Nitrogen Dynamics are a Key Factor in Explaining Global Land Carbon Sink
Tuesday, 15 December 2015: 09:30
2022-2024 (Moscone West)
Deborah N Huntzinger1, Anna M Michalak2, Christopher Schwalm3, Philippe Ciais4, Kevin M Schaefer5, Anthony W King6, Yaxing Wei6, Robert B Cook6, Joshua B Fisher7, Daniel J Hayes8, Maoyi Huang9, Akihiko Ito10, Atul K Jain11, Huimin Lei12, Chaoqun Lu13, Fabienne Maignan4, Jiafu Mao6, Nicholas Parazoo14, Shushi Peng4, Benjamin Poulter15, Daniel M Ricciuto6, Xiaoying Shi6, Hanqin Tian16, Weile Wang17, Ning Zeng18 and Fang Zhao18, (1)Northern Arizona University, Flagstaff, AZ, United States, (2)Carnegie Institution for Science, Department of Global Ecology, Stanford, CA, United States, (3)Woods Hole Research Center, Woods Hole, MA, United States, (4)LSCE Laboratoire des Sciences du Climat et de l'Environnement, Gif-Sur-Yvette Cedex, France, (5)University of Colorado, National Snow and Ice Data Center, Boulder, CO, United States, (6)Oak Ridge National Laboratory, Oak Ridge, TN, United States, (7)Jet Propulsion Lab, Pasadena, CA, United States, (8)University of Maine, Orono, ME, United States, (9)Pacific Northwest National Laboratory, Atmospheric Sciences and Global Change Division, Richland, WA, United States, (10)National Institute for Environmental Studies, Tsukuba, Japan, (11)University of Illinois at Urbana Champaign, Urbana, IL, United States, (12)Pacific NW Nat'l Lab-Atmos Sci, Richland, WA, United States, (13)Iowa Sate University, Ames, IA, United States, (14)NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States, (15)Montana State University, Bozeman, MT, United States, (16)Auburn University, International Center for Climate and Global Change Research and School of Forestry and Wildlife Sciences, Auburn, AL, United States, (17)NASA Ames Research Center, Moffett Field, CA, United States, (18)University of Maryland College Park, College Park, MD, United States
Abstract:
The terrestrial carbon cycle plays a critical role in regulating the amount of anthropogenic emissions that remain in the atmosphere. Yet, land-atmosphere carbon dynamics are one of the largest sources of uncertainty in projections of future climate. Reducing this uncertainty requires understanding the relative role of various drivers to land carbon uptake. We use an ensemble of land surface models to quantify the influence of climate, land use history, atmospheric CO2, and nitrogen deposition on the strength of the net land sink over the past 110 years. Each model can be thought of as one realization of terrestrial carbon cycling and the factors most important in controlling land sink strength. Using a series of sensitivity simulations, we identify the dominant drivers to the net land sink that emerge consistently across models, both globally and regionally. We find that the relative importance of external forcing factors on the strength of net land carbon uptake varies considerably across models and depends strongly on whether nitrogen cycling is explicitly simulated. Models without a nitrogen cycle estimate cumulative land carbon uptake (since 1959) that is 3 times greater (93.3 ± 84.1 PgC) than global mass balance constraints (34.6 ± 41.6 PgC). Surprisingly, the greatest impacts are seen in the tropics, where coupled carbon-nitrogen cycle models estimate CO2 fertilization and climate affects that are ~60% weaker than models without a nitrogen cycle. The results highlight the importance of model structure on the inferred sensitivity of land carbon uptake to external forcing factors. The range in sensitivity across models is important for future climate projections since the differences in the processes that explain trends in net land sink strength between models with and without nitrogen dynamics can lead to very different future trajectories of atmospheric CO2 and thus climate.
