Large uncertainty in ecosystem carbon dynamics resulting from ambiguous numerical coupling of carbon and nitrogen biogeochemistry: A demonstration with the ACME land model

Abstract

Most Earth System Models (ESMs) have incorporated, or are incorporating, coupled carbon and nutrient dynamics in their land modules. We show here that different numerical implementations of nutrient controls may imply different ecological mechanisms not recognized in the original model design and can have first order impacts on predicted terrestrial carbon cycling. Using ALM, the land module of the DOE ESM (ACME), we analysed land-atmosphere CO<sub>2</sub> exchange with coupled carbon and nitrogen dynamics through three commonly-applied numerical implementations of nitrogen limitation: (1) Mineral Nitrogen based Limitation (MNL), (2) Net nitrogen Uptake based Limitation (NUL), and (3) Proportional Nitrogen flux based Limitation (PNL). By the last decade of the contemporary period (1850–2000), the three schemes resulted in very similar global terrestrial carbon and nitrogen distributions. However, under an RCP4.5 CO<sub>2</sub> concentration forcing, the implementations resulted in wildly diverging 2001–2300 land-atmosphere CO<sub>2</sub> exchanges. Quantitatively, the divergence is as large as that of the CMIP5 models by 2100 and is about 1900 Pg C (~ 890 ppmv) by 2300. Our analysis suggests that these differences result from: (1) the typically high predicted terrestrial ecosystem carbon to nitrogen ratios (i.e., nutrient constrained conditions) and (2) the schemes predict different levels of nitrogen limitation to the carbon cycle, so that the PNL scheme leads to larger nitrogen loss through aerobic and anaerobic denitrification and surface and subsurface hydrological transport. We also found significant sensitivity of model predictions to initial conditions and numerical time step size but insignificant sensitivity to the sequence of numerical oxygen and nitrogen limitation or the ordering of reaction and chemical transport. We conclude that inconsistencies in imposing nutrient limitations will very likely lead to large uncertainties in predicted carbon stocks and long-term carbon-climate feedbacks. Finally, we recommend approaches to systematically alleviate these uncertainties.

Impacts of microtopographic snow redistribution and lateral subsurface processes on hydrologic and thermal states in an Arctic polygonal ground ecosystem: a case study using ELM-3D v1.0

“…Our goals in this study include (1) analyzing the effects of spatially heterogeneous snow in polygonal ground on soil temperature and moisture and surface processes (e.g., surface energy budgets); (2) analyzing how model predictions are affected by the inclusion of lateral subsurface hydrologic and thermal processes; and (3) developing and testing a threedimensional version of the E3SM Land Model (ELM; Tang and Riley, 2016;Zhu and Riley, 2015), called ELM-3D v1.0 (hereafter ELM-3D). We then applied ELM-3D to a transect across a polygonal tundra landscape at the Barrow Environmental Observatory in Alaska.…”

Reply

“…This is in strong contrast with other branches of earth system modeling, e.g. atmospheric chemistry (Sandu, 200;Wan et al, 2013), atmospheric physics (Arakawa, 1966), hydrology (Tang et al, 2015) and marine biogeochemistry (Broekhuizen et al, 2008). These studies suggest that if the numerical implementation is not consistently C2…”

“…Response: Please see P3, L9-12 in first submission for a description of these issues. Following Tang et al (2013), we solved the transport as a separate processes, and equation 1only represents the competing processes during decomposition. In this study, all schemes assume equal competition between plants and microbes (which is however unlikely to be true as we will present elsewhere).…”