Articles | Volume 13, issue 17
Biogeosciences, 13, 5121–5137, 2016
Biogeosciences, 13, 5121–5137, 2016

Research article 14 Sep 2016

Research article | 14 Sep 2016

Role of CO2, climate and land use in regulating the seasonal amplitude increase of carbon fluxes in terrestrial ecosystems: a multimodel analysis

Fang Zhao1,2, Ning Zeng1,3, Ghassem Asrar4, Pierre Friedlingstein5, Akihiko Ito7, Atul Jain8, Eugenia Kalnay1, Etsushi Kato9, Charles D. Koven10, Ben Poulter11, Rashid Rafique4, Stephen Sitch6, Shijie Shu8, Beni Stocker12, Nicolas Viovy13, Andy Wiltshire14, and Sonke Zaehle15 Fang Zhao et al.
  • 1Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA
  • 2Potsdam Institute for Climate Impact Research, Telegraphenberg, 14412 Potsdam, Germany
  • 3Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA
  • 4Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD 20742, USA
  • 5University of Exeter, College of Engineering Mathematics and Physical Sciences, Exeter, EX4 4QF, UK
  • 6University of Exeter, College of Life and Environmental Sciences, Exeter, EX4 4QF, UK
  • 7Center for Global Environmental Research, National Institute for Environmental Studies, 305-0053 Tsukuba, Japan
  • 8Department of Atmospheric Sciences, University of Illinois, Urbana, IL 61801, USA
  • 9Global Environment Program Research & Development Division, the Institute of Applied Energy (IAE), 105-0003 Tokyo, Japan
  • 10Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
  • 11Institute on Ecosystems and Department of Ecology, Montana State University, Bozeman, MT 59717, USA
  • 12Climate and Environmental Physics, Physics Institute, University of Bern, 3012 Bern, Switzerland
  • 13Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, 91191 Gif-sur-Yvette, France
  • 14Hadley Centre, Met Office, Exeter, EX1 3PB, UK
  • 15Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, P.O. Box 10 01 64, 07701 Jena, Germany

Abstract. We examined the net terrestrial carbon flux to the atmosphere (FTA) simulated by nine models from the TRENDY dynamic global vegetation model project for its seasonal cycle and amplitude trend during 1961–2012. While some models exhibit similar phase and amplitude compared to atmospheric inversions, with spring drawdown and autumn rebound, others tend to rebound early in summer. The model ensemble mean underestimates the magnitude of the seasonal cycle by 40 % compared to atmospheric inversions. Global FTA amplitude increase (19 ± 8 %) and its decadal variability from the model ensemble are generally consistent with constraints from surface atmosphere observations. However, models disagree on attribution of this long-term amplitude increase, with factorial experiments attributing 83 ± 56 %, −3 ± 74 and 20 ± 30 % to rising CO2, climate change and land use/cover change, respectively. Seven out of the nine models suggest that CO2 fertilization is the strongest control – with the notable exception of VEGAS, which attributes approximately equally to the three factors. Generally, all models display an enhanced seasonality over the boreal region in response to high-latitude warming, but a negative climate contribution from part of the Northern Hemisphere temperate region, and the net result is a divergence over climate change effect. Six of the nine models show that land use/cover change amplifies the seasonal cycle of global FTA: some are due to forest regrowth, while others are caused by crop expansion or agricultural intensification, as revealed by their divergent spatial patterns. We also discovered a moderate cross-model correlation between FTA amplitude increase and increase in land carbon sink (R2 =  0.61). Our results suggest that models can show similar results in some benchmarks with different underlying mechanisms; therefore, the spatial traits of CO2 fertilization, climate change and land use/cover changes are crucial in determining the right mechanisms in seasonal carbon cycle change as well as mean sink change.

Short summary
The increasing seasonality of atmospheric CO2 is strongly linked with enhanced land vegetation activities in the last 5 decades, for which the importance of increasing CO2, climate and land use/cover change was evaluated in single model studies (Zeng et al., 2014; Forkel et al., 2016). Here we examine the relative importance of these factors in multiple models. Our results highlight models can show similar results in some benchmarks with different underlying regional dynamics.
Final-revised paper