labs_title

Uncertainty in the response of terrestrial carbon sink to environmental drivers undermines carbon-climate feedback predictions

D.N. Huntzinger, A.M. Michalak, C. Schwalm, P. Ciais, A.W. King, Y. Fang, K. Schaefer, Y. Wei, R.B. Cook, J.B. Fisher, D. Hayes, M. Huang, A. Ito, A.K. Jain, H. Lei, C. Lu, F. Maignan, J. Mao, N. Parazoo, S. Peng, B. Poulter, D. Ricciuto, X. Shi, H. Tian, W. Wang, N. Zeng and F. Zhao

This paper examines the relative roles of changing climate, rising atmospheric CO2, land cover land use change, and nitrogen deposition on the growth of the terrestrial carbon sink over the 20th century. Whereas some recent studies have argued for the primary importance of one of these factors, we find that there is in fact substantial disagreement across models about the importance of each driver. We also find evidence of an implicit tradeoff in models between sensitivity to climate and atmospheric CO2. This work highlights current limitations to the predictability of global carbon dynamics under changing climate conditions, and demonstrates the benefits of a probabilistic multi-model approach to understanding carbon cycle drivers and feedbacks.


Figure: Attribution of net global carbon land sink. (a) Attribution of the cumulative net land sink (red circles) by model and for the two sub-ensemble means (models with and without a coupled carbon-nitrogen cycle) from 1959 to 2010 compared to the cumulative sink (black dashed line) and associated uncertainty (grey shaded region) estimated from the Global Carbon Project. The cumulative sink is decomposed into the influence of time varying climate (blue), land-cover change history (brown), atmospheric CO2 (orange), and nitrogen deposition (green). (b) Attribution of the cumulative net land carbon sink (red circles) over period 1959 to 2010 by region for ensemble mean of models with (left) and without (right) a coupled carbon-nitrogen cycle. Negative values indicate carbon loss from the terrestrial biosphere, where positive values indicate a carbon gain by the terrestrial biosphere.

Abstract

Terrestrial ecosystems play a vital role in regulating the accumulation of carbon (C) in the atmosphere. Understanding the factors controlling land C uptake is critical for reducing uncertainties in projections of future climate. The relative importance of changing climate, rising atmospheric CO2, and other factors, however, remains unclear despite decades of research. Here, we use an ensemble of land models to show that models disagree on the primary driver of cumulative C uptake for 85% of vegetated land area. Disagreement is largest in model sensitivity to rising atmospheric CO2 which shows almost twice the variability in cumulative land uptake since 1901 (1 s.d. of 212.8 PgC vs. 138.5 PgC, respectively). We find that variability in CO2 and temperature sensitivity is attributable, in part, to their compensatory effects on C uptake, whereby comparable estimates of C uptake can arise by invoking different sensitivities to key environmental conditions. Conversely, divergent estimates of C uptake can occur despite being based on the same environmental sensitivities. Together, these findings imply an important limitation to the predictability of C cycling and climate under unprecedented environmental conditions. We suggest that the carbon modeling community prioritize a probabilistic multi-model approach to generate more robust C cycle projections.

Huntzinger, D.N., A.M. Michalak, C. Schwalm, P. Ciais, A.W. King, Y. Fang, K. Schaefer, Y. Wei, R.B. Cook, J.B. Fisher, D. Hayes, M. Huang, A. Ito, A. K. Jain, H. Lei, C. Lu, F. Maignan, J. Mao, N. Parazoo, S. Peng, B. Poulter, D. Ricciuto, X. Shi, H. Tian, W. Wang, N. Zeng, F. Zhao (2017) "Uncertainty in the response of terrestrial carbon sink to environmental drivers undermines carbon-climate feedback predictions", Scientific Reports, 7 (4765), doi:10.1038/s41598-017-03818-2.