A Large Committed Long‐Term Sink of Carbon due to Vegetation Dynamics

T A M Pugh,C Huntingford,M Lomas,C Burton,E Robertson,V Brovkin,S Sitch,C D Jones,S L Piao,P Ciais,A Arneth

doi:10.1029/2018ef000935

Abstract

AbstractThe terrestrial biosphere shows substantial inertia in its response to environmental change. Hence, assessments of transient changes in ecosystem properties to 2100 do not capture the full magnitude of the response realized once ecosystems reach an effective equilibrium with the changed environmental boundary conditions. This equilibrium state can be termed the committed state, in contrast to a transient state in which the ecosystem is in disequilibrium. The difference in ecosystem properties between the transient and committed states represents the committed change yet to be realized. Here an ensemble of dynamic global vegetation model simulations was used to assess the changes in tree cover and carbon storage for a variety of committed states, relative to a preindustrial baseline, and to attribute the drivers of uncertainty. Using a subset of simulations, the committed changes in these variables post‐2100, assuming climate stabilization, were calculated. The results show large committed changes in tree cover and carbon storage, with model disparities driven by residence time in the tropics, and residence time and productivity in the boreal. Large changes remain ongoing well beyond the end of the 21st century. In boreal ecosystems, the simulated increase in vegetation carbon storage above preindustrial levels was 20–95 Pg C at 2 K of warming, and 45–201 Pg C at 5 K, of which 38–155 Pg C was due to expansion in tree cover. Reducing the large uncertainties in long‐term commitment and rate‐of‐change of terrestrial carbon uptake will be crucial for assessments of emissions budgets consistent with limiting climate change.

Highlights

Terrestrial ecosystems respond to changes in the environment in which they exist, but they do so with a substantial lag
In the temperate and tropical regions (Figures 2g and 2j) these differences are primarily driven by the simulations from HYL, LPJ, and ORC being for potential natural vegetation, whilst HadGEM2-ES, LPJ-GUESS, and MPI-Earth System Models (ESMs) used 1850 land use at ΔT = 0 K, and omitting substantial areas of agricultural land use in temperate and tropical regions
The very strong positive response of HadCM3LC may be a result of land-atmosphere coupling within the ESM, a localized amplification of warming driven by decreases in surface albedo due to expanding forest cover (Falloon et al, 2012)

Summary

Introduction

Terrestrial ecosystems respond to changes in the environment in which they exist, but they do so with a substantial lag This fact has long been acknowledged (e.g., Smith & Shugart, 1993), leading eventually to the development of dynamic global vegetation models (DGVMs) to explore dynamic changes in global terrestrial ecosystem state and function under different environmental conditions (Cramer et al, 2001). Such DGVMs have shown success in reproducing the main features of current global vegetation (e.g., Hickler et al, 2006; Piao et al, 2013; Sitch et al, 2015, 2008; Zhu et al, 2015) and are widely used to simulate changes in. Transient ecosystem states at the point of cessation of climate change will not immediately reveal the committed ecosystem state in equilibrium with any stabilized climate

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Discussion

Conclusion