Low terrestrial carbon storage at the Last Glacial Maximum: constraints from multi-proxy data

Aurich Jeltsch-Thömmes,Fortunat Joos,Gianna Battaglia,Olivier Cartapanis,Samuel L Jaccard

doi:10.5194/cp-15-849-2019

Aurich Jeltsch-Thömmes, Fortunat Joos + Show 3 more

Open Access

https://doi.org/10.5194/cp-15-849-2019

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Abstract

Abstract. Past changes in the inventory of carbon stored in vegetation and soils remain uncertain. Earlier studies inferred the increase in the land carbon inventory (Δland) between the Last Glacial Maximum (LGM) and the preindustrial period (PI) based on marine and atmospheric stable carbon isotope reconstructions, with recent estimates yielding 300–400 GtC. Surprisingly, however, earlier studies considered a mass balance for the ocean–atmosphere–land biosphere system only. Notably, these studies neglect carbon exchange with marine sediments, weathering–burial flux imbalances, and the influence of the transient deglacial reorganization on the isotopic budgets. We show this simplification to significantly reduce Δland in simulations using the Bern3D Earth System Model of Intermediate Complexity v.2.0s. We constrain Δland to ∼850 GtC (median estimate; 450 to 1250 GtC ±1SD) by using reconstructed changes in atmospheric δ13C, marine δ13C, deep Pacific carbonate ion concentration, and atmospheric CO2 as observational targets in a Monte Carlo ensemble with half a million members. It is highly unlikely that the land carbon inventory was larger at LGM than PI. Sensitivities of the target variables to changes in individual deglacial carbon cycle processes are established from transient factorial simulations with the Bern3D model. These are used in the Monte Carlo ensemble and provide forcing–response relationships for future model–model and model–data comparisons. Our study demonstrates the importance of ocean–sediment interactions and burial as well as weathering fluxes involving marine organic matter to explain deglacial change and suggests a major upward revision of earlier isotope-based estimates of Δland.

Highlights

Atmospheric CO2 varied between about 180 and 300 ppm over the past 800 000 years (Neftel et al, 1982; Siegenthaler et al, 2005; Lüthi et al, 2008; Marcott et al, 2014; Bereiter et al, 2015)
We used a Bayesian approach to constrain the change in the land biosphere carbon inventory between the Last Glacial Maximum and the preindustrial period
Nutrient, alkalinity, and carbon isotope exchange with the lithosphere and ocean sediments is explicitly taken into account, in contrast to earlier studies that applied the isotopic budget of 13C to estimate deglacial change in land biosphere carbon

Summary

Introduction

Atmospheric CO2 varied between about 180 and 300 ppm over the past 800 000 years (Neftel et al, 1982; Siegenthaler et al, 2005; Lüthi et al, 2008; Marcott et al, 2014; Bereiter et al, 2015) These CO2 variations were tightly coupled to glacial–interglacial climate change (Petit et al, 1999; Siegenthaler et al, 2005) and amplified orbitally driven climate variations (Jansen et al, 2007). To make progress in this research area requires the combination of multiple lines of evidence whereby proxy reconstructions are compared to quantitative model analyses This will help to constrain underlying processes and to quantify their contribution

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