Holocene peatland and ice-core data constraints on the timing and magnitude of CO₂emissions from past land use

Abstract

CO 2 emissions from preindustrial land-use change (LUC) are subject to large uncertainties. Although atmospheric CO 2 records suggest only a small land carbon (C) source since 5,000 y before present (5 kyBP), the concurrent C sink by peat buildup could mask large early LUC emissions. Here, we combine updated continuous peat C reconstructions with the land C balance inferred from double deconvolution analyses of atmospheric CO 2 and δ 13 C at different temporal scales to investigate the terrestrial C budget of the Holocene and the last millennium and constrain LUC emissions. LUC emissions are estimated with transient model simulations for diverging published scenarios of LU area change and shifting cultivation. Our results reveal a large terrestrial nonpeatland C source after the Mid-Holocene (66 ± 25 PgC at 7-5 kyBP and 115 ± 27 PgC at 5-3 kyBP). Despite high simulated per-capita CO 2 emissions from LUC in early phases of agricultural development, humans emerge as a driver with dominant global C cycle impacts only in the most recent three millennia. Sole anthropogenic causes for particular variations in the CO 2 record (∼20 ppm rise after 7 kyBP and ∼10 ppm fall between 1500 CE and 1600 CE) are not supported. This analysis puts a strong constraint on preindustrial vs. industrial-era LUC emissions and suggests that upper-end scenarios for the extent of agricultural expansion before 1850 CE are not compatible with the C budget thereafter.carbon cycle | Anthropocene | agriculture | peatland | ice core T he Earth's functioning is now so significantly affected by human activities that their impacts will leave long-lasting fingerprints in environmental records. This has motivated the definition of a new, human-dominated geological epoch-the Anthropocene (1, 2). However, human impacts on the Earth's environmental history throughout the preindustrial Holocene are less clear. Although a causal link between anthropogenic greenhouse gas (GHG) emissions and the rapid rise in their concentrations since industrialization is unequivocal, CO 2 and CH 4 concentrations already exhibited a particular, albeit slow and not synchronous increase millennia earlier. This may give rise to the possibility of far-reaching global environmental change induced by early agriculture (3). Similarly, variations in atmospheric CO 2 during the last millennium have been linked to agricultural expansion and collapse (2). However, any association between a particular change in the GHG records and a coincidental continental or global socio-economic change hinges on a causal relationship between the two and on the plausibility of bottom-up estimates to imply a sufficient impact on atmospheric GHG concentrations.In all cases where such a connection has been claimed, causality is strongly debated (4-8). For example, the link between the particular CO 2 increase after 7 kyBP and the Neolithic Revolution is motivated by a multitude of local-scale paleoecological and archaeological archives documenting an early onset of a human influence on the landscape wit...

Do we need to include anthropogenic land-use and land-cover changes in paleoclimate simulations?

“…The idea that greenhouse gas emissions associated with Neolithic LULC changes were sufficiently large to offset climate cooling (the overdueglaciation hypothesis: Ruddiman 2003) has been challenged on multiple grounds (e.g. EPICA Community Members 2004; Stocker et al 2017) but a LULC impact on climate in more recent millennia appears more plausible. Model studies have shown that prescribed Holocene LULC changes had detectable impacts on regional temperature and precipitation and even had a significant effect beyond the major agricultural regions (e.g.…”

Reply to review of J.-F. Exbrayat

“…However, the partitioning of this land-atmosphere flux to effects from humaninduced land-use and land-cover change (LULCC) and the transient change in the residual terrestrial sink remains highly debated (Schimel et al, 2015). It is estimated that approximately a third of the cumulative anthropogenic CO 2 emissions in the industrial period stem from the effects of LULCC Brovkin et al, 2013;Houghton and Nassikas, 2017;McGuire et al, 2001;Mahowald et al, 2017;Pongratz and Caldeira, 2012;Sitch et al, 2015;Strassmann et al, 2008;Stocker et al, 2017Stocker et al, , 2014Peng et al, 2017). A better understanding of the mechanisms of the historical terrestrial carbon cycle is vital for more accurate future projections of the global carbon cycle and climate.…”

“…The assimilation of observations should be an integral part of model development. Various approaches to incorporate constraining data exist, such as variational approaches minimizing a cost function using the adjoint of the model Kaminski et al, 2013) or the use of ensemble Kalman filters (Lorenc, 2003;Gerber and Joos, 2013;Stöckli et al, 2011;Ma et al, 2017). A drawback of these methods is that the sampling process is dependent on the choice of the cost function, the design of which is not trivial when assimilating multiple observations simultaneously.…”

Peatland area and carbon over the past 21 000 years – a global process based model investigation

“…The treatment of acrotelm and catotelm as single carbon pools, and the absence of strong disturbances such as peat fires, constitute limits on the comparability of the model results to peat core carbon profiles. This simplified representation however has been shown to reproduce peatland area and carbon accumulation well within the observational constraints (Wania et al, 2009a;Spahni et al, 2013;Stocker et al, 2014Stocker et al, , 2017) while using a minimal set of free paramters. Our efficient representation allows for long transient paleo simulations and sensitivity studies as we present them here.…”