Quantifying the influence of the terrestrial biosphere on glacial–interglacial climate dynamics

Abstract

Abstract. The terrestrial biosphere is thought to be a key component in the climatic variability seen in the palaeorecord. It has a direct impact on surface temperature through changes in surface albedo and evapotranspiration (so-called biogeophysical effects) and, in addition, has an important indirect effect through changes in vegetation and soil carbon storage (biogeochemical effects) and hence modulates the concentrations of greenhouse gases in the atmosphere. The biogeochemical and biogeophysical effects generally have opposite signs, meaning that the terrestrial biosphere could potentially have played only a very minor role in the dynamics of the glacial-interglacial cycles of the late Quaternary. Here we use a fully coupled dynamic atmosphereocean-vegetation general circulation model (GCM) to generate a set of 62 equilibrium simulations spanning the last 120 kyr. The analysis of these simulations elucidates the relative importance of the biogeophysical versus biogeochemical terrestrial biosphere interactions with climate. We find that the biogeophysical effects of vegetation account for up to an additional −0.91 • C global mean cooling, with regional cooling as large as −5 • C, but with considerable variability across the glacial-interglacial cycle. By comparison, while opposite in sign, our model estimates of the biogeochemical impacts are substantially smaller in magnitude. Offline simulations show a maximum of +0.33 • C warming due to an increase of 25 ppm above our (pre-industrial) baseline atmospheric CO 2 mixing ratio. In contrast to shorter (century) timescale projections of future terrestrial biosphere response where direct and indirect responses may at times cancel out, we find that the biogeophysical effects consistently and strongly dominate the biogeochemical effect over the inter-glacial cycle. On average across the period, the terrestrial biosphere has a −0.26 • C effect on temperature, with −0.58 • C at the Last Glacial Maximum. Depending on assumptions made about the destination of terrestrial carbon under ice sheets and where sea level has changed, the average terrestrial biosphere contribution over the last 120 kyr could be as much as −50 • C and −0.83 • C at the Last Glacial Maximum.

Deglacial carbon cycle changes observed in a compilation of 127 benthic <i>δ</i><sup>13</sup>C time series (20–6 ka)

“…The volume-weighted global δ 13 C stack increases from 19 to 6 ka and likely reflects terrestrial biosphere growth, in agreement with model simulations (Kaplan et al, 2002;Joos et al, 2004;Davies-Barnard et al, 2017). To constrain the timing of the end of terrestrial biosphere expansion, future work should focus on extending the global stack through the Late Holocene.…”

“…Simulations from HadCM3 estimated that 45 %-70 % of terrestrial biosphere expansion occurred between 18-14 ka (Davies-Barnard et al, 2017). Dramatically different trends were observed from 14-6 ka in simulations with different assumptions about carbon storage under glacial ice sheets and on continental shelves.…”

“…More recently, the potential effects of changes in poorly constrained carbon reservoirs (e.g., beneath ice sheets and on continental shelves) were evaluated using deglacial simulations of biogeophysical and land carbon changes from the HadCM3 general circulation model (GCM). The model simulated a rapid increase in terrestrial carbon storage from 20-14 ka, different responses between 14-11 ka depending on the model scenario, and then steady, gradual change from 11-4 ka (Davies-Barnard et al, 2017).…”

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

“…Dissolved inorganic carbon and semilabile organic carbon (DIC, DOC), the corresponding isotopic forms (DI 13 C, DO 13 C, DI 14 C, DO 14 C), and alkalinity (Alk), phosphate (PO 4 ), oxygen (O 2 ), iron (Fe), silica (Si), and an ideal age tracer are explicitly transported by advection, diffusion, and convection. New production of organic matter is limited to the euphotic zone in the uppermost 75 m and calculated as a function of light availability, temperature, and phosphate and iron availability (Doney et al, 2006). Bacteria and plankton are not explicitly represented.…”

A global assessment of the drivers of threatened terrestrial species richness

“…A measure of temporal climate stability since the last interglacial period (present to 125,000 years ago) was derived from palaeo-climatic data made available by the Bristol Research Initiative for the Global Environment (BRIDGE, http://www.bridge.bris.ac.uk/). Details of the calculation of the model used to derive the paleo-climate data are provided by 62,63 . Data on precipitation and temperature were sampled at 4,000 year intervals.…”