A new multivariable benchmark for Last Glacial Maximum climate simulations

Abstract

Abstract. We present a new global reconstruction of seasonal climates at the Last Glacial Maximum (LGM, 21 000 years BP) made using 3-D variational data assimilation with pollen-based site reconstructions of six climate variables and the ensemble average of the PMIP3—CMIP5 simulations as a prior (initial estimate of LGM climate). We assume that the correlation matrix of the uncertainties in the prior is both spatially and temporally Gaussian, in order to produce a climate reconstruction that is smoothed both from month to month and from grid cell to grid cell. The pollen-based reconstructions include mean annual temperature (MAT), mean temperature of the coldest month (MTCO), mean temperature of the warmest month (MTWA), growing season warmth as measured by growing degree days above a baseline of 5 ∘C (GDD5), mean annual precipitation (MAP), and a moisture index (MI), which is the ratio of MAP to mean annual potential evapotranspiration. Different variables are reconstructed at different sites, but our approach both preserves seasonal relationships and allows a more complete set of seasonal climate variables to be derived at each location. We further account for the ecophysiological effects of low atmospheric carbon dioxide concentration on vegetation in making reconstructions of MAP and MI. This adjustment results in the reconstruction of wetter climates than might otherwise be inferred from the vegetation composition. Finally, by comparing the uncertainty contribution to the final reconstruction, we provide confidence intervals on these reconstructions and delimit geographical regions for which the palaeodata provide no information to constrain the climate reconstructions. The new reconstructions will provide a benchmark created using clear and defined mathematical procedures that can be used for evaluation of the PMIP4–CMIP6 entry-card LGM simulations and are available at https://doi.org/10.17864/1947.244 (Cleator et al., 2020b).

The role of land cover on the climate of glacial Europe

“…Still, a few locations show considerable differences between the pollen-based and model-based climate anomalies, in line with similar findings mentioned in earlier studies (e.g. ; Beghin et al, 2016;Ludwig et al, 2016;Cleator et al, 2020). These differences could be associated with shortcomings within the RCM and/or uncertainties in the proxy reconstructions (Bartlein et al, 2011;Ludwig et al, 2019;Cleator et al, 2020).…”

“…; Beghin et al, 2016;Ludwig et al, 2016;Cleator et al, 2020). These differences could be associated with shortcomings within the RCM and/or uncertainties in the proxy reconstructions (Bartlein et al, 2011;Ludwig et al, 2019;Cleator et al, 2020). Kageyama et al (2006) suggested that terrestrial paleoclimate proxies may be more sensitive to climatic extremes than to the climatological mean state, which could lead to part of the discrepancies between pollen-based reconstructions and the model simulations.…”

“…The Last Glacial Maximum (LGM, 21 ka; Yokoyama et al, 2000;Clark et al, 2009;Van Meerbeeck et al, 2009) is a period of focus for Earth system modelling because it represents a time when boundary conditions were very different from the present and is therefore a good testbed of models' ability to faithfully reproduce a range of climate states (e.g., Mix et al, 2001;Janská et al, 2017;Cleator et al, 2020). In Europe, the LGM is also an interesting period in human history, because small groups of highly mobile Upper Paleolithic hunter-gatherers persisted in the face of inhospitable climate, while Neanderthals disappeared (Finlayson, 2004;Finlayson et al, 2006;Finlayson, 2008;Burke et al, 2014;Maier et al, 2016;Baena Preysler et al, 2019).…”

Seasonal temperature and moisture changes in interior semi‐arid Spain from the last interglacial to the Late Holocene

“…Much of the discussion about the lack of modern analogues has focused on interpretation of assemblages of species that are not found together today (Overpeck et al, 1985;Jackson and Williams, 2004;Williams and Shuman, 2008). However, one important non-analogue situation that is ignored in all previous statistical reconstructions is the impact of [CO 2 ] different from today on plant assemblages, with important effects on apparent climatic moisture inferred from pollen data (Prentice et al, 2017;Cleator et al, 2019). Taking the impact of [CO 2 ] into account in our reconstructions reduces the variability of MI during glacial intervals.…”

The role of land cover in the climate of glacial Europe

“…Studies have demonstrated that a realistic representation of surface conditions is essential for the accuracy of the simulated regional climate as they play a crucial role in regulating water and energy fluxes between the land surface and the atmosphere (e.g. Crowley and Baum, 1997;Kjellström et al, 2010;Strandberg et al, 2011Strandberg et al, , 2014Gómez-Navarro et al, 2015;Jia et al, 2019a;Ludwig et al, 2017).…”