Abstract
Abstract. Earth system models show wide disagreement when simulating the climate of the continents at the Last Glacial Maximum (LGM). This disagreement may be related to a variety of factors, including model resolution and an incomplete representation of Earth system processes. To assess the importance of resolution and land–atmosphere feedbacks on the climate of Europe, we performed an iterative asynchronously coupled land–atmosphere modelling experiment that combined a global climate model, a regional climate model, and a dynamic vegetation model. The regional climate and land cover models were run at high (18 km) resolution over a domain covering the ice-free regions of Europe. Asynchronous coupling between the regional climate model and the vegetation model showed that the land–atmosphere coupling achieves quasi-equilibrium after four iterations. Modelled climate and land cover agree reasonably well with independent reconstructions based on pollen and other paleoenvironmental proxies. To assess the importance of land cover on the LGM climate of Europe, we performed a sensitivity simulation where we used LGM climate but present-day (PD) land cover. Using LGM climate and land cover leads to colder and drier summer conditions around the Alps and warmer and drier climate in southeastern Europe compared to LGM climate determined by PD land cover. This finding demonstrates that LGM land cover plays an important role in regulating the regional climate. Therefore, realistic glacial land cover estimates are needed to accurately simulate regional glacial climate states in areas with interplays between complex topography, large ice sheets, and diverse land cover, as observed in Europe.
Highlights
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 a good test bed 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)
To create the best possible estimate of European land cover for the LGM, we used an iterative asynchronous coupling design that combines CCSM4/Weather Research and Forecasting (WRF) with the LPJ-LMfire model. This coupling design consists of four steps: (i) the fully coupled CCSM4 provides atmospheric variables for the LGM to generate the first approximation of LGM land cover with LPJLMfire at a horizontal grid spacing of 1.25◦ × 0.9◦; (ii) WRF is driven by the CCSM4 with LGM conditions and the first approximation of LGM land cover created in step (i) to generate the first downscaled atmospheric variables for the LGM at 54 and 18 km grid spacing; (iii) LPJ-LMfire is run with the downscaled LGM atmospheric variables to regenerate the LGM land cover at the regional climate models (RCMs) resolutions; and (iv) same as (ii) but WRF uses the land-surface boundary conditions simulated at 54 and 18 km
Thereby, it is important that the land cover and the climate is in quasi-equilibrium (Strandberg et al, 2011) in order to discard the source of uncertainty related to an unbalanced climate system
Summary
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 a good test bed 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). Ludwig et al (2019) found that downscaling using an RCM offers a clear benefit to answer paleoclimate research questions and to improve interpretation of climate modelling and proxy reconstructions They found that the regional climate models require appropriate surface boundary conditions to properly represent the lower troposphere. We evaluate the results of our coupled model experiment using independent reconstructions of land cover and climate, and we perform a sensitivity test to better understand the importance of land cover for LGM climate in Europe by forcing the RCM with an alternative set of land-surface boundary conditions
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