Aim Insight into global biome responses to climatic and other environmental changes is essential to address key questions about past and future impacts of such changes. By simulating global biome patterns 140 ka to present we aimed to address important questions about biome changes during this interval.
Methods Using the LPJ-GUESS dynamic global vegetation model, we made 89 simulations driven using ice-core atmospheric CO2 concentrations, Earth’s obliquity, and outputs from a pre-industrial and 88 palaeoclimate experiments run using HadCM3. Experiments were run for 81 time slices between 1 ka and 140 ka, seven ‘hosing’ experiments also being run, using a 1 Sv freshwater flux to the North Atlantic, for time slices corresponding to Heinrich Events H0 – H7. Using a rule-based approach, based on carbon mass and leaf area index of the LPJ-GUESS plant functional types, the biome was inferred for each grid cell. Biomes were mapped, and the extent and total vegetation biomass of each biome, and total global vegetation biomass, estimated.
Results Substantial changes in biome extents and locations were found on all vegetated continents. Although the largest-magnitude changes were in Eurasia, important changes were seen in tropical latitudes and the Southern Hemisphere. Total global extent of most biomes varied on multi-millennial (orbital) time scales, although some (e.g. Tropical Raingreen Forest) responded principally to the ca. 100 kyr glacial–interglacial cycle and others (e.g. Temperate Broad-leaved Evergreen Forest) mainly to the ca. 20 kyr precession cycle. Many also responded to millennial contrasts between stadial (‘hosing’) and interstadial climates, with some (e.g. Tropical Evergreen Forest) showing stronger responses than to the multi-millennial changes.
Main conclusions No two time slices had identical biome patterns. Even equivalent Holocene and last interglacial time slices, and the last and penultimate glacial maxima, showed important differences. Only a small proportion of global land area experienced no biome change since 140 ka; many places experienced multiple biome changes. These modelling experiments provided little evidence for long-term biome stability.
he CMass and LAI files are derived from outputs produced by LPJ-GUESS simulations. These simulations were driven using palaeoclimates derived from a series of GCM runs, along with the associated atmospheric carbon dioxide concentration and orbital obliquity. Full details and relevant citations are given in Allen et al. (2020). The LPJ-GUESS output files were opened (by JRMA) using Microsoft Excel, a header line added and, in the case of the CMass files, columns added giving the grid cell areas and ice-free land fractions, these being copied from the relevant columns of the Ice-free_land_fraction_89_time-slices file. A second worksheet was then added to the CMass files and values for grid cell CMass calculated from the LPJ-GUESS CMass per unit area values output and the product of the grid cell area and ice-free land fraction values. The second worksheet was then saved in comma-delimited (.csv) format.
The Biome_assignments_V1.1_89_time-slices.csv file is derived from the primary output file generated by the FORTRAN program BiomiseLPJ_V1.1. This program was written by BH and the source code is provided in the Supplementary Information to Allen et al. (2020). The program output file was opened (by BH) using Microsoft Excel, a header line added, and the resulting worksheet saved in comma-delimited (.csv) format.
The Biome_extents_V1.1_89_time-slices and PFT_CMass_by_biome_V1.1_89_time-slices are also derived from output files generated by BiomiseLPJ_V1.1. Minor editing was performed (by BH) using WordPad in order consistently to separate the blocks of data in the files and to ensure that all columns had labels in the header rows, thus rendering the files easier to open using software such as R.
The primary data underlying the Ascii grid files are the ETOPO1 1-minute topography and bathymetry dataset, and the ICE-6G 1-degree ice fraction data files, both datasets being downloaded from their respective online locations. In addition, eustatic sea levels prior to 34 ka were estimated using a multiple regression model relating eustatic sea levels 0-34 ka to ocean delta-18O values and a relative sea-level curve, both of which are available extending back to 140 ka and beyond. Fractional ice cover for time slices prior to the Last Glacial Maximum (LGM) was taken to be that of the post-LGM time slice with the closest matching eustatic sea level. These various input sources were processed using FORTRAN programs written by BH for the purpose to generate the 6-minute datasets for land, ocean, ice cover and ice-fraction. Further details are provided in the readme_first.pdf file and in Appendix S2 of the Supplementary Information to Allen et al. (2020).
The data in the Ice-free_land_fraction_89_time-slices file was generated from the 1-minute ETOPO1 topography and bathymetry dataset and the ICE-6G data using the same approach as was used for the 6-minute land and ice-fraction datasets, implemented in a FORTRAN program written by BH. In this case the file coverage is only those half-degree grid cells on the current land areas and shelf areas exposed at the LGM, whereas the 6-minute files extend over the entire global surface.