COMPARING AND CONTRASTING VIEWS OF ENVIRONMENTAL CHANGE BETWEEN CLIMATE MODELS WITH POLLEN DATA FOR SOUTH AMERICA

The biomisation method is used to reconstruct vegetation from fossil pollen data across South American at 6000 and 18000 radiocarbon years before present (yr BP). Tests using modern pollen from 381 samples derived from 296 locations were able to broadly reproduce vegetation distribution as reflected in a map of potential natural vegetation, including the strong environmental gradients associated with the Andes. For the time period 6000 ± 1000 yr BP, 255 samples derived from 114 pollen records comprise the data set. The main differences between the modern and the 6000 yr BP reconstruction is a transition to biomes characteristic of a slightly drier climate. For the time period 18,000 ± 2000 yr BP, 61 samples derived from 38 pollen records comprise the data set. At 18,000 yr BP the pattern of vegetation change is more pronounced and describes a generally cool and dry environment reflected in dry forest, tropical seasonal forest and cool grass / shrub biomes being common.

Focusing our analysis at 18,000 yr BP, we compared the results from the pollen-based reconstruction against output from two global climate models scaled to South America - GENISIS-IBIS and LMD-5. To enable a comparison, model run outputs are portrayed as vegetation via the BIOME 4.2 vegetation model, this first having undergone a comparison between the modern reconstructed and the potential vegetation. Both GENISIS-IBIS and LMD-5 show the presence of a dry corridor at 18,000 yr BP, breaking the present day expansion of Amazonian tropical rain forest. GENISIS-IBIS records more extensive expansion of tropical dry forest to the north of the Amazonian region where as LMD-5 indicates a greater expansion of tropical dry forest to the south of the Amazonian region, as seen from the data of southeast Brasil. Both models indicate continued presence of tropical rain forest in the upper Amazon region, this is not apparent from the data where there is the increased presence of Andean floral elements at low altitudes under a late glacial climate. Although South America probably has the greatest concentration of palaeoecological records dated to the LGM, efforts to compare with climate models remains to be hampered by low site density.

This abstract is coauthored with Latin American Pollen Database members, Latin American Pollen Database, http://www.ngdc.noaa.gov/paleo/lapd.html.