Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling

Abstract

In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were higher (1-5 degrees C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most of Eurasia and Africa, with a significant warming from similar to 3 to 5 degrees C over northwestern and southern Europe, southern Africa, and eastern Africa. In contrast, summers were 1-3 degrees C cooler than present in the Mediterranean lowlands and in a band from the eastern Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation was similar to 200-1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were similar to 10-17 degrees C lower than today in Eurasia with a more significant decrease in northern regions. In Africa, winter temperature was similar to 10-15 degrees C lower than present in the south, while it was only reduced by similar to 0-3 degrees C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency

A few prospective ideas on climate reconstruction: from a statistical single proxy approach towards a multi-proxy and dynamical approach

“…The reconstructed anomalies under low CO 2 concentration are not significantly different from the reconstructed anomalies under high CO 2 concentration. Wu et al (2007a) found a clear bias for winter temperature, i.e. about 10 • C colder under higher CO 2 , but nothing for summer temperature.…”

“…This bias reaches 3 • C in southernmost sites but not more than 1 • C in nothernmost ones, meaning that CO 2 becomes more limiting than temperature far away from the ice cap. Wu et al (2007a) have improved the method. First BIOME3 has been replaced by BIOME4 .…”

“…There is a large dispersion which can mainly be explained by a large dispersion of the elevations. Wu et al (2007a) have shown a strong altitudinal gradient of precipitation. For the modern level of CO 2 , one cannot fit a linear relationship of temperature to latitude in Africa, but yet in Europe, the relationship is negative: high latitude sites had a temperature anomaly of about −12 • C and southern sites anomalies of −10 to −5 • C. The gradient is negative while it was positive in Fig.…”

“…In fact, Wu et al (2007b) have shown that the disappearance of forest above 2000 m elevation can be explained partly by a precipitation decrease and partly by a CO 2 lowering. Wu et al (2007a) analysed the water stress variable α, which is the ratio of actual and potential evapotranspiration and is closely related to the stomatal area and the water use efficiency. They found that its maximum probability ranges within [−40, −28%] for high CO 2 and within [−40, −8%] for low CO 2 .…”

Northern hemisphere storm tracks during the last glacial maximum in the PMIP2 ocean-atmosphere coupled models: energetic study, seasonal cycle, precipitation

“…In this context, the Last Glacial Maximum (LGM) is particularly interesting since it concerns a significantly different climate compared to present and since great efforts have been put and are still on the way concerning reconstructions of oceanic conditions (CLIMAP 1981;Mix et al 2001;Kucera et al 2005) and of continental climate (e.g. Wu et al 2007). The comparison between the different models is also very instructive and helps evaluate the consistency of model results.…”