Sunday, September 7, 2014

New paper shows the Holocene Climate Optimum ~6000 years ago was much wetter and warmer

A new paper published in Quaternary Science Reviews reconstructs the Indian Summer Monsoon, a "major global climatic phenomenon," over the past 18,000 years and finds the mid-Holocene Climate Optimum [HCO] in India was characterized by 20% more precipitation in comparison to the pre-industrial period ~1850. Most global precipitation records show a slight increase of precipitation since the pre-industrial period of only a few percent, thus the study suggests the Holocene Climate Optimum was considerably wetter and warmer than present-day climate, as also has been shown by hundreds of other paleoclimate papers finding the HCO was 2-3C warmer and wetter than currently, and thus the probability of drought in a warmer climate would tend to decrease, not increase as many alarmists have claimed. 

Multiple proxies of precipitation [and temperature] over the past 18,000 years show the Early Holocene Climate Optimum [EHCO] was much wetter and warmer than at the end of the record in the preindustrial period. Warm periods in red, cold in blue, intermediate in brown. 


We provide a continuous history of the ISM in the Bengal region for the last 18 ka.
Precipitation and vegetation are reconstructed from alkane δD and δ13C composition.
Paleo-precipitation δD was driven by the amount effect reflecting monsoon intensity.
Paleo-vegetation composition was driven by precipitation variability since 15 ka BP.
Comparison with numerical model results suggests coherent regional ISM variability.


The Indian Summer Monsoon (ISM) is a major global climatic phenomenon. Long-term precipitation proxy records of the ISM, however, are often fragmented and discontinuous, impeding an estimation of the magnitude of precipitation variability from the Last Glacial to the present. To improve our understanding of past ISM variability, we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower Ganges-Brahmaputra-Meghna catchment and the Indo-Burman ranges over the last 18,000 years (18 ka). The records derive from a marine sediment core from the northern Bay of Bengal (NBoB), and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the Bengal region. The isotopic composition of terrestrial plant waxes (δD and δ13C of n-alkanes) are compared to results from an isotope-enabled general atmospheric circulation model (IsoCAM) for selected time slices (pre-industrial, mid-Holocene and Heinrich Stadial 1). Comparison of proxy and model results indicate that past changes in the δD of precipitation and plant waxes were mainly driven by the amount effect, and strongly influenced by ISM rainfall. Maximum precipitation is detected for the Early Holocene Climatic Optimum (EHCO; 10,500-6,000 years before the present), whereas minimum precipitation occurred during the Heinrich Stadial 1 (HS1; 16.9–15.4 ka BP). The IsoCAM model results support the hypothesis of a constant moisture source (i.e. the NBoB)[northern Bay of Bengal] throughout the study period. Relative to the pre-industrial period the model reconstructions show 20% more rain during the mid-Holocene (6,000 years before the present) and 20% less rain during the Heinrich Stadial 1 (HS1), respectively. A shift from C4-plant dominated ecosystems during the glacial to subsequent C3/C4-mixed ones during the interglacial took place. Vegetation changes were predominantly driven by precipitation variability, as evidenced by the significant correlation between the δD and δ13C alkane records. When compared to other records across the ISM domain, precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ISM from the Last Glacial to the present.

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