Climate change 4.2 ka BP most likely responsible for the termination of the Indus valley civilization, study finds

In this post we present selected parts of the very interesting paper titled “Climate change at the 4.2 ka BP termination of the Indus valley civilization and Holocene south Asian monsoon variability“, by M. Staubwasseret al.

Introduction Northern hemisphere records reveal millennial-scale climate changes during the Holocene, which may have been tuned to solar radiation variability [Sirocko et al., 1996; deMenocal et al., 2000; Bond et al., 2001]. The most pronounced changes in northern Africa and western Asia occurred at the mid-late Holocene transition between 5.5 and 4.0 ka cal. BP, where one or several successive shifts towards dryer conditions are well documented [Bar-Matthews et al., 1997; deMenocal et al., 2000; Gasse, 2000].

Around 4.2 ka BP the ancient civilizations of Egypt and Mesopotamia suffered from sustained drought or even collapsed entirely [Weiss et al., 1993; Hassan, 1997; Cullen et al., 2000]. At the same time the Harappan civilization of the Indus valley (Pakistan) transformed from a highly organized urban phase to a post-urban phase of smaller settlements accompanied by a southeastward migration of the population [Possehl, 1997]. However, previous climate records are inconclusive on the timing of south Asian Holocene climate change, the underlying mechanisms of such change, and whether the Harappan civilization decline was the result of climate change [Singh et al., 1990; Possehl, 1997; Enzel et al., 1999; Weiss, 2000].

The 4.2 ka BP drought Event The δ¹⁸O record of surface dwelling planktonic foraminifer Globigerinoides ruber shows little change in the mid-Holocene, but enhanced variability in the late Holocene with δ¹⁸O values between -1.7%to -2.1%. Between ∼1.0 ka BP and ∼0.4 ka BP detailed observations are somewhat compromised by intermittent absence of G. ruber in the core. The relatively variable late Holocene appears to begin with a (positive) shift to heavier δ¹⁸O at 4.2 ka BP. This shift in G. ruber δ¹⁸O may reflect any combination of cooler sea surface temperatures (SST) and heavier δ¹⁸O of the ambient water due to enhanced salinity [Bemis et al., 1998]. A comparison with the δ¹⁸O record (G. ruber) of core M5-422 from the GO shows opposing δ¹⁸O trends across the northern AS at the 4.2 ka BP event. Under the dominant monsoon forcing of AS surface temperature, opposing δ¹⁸O trends across the northern AS are difficult to explain in terms of SST changes. In addition, a late – mid-Holocene alkenone record from the northeastern AS does not indicate significant SST change between 4.5–3.5 ka BP [Doose-Rolinski et al., 2001].

Around the 4.2 ka BP shift in 63 KA δ¹⁸O the Harappan civilization in the Indus valley transformed from a highly urban urban phase to a rural post urban phase [Possehl, 1997]. In particular, cultural centers, such as the large cities of Mohenjo-Daro and Harappa, were almost completely abandoned while locations in northern India grew in population. The concordance of Harappan habitat tracking with the 4.2 ka Indus discharge event suggests a causal relationship. A possible explanation is that a reduction of the average annual rainfall over the Indus river watershed restricted Harappan farming in the Indus valley and left large city populations unsustainable.

Causes of Holocene Monsoon Change A reduction in annual rainfall over the Indus watershed must not necessarily be the result of a change in the amount of summer monsoon rain alone. Although Indus discharge is extremely biased towards the summer monsoon season [Milliman et al., 1984], the δ¹⁸O of Indus water is lighter than monsoon rainwater and significantly affected by melt water from glaciers and snowfields in the Karakoram and western Himalayas, which are largely fed by winter and spring precipitation [Mook, 1983; Wake, 1989; also The Global Precipitation Climatology Centre, available at http://www.dwd.de/research/gpcc, 1998, hereinafter referred to as GPCC, 1998]. Advanced glaciers in the Karakoram mountains as well as pollen assemblages and lake-levels in the Thar desert suggest that winter/spring rain was also higher than today during the mid Holocene and extended further southwards than at present [Singh et al., 1990; Enzel et al., 1999; Phillips et al., 2000]. It is therefore likely, that Indus discharge was then distributed more evenly over the year, and that an enhanced annual Indus discharge rate led to lighter δ¹⁸O values in the northeastern AS. At the time of the Indus discharge reduction at 4.2 ka BP, dust flux from northern Arabia and Mesopotamia also increased, and rainfall in the Eastern Mediterranean was reduced [Bar-Matthews et al., 1997; Cullen et al., 2000]. This regional pattern suggests a significance of the 4.2 ka BP drought event beyond the south Asian summer monsoon regime into the winter rain dominated extra tropics of the northern hemisphere. It is likely, that the drought induced decline of ancient civilizations in Mesopotamia and south Asia at ∼4.2 ka BP is the consequence of altered extra tropical airflow during winter and a change in monsoon seasonality.

Recently, a relationship between global climate change during the Holocene and the variation in solar radiation has been demonstrated [Neff et al., 2001; Bond et al., 2001]. Solar radiation variability is generally inferred from cosmogenic ¹⁴C production records, where enhanced production rates are attributed to lower solar radiation intensity and a reduced shielding of the earth from cosmic particles by the solar magnetic field [Stuiver and Braziunas, 1993]. The coherent quasi-periodic pacing of Indus discharge and ¹⁴C production on the multi-centennial band (630–780 years) during the late Holocene following the 4.2 ka BP event suggests a link between solar variability and south Asian climate change.

A welldated dry spell has been observed in the Thar desert around 4.7 ka cal BP [Enzel et al., 1999], which coincides with a ¹⁴C  production minimum.

Spectral analysis of the AS 63KA and Oman stalagmite δ¹⁸O records suggests a significant contribution of solar forcing to early Holocene south Asian climate variability [Neff et al., 2001; Staubwasser et al., 2002].

Subtropical Asian climate in general is relatively sensitive to changes in solar radiation. Climate modeling has demonstrated a non-uniform global response of annual surface temperatures to changes in solar radiation, with a relative warming of the east African tropics and the Tibetan Plateau occurring at high solar radiation [Cubasch et al., 1997]. This would have a direct effect on the fundamental cause of the south Asian summer monsoon, i.e. the heat gradient between the warm Tibetan Plateau and the cool southern Indian Ocean. However, a mismatch of amplitudes in the monsoon and solar radiation records suggests that any such link would be complex. On inter-annual time-scales, changes in boundary conditions due to non-solar causes result in large amplitude variations of the monsoon [Webster et al., 1998].

There is a tendency of a strong south Asian monsoon to be followed by a relatively weak one [Meehl, 1994]. This is part of the large scale tropospheric biennial oscillation, in which tropical/subtropical summer convection over east Africa, south Asia and the Indo-Pacific Ocean is coupled with extra tropical atmospheric flow over Asia in a way that modulates air flow direction, temperature and precipitation over south and central Asia in winter. It is possible that higher levels of solar energy output during the Holocene may have enhanced inter-annual summer monsoon variability, altered winter airflow over south Asia, and changed total annual precipitation over the Indus watershed.

Research-Selection for NovoScriptorium: Isidoros Aggelos