Environmental Cluedo

It's like environmental Cluedo; we have the victim - Harappan Society, we have the location - the Indus Valley, we just need to find out who did it...The Holocene has played host to some of the most iconic civilisations seen on Earth, evidence of which litters the globe, but many have found their demise at the hands of Mother Nature (McMichael, 2012; Figure 1). Today I want to look at human civilisation in the South Asian monsoonal regions through the Holocene, with a focus on the Harappan Civilisation.

Comparison of historical and climate events since 8 ka (Clift and Plumb, 2008:199)

The earliest evidence of human settlement in the Indian subcontinent is all found in western Pakistan and eastern Afghanistan and dates from 7000 BCE. These settlements, known as the Mehrgarh Culture, were Neolithic in their nature and there is evidence of farming of wheat and barley alongside herding of livestock (Clift and Plumb, 2008:207). The bulk of the evidence, or distinct lack of, points towards a hunter-gatherer society during the Early Holocene (Clift and Plumb, 2008:200) until the Mid Holocene (~7000 - 6000 BCE), when the climate became much drier and human societies formed as a result of the need to work together to find food (deMenocal, 2001; Clift and Plumb, 2008:200). 

It is from the Mehrgarh Culture that the Harappan Civilisation emerged around 3300 BCE during a period of increasing aridity (Madella and Fuller, 2005); by 2600 BCE it had become a complex civilisation with grid format cities, written script, water supply systems and the world's first urban sanitation systems (Clift and Plumb, 2008:227). Along with well developed agricultural systems, complex trade is also evident in Harappan society, an example of which are the ruins of Lothan Port (Figure 2). Indeed, Harappan artefacts such as carnelian, pearls, lapis-lazuli and woods have been found as far as the Akkadian empire of Mesopotamia (MacDonald, 2009). 
  

The Harappan engineers must have possessed great knowledge of tides and hydrology as they built this structure (assumed to be a port) on the Sabarmati river and included both inlet and outlet flows and wooden gate systems to maintain the water levels (Image).

Then an apparent threshold was breached around 2200 BCE...the 'Urban Harappan' period, where the population lived in organised cities, moved to the 'Post-Urban Harappan' period, where the population moved to smaller settlements and migrated south-eastward (Clift and Plumb, 2008:208). Madella and Fuller. (2006) have also suggested an increase in Harappan rural settlements in the wetter foothills of the Himalayas and also the Ganges watershed. It was also at this time that the ancient civilisations of Egypt and Mesopotamia came near to or did collapse (Straubwasser et al., 2003) so what was the cause? 

A common suggestion is an intense drought associated with a weakened monsoon. Straubwasser et al. (2003) explored this possibility, with the use of oxygen isotope analysis (δ¹⁸O) of planktonic foraminifera from cores around the Indus delta mouth. They linked a change in the δ¹⁸O to increased salinity in the ocean around the Indus delta, the cause of which was identified as reduced flow from the Indus river linked to a rapid weakening of the SW monsoon. Solar variability (from ¹⁴C records) has been proposed as a driver for this (Straubwasser et al., 2003) and earlier events (Neff et al., 2001), as has changes in the Pacific Ocean in the form of ENSO events (MacDonald, 2009) but both still have uncertainties attached. 

Another interesting possibility involves the disappearance of rivers...the Saraswati River in particular. This river ran sub-parallel to the Indus and drained the western Himalayas, transporting water to the Arabian Sea. The river is mentioned in the Sanskrit Hindu holy text - Rig Veda - 72 times and is noted to be of a similar size to the Indus itself, and yet this river does not exist today. Of the 2600 Harappan sites discovered thus far, 2000 of them were located on on the palaeo-channel of the Saraswati River (Clift and Plumb, 2008:210). Regardless of what drove this change, the results of the drainage piracy/drainage capture of the Saraswati River would have impacted the populations it sustained dramatically, although further study is again needed to calibrate this to the historic events (Figure 3). 

Satellite map of the Indus River Valley with the modern day Indus River and the palaeo-channel of the Saraswati River. The hexagons are Harappan sites and the triangles are early farming sites of the Mehrgarh Culture (Clift and Plumb, 2008:206)

Undoubtedly the change in climate would have had a detrimental affect upon the ability of the Harappan Civilisation to sustain themselves, but how much impetus we place on environmental determinism is open to debate. To definitively say that climate caused the collapse is impossible as it is merely an assumption of the proxy data, but I tend to air towards climate being a major influence with its effects dependent upon societal susceptibility and exacerbated by a mix of other factors. Two big questions remain...firstly, how will future changes in the monsoon impact upon relations between various nations and secondly, do you think we have solved the clues and won the game....or is the culprit still out there?