Past Monsoonal Changes and Anthropogenic Impact in the Thar.

The Thar Desert, also known as the Great Indian Desert or the ‘place of death’ by locals, occupies around 2.34 million km² of the northwest of the Indian subcontinent and the adjoining area of Pakistan (Laity, 2008). The desert has a number of striking features such as its large covering of shrubs, bushes and trees, frequent scatterings of civilization, and intensive farming of cattle, goats and camels. The high desert population, 100 per km² in Rajasthan, coupled with its association with the southwest monsoon, has placed anthropogenic and climatic pressures on the landscape (Goudie, 2002).

Southwest monsoon variability has played a critical role in shaping the characteristic features of the Thar Desert. Fluvial, aeolian and lacustrine deposits found in the Thar region reflect the sensitivity of the environment and illustrate the influence climate has had on geomorphic processes and systems (Kar et al, 2001). The southwest monsoon is important in determining the amount of rainfall that the Thar receives. In turn the amount of rainfall plays a critical role in the spatial and temporal nature of desert margin shift. Substantially regulated by changes in solar insolation and the temperature gradient found between the land and sea - precipitation, evaporation and wind changes can be identified from the sedimentary record. Juyal et al (2006) used sedimentology and luminescence dating to suggest the existence of a river system as a result of enhanced monsoon conditions 130-120 ka. Since then changes in the characteristics of the sediment indicate fluctuating monsoonal strength at 120-100 ka, 100-70 ka, 70-60 ka and 60-30 ka with periods of intense and reduced precipitation. Aeolian sedimentation post 30 ka suggests much drier conditions.

The fluctuation between aeolian and humid conditions provides great scope from congruent changes in vegetation. Arid and hyper-arid desert conditions provide a biologically stressful environment and species are forced to make adaptations or die. Water is usually the most important factor governing plant growth. Hence the monsoon is important in determining the type of vegetation found in the Thar through time. Various studies using values of δ¹³C of organic carbon in lakes have shown the relative abundance of C₃ versus C₄ vegetation during periods of increased precipitation or strengthening of the monsoon (Enzel et al, 1999).

High desert population has permitted extensive anthropogenic influence on the environment. Yadav and Rajamani (2004) have studied the geochemistry of aerosols in the Thar and stressed their importance in terms of climate change, nutrient dynamics and environmental health. Anthropogenically produced aerosols have been shown to have the potential of contributing to ongoing desertification. Wada et al (1995) also link human activities to desertification in the Thar suggesting that overgrazing by large mammals can significantly alter natural plant succession and ultimately reduced vegetative cover. As Charney’s hypothesis suggests, this can trigger extensive feedback mechanisms. Subdivision and fragmentation of land holdings have also been identified as a major cause of desertification in the Thar Desert. Shrinking land holdings have induced constant cultivation resulting in soil infertility and reduced long term productive potential (Ram et al, 1999).

References

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell