Vedic Period During last glacial period (110,000-11,000 years before present or BP) it appears that even Lower Himalaya was covered with ice and snow. The glacial period ended around 11,000 BP and in the following 1,000 years the near surface temperature of the northern hemisphere rose from 11°C to 15°C. From glacier/ice/snow melts rivers flowed through northwest India – one of them being the Sarasvati. Originating near Shimla (Himachal Pradesh) Sarasvati flowed through present day Punjab, Rajasthan and Gujarat (parallel to the India and Pakistan border) and debouched on to the Arabian Sea. The monsoon current was very weak, sometimes absent, but the moderate climate, plain land, fertile soil and ample good quality water provided an amicable environment for the Vedic Civilization to flourish on the banks of Sarasvati. With Yamuna, Satluj, Beas, Ravi and Chenab as its tributaries, the Sarasvati was a perennial river. People living on its bank practiced agriculture and reared domestic animals (cow, buffalo, horse etc.), spoke Sanskrit and knew critical details of nature from the atom to the universe. Observing nature, describing its importance and usage in lucid shlokas, memorising them and transferring the generated and accumulated knowledge orally to the next generation were important components of education in the guru-protégé lineage. Sixteen shlokas in the Rig Veda provide description of large fluctuations in the behaviour of the Vedic Sarasvati: heavy floods, low flow, to no flow and even changes in its course are documented. Due to persisting temperatures of about 15°C the ice/snow cover was exhausted in the lower and middle Himalayas and the Sarasvati turned into a dry channel around 8,000 BP. People migrated towards the east along the banks of River Ganga and towards the south – in due course the Vedic Civilization that existed for more than 1,000 years waned to an end.
Ramayana Period: River Ganga rose to prominence after the collapse of the Vedic Civilization. The temperatures now ranged between 15°C and 15.5°C – the monsoon circulation and the rainfall being almost similar to the present day. Except north-western parts, the entire Indian subcontinent was inhabited including Sri Lanka. It appears landscape and drainage pattern of northern India started developing through pluvial (rainfall) processes during this period. Archaeological evidences suggest that rice was an important crop in the Gangetic Plains. In feudal system, the society was divided into three tiers – the mighty king and his administrators and army living in cities and towns at the topmost; the gurus and the protégés associated with gurukul system of education living in hermitages in the forest at the second rung; and farmers, traders, artisans and labourers living in rural areas at the third. Sanskrit was the language of rulers and the elite while workers and labourers spoke varied dialects. Besides imparting Vedic knowledge, the hermitages were also training warriors. The Ramayana Culture flourished in the Indo Gangetic Plains for ~1,000 years. Around 7,000 BP the temperature started rising above 15.5°C. Intense monsoon rainfall shifted westward due to a westward shift in two important components of the summer monsoon circulation i.e. surface ‘heat low’ and the Tibetan Anticyclone in the upper troposphere. The area between Delhi and Kandahar (Afghanistan) experienced much greater rainfall compared to the present day and the Gangetic Plains frequent droughts. People from Ganga belt started migrating towards west and the Ramayana Culture started waning.
Mahabharata Period During 7,000-6,000 BP the temperatures were greater than 16oC. The atmosphere was hottest compared to other parts of the Holocene period and monsoon was intense due to the heating of the Afro-Eurasian dry province and the Tibetan-Himalayan highlands. The northwestern Indian subcontinent experienced a much wetter monsoon compared to the present day with perennial rivers and thick vegetation proliferating in the region. Two narrations in the saga of Mahabharata talks about heavy rain in the area. The first is when Krishna is born, and the other is when the people of Gokul denied worship to Lord Indra, who in a fit of anger started pouring rain; Krishna intervened to saved the rain ravaged people by lifting the Govardhan Hill and sheltering them beneath it. There was re-emergence of the Sarasvati – but this time it was a rain fed river. The Mahabharata Culture flourished – spreading over Delhi and Kandahar. A pilgrimage by Balarama (the elder brother of Krishna) from Dwarka to Mathura is mentioned in the texts – along the rivers Sarasvati and Yamuna, providing the last description of the Sarasvati available in mythological scriptures. The river might have changed its course westward and eastward due to west-east fluctuations of the monsoon rainfall associated with rise and fall of the northern hemisphere temperatures, and the effect of climatic changes on the river flows might have altered the drainage pattern of the Vedic Sarasvati.
Sanskrit was still the language of the upper classes and commoners spoke dialects. The society was prosperous and the gurukul education expanded to train warriors on a much larger scale. In the Great Mahabharata War, which broke out around 6,000 BP nearly 2 million soldiers were killed in merely 18 days. Around the same time the temperature started falling sharply and was below 15°C. Frequent monsoon failures and droughts escalated the miseries of the war torn people. The Sarasvati became a dry channel. This condition prevailed for about 1,000 years, and people started migrating eastward and southward with the Mahabharata culture coming to an end. The Great War coupled with dry climatic conditions shattered the human society and it took nearly 2,000 years for the society to resume normalcy around 3,500 BP.
Indus Valley Civilization During cool and dry epoch of 1,000 years (6,000-5,000 BP) there was large migration of people from middle east and central Asia in search of food and water into the Indus Valley. Around 4,500 BP the northern hemisphere temperatures rose to around 16°C and northwestern Indian subcontinent started experiencing ample monsoonal rainfall. The Sarasvati started flowing a third time but as an ephemeral (seasonal) river. This emergence might have altered the drainage pattern of the Sarasvati during the Mahabharata Period. Hence, the LANDSAT imageries can only decipher the drainage pattern of the Sarasvati during the Indus Valley Period. People lived in the Indus Valley during 5,000 to 4,000 BP, and excelled in education, agriculture, art, architecture, trade, commerce and urban planning. Around 4,000 BP the temperatures decreased to less than 15°C and monsoon rains started failing frequently. Northwest India became a dry province and a large majority of the Indus Valley population lost their lives to hunger and epidemics, and the rest migrated to the east and south.
- 1813-1869 (dry): India experienced frequent droughts and famines due to the failure of monsoon caused by the little ice age. Routine instrumental meteorological observations started at Chennai (Madras). The most important event of this period was the start of railways in response to the demand to transport food grains from one part of the country to another.
- 1870-1894 (wet): Control of all the meteorological observations across the country was taken
over by the central government
and India meteorological Department (IMD) started functioning from 1875 onwards for all weather and climate related purposes. Attempts were made for the first time to foreshadow the performance of the summer monsoon to predict rainfall.
- 1895-1941 (dry): The country experienced frequent droughts and expansion of Thar Desert towards east and southeast was reported. A large number of palaces, forts, monuments was built under the food for work programme. Large scale western style education started in the country. Operational long range forecast of summer monsoon rainfall was started by the IMD under the leadership of Sir G T Walker.
- 1942-1964 (wet): Construction of large reservoirs, dams and
canal network started. India’s population grew at a faster rate during this period.
- 1965-1994 (fluctuating): Numerous measures were adopted by the federal government under the banner of ‘Green Revolution’ to meet the food grains demand of the growing population. Agricultural production increased sharply due to management of water resources and adoption of advanced technology for agricultural purposes. Effective systems were developed for large scale storage of the food grains.
- 1995-2008 (dry): Despite the rising temperature in the northern hemisphere the performance of summer monsoon rainfall remained weaker. It is a paradoxical situation as the Asian summer monsoon is a thermally driven circulation system. Investigations reveal that from 1965 onwards temperature of the lower troposphere (surface to 4 km) is rising, 4 to 6 km is thermally stable but above 6 km the atmosphere is cooling. During warmer upper troposphere (6-16 km layer) the Tibetan anticyclone, one of the important components of the Asian summer monsoon circulation, lies over western Tibet-Afghanistan-Iran sector (40oN latitude). But due to recent cooling of the upper atmosphere the Tibetan anticyclone is weaker and frequently located south of its normal position; roughly over northwest India-Pakistan-Afghanistan sector (30oN latitude).
Post-Buddha Period During 2,600 to 2,300 BP the northern hemisphere atmosphere was cooler (~14oC) and the Indian monsoon drier and frequent and severe droughts occurred over the Gangetic Plains. The longest drought of reportedly a 100 years, occurred around 400 BC. The first rainguage was invented by Chanakya during this period. Kautilya’s ‘Arthashastra’ provides an elaborate description of rainfall distribution across the country, particularly over forests of Bihar, west coast and Himalayan region and over Ujjain and Maharashtra. Due to frequent droughts, rainfall was somewhat lesser compared to the present. This period was the witness to the rise and fall of five outstanding historical personalities – Gautama Buddha, Alexander the Great, Chandragupta Maurya, Vishnugupta Chanakya and Ashoka the Great. Buddhism emerged as one of the greatest religions.
Around 2,000 BP the temperature was greater than 15.5oC. The monsoon was wetter over the Indian subcontinent. Some of the most important historical events took place during this period such as rise and fall of the mighty Roman Empire and the origin of the Christianity. Around 1,600 BP the northern hemisphere temperature was below 15oC. During this period Prophet Mohammad propounded Islam. During AD 800-1300 the temperature rose beyond 15oC. The period is widely known as medieval warm period. India experienced a wetter monsoon and a wealthy society. Mohammad Bin Kassim invaded India during this period and Mohammad Gazanavi attacked India 17 times during 1001-1026 AD.
The Little Ice Age (LIA) The LIA started around 1250 AD. India was invaded by Portuguese, French, Mongols, Turks, Arab, Afghans and British and was ruled by foreign rulers for more than 700 years. India witnessed a mixing of a large number of European, African and Asian cultures. After Sanskrit and Tamil the third literary language Urdu was born from a mixture of Sanskrit, Persian, Arabic, Turkish and number of dialects spoken by the people in and around Delhi. Literary development of Bengali started with arrival of British during 16-17th centuries. The development of Malayalam, Telugu, Kannada, Marathi, Gujarati, Oriya, Assamese, Punjabi and Kashmiri took place during 17-19th centuries. Development of Hindi as a literature started with the fall of the Mughal Empire and the ascension of British supremacy to assume full administrative control of India in the middle of 19th century.
India started becoming wealthier state with good rainfall associated with rising trend in the northern hemisphere temperature since 1850. Instrumental rainfall data for India is available from 1813 onwards.
The northern hemisphere temperature of 15°C is critical for the Indian summer monsoon – temperature less than that would lead to drier conditions over the Indo-Gangetic Plains (IGP). Temperature between 15°C and 15.5°C would result in a wetter monsoon (similar to the present day) over the IGP. Temperature greater than 15.5°C would induce a westward shift in monsoon rainfall activities with relatively drier condition prevailing over the Gangetic Plains. This relationship occurs on centuries and millennia scales rather annual and decadal scales. During cooler atmospheric conditions and drier monsoon – spirituality, religion, philosophy and literature dominated the psyche of the Indian masses while materialism, power, wealth, politics, economics, trade and commerce dominated during warmer and thus wetter monsoon. From Vedic Period (10,000 BP) to Indus Valley Period (4,000 BP) the warmer and cooler epochs were of longer duration. However, the epochs were of shorter duration with less intensity in the later period. This suggests that the recent rising trend in atmospheric temperature is not likely to continue for a longer period.