Weird Science | A research paper aims to show the connection between monsoon and invasions on the Indian Subcontinent…
…invasions from Central Asian regions coincided with weak precipitation in their regions, indicating that climate has always played a big role in geopolitics
To think that the monsoon played a role in the course of history might seem strange, but isn’t a leap of the imagination. Researchers at the Indian Institute of Tropical Meteorology (IITM), Pune, under the Union earth sciences ministry and the Department of Remote Sensing, Birla Institute of Technology, Ranchi, have been studying the waxing and waning of the Indian summer monsoon and the impact it has had on invasions in the Indian subcontinent. By connecting climatic rhythms to historical conquests, their research highlights how weather influenced warfare, prosperity, and ultimately, the fate of empires.
The researchers studied climatic changes alongside the socio-political landscape of the Indian subcontinent from the 6th century (Before the Common Era) BCE to the 16th century CE (Common Era or AD).
The team, led by Dr Naveen Gandhi, scientist, IITM, assessed paleoclimate records spanning 2,500 years (using natural records such as ice cores, tree rings, fossil pollen, and sediment layers), to map out a timeline of climatic conditions that corresponded with periods of significant invasions across South and Central Asia.
“We found a striking correlation between periods of favourable monsoon, agricultural prosperity, and the timings of these invasions. The favourable climate of the Indian region attracted invaders from central Asia, who sought to capitalise on its wealth, while their own (Central Asian) harsh climate limited their economic opportunities,” Gandhi said.
So far, so logical.
The thing to note is the impact of climatic conditions on historical military campaigns often from places where the monsoon was not robust. The monsoon, crucial for agriculture production and food security, historically determined the prosperity of regions within the Indian subcontinent — and the attraction this held for invaders.
“In the pre-industrial era, the economy revolved around the agricultural sector, which was heavily influenced by climatic conditions. The Indian subcontinent flourished economically thanks to its favourable climate, which supported robust agricultural production. This prosperity led to the accumulation of wealth through trade routes connecting the region with the rest of the world. In contrast, central Asian regions experienced adverse climatic conditions, resulting in limited agricultural output and trade opportunities,” said Gandhi.
Out of the 11 major invaders between the 6th century BCE and the 16th century CE, nine invasion events occurred during good Indian monsoon events. “For instance, the initial Achaemenid invasion led by Cyrus the Great in the late 6th century BCE coincided with a drought in Central Asia and a strong monsoon period in India. Our findings suggest that invaders might have been motivated by the relative prosperity in India due to favourable climatic conditions,” Gandhi said.
During Muhammad ibn al-Qasim's invasion of the Sindh region in 712 CE, the abundant monsoon contributed to the prosperity and appeal of the region, making it a strategic target for the Umayyad Caliphate's expansion. Similarly, Mahmud of Ghazni’s multiple raids between 1001 and 1026 CE (turn of the first millennium) exploited the economic stability and wealth generated by strong monsoons in rich temple towns.
The Mongol invasions led by Genghis Khan in the 13th century CE also occurred during periods of favourable monsoon, which supported the sustenance and movement of large armies across the region. Favourable Indian monsoon conditions in 1220–1270 CE coincided with weak precipitation in Central Asia, and this period involved a series of battles fought between Mohammad of Ghori and Prithviraj Chauhan III. Babur’s invasion in the 16th century CE coincided with excellent monsoon conditions in India.
“Only Alexander the Great and Timur (Turco-Mongol conqueror) invaded India during unfavourable monsoonal conditions. Their primary objective was global conquest rather than immediate economic or strategic interests. Therefore, these two invasions may be considered different from the rest,” added Gandhi.
The trade routes, which facilitated the exchange of goods such as silk, spices, textiles, precious stones, and metals between diverse regions—from East Asia through Central Asia to Europe and the Middle East, helped spread the news of prosperity. Routes like the Silk Route and the Spice Route played a critical role in facilitating not just commerce and the movement of armies, but also the spread of ideas and information.
"These routes were lifelines that connected disparate empires and facilitated invasions when climatic conditions made the regions economically prosperous and thus ripe for conquest," added Dr P Mukhopadhyay, scientist, IITM, and coauthor.
“Information about the prosperity of the Indian subcontinent likely spread to central Asian regions via these trade routes, enticing them to invade in search of wealth. The primary motivation behind these invasions was to acquire wealth rather than territorial expansion, with exceptions such as Alexander the Great and Timur,” Gandhi added.
To investigate the potential linkage between climatic conditions and historical invasions on the Indian subcontinent, the research team analysed records from various locations across South and Central Asia.
The Indian subcontinent, as defined in the study, includes present-day countries such as India, Pakistan, Bangladesh, Nepal, Bhutan, Sri Lanka, the Maldives, and the eastern part of Afghanistan. This area is bordered by the Himalayas to the north, the Hindu Kush to the west, and the Indo-Burma Range to the east, providing a natural barrier that historically shaped the socio-political and cultural exchanges through select mountain passes and along the coastlines. Central Asia, mentioned in the study, covers areas including modern-day Turkmenistan, Uzbekistan, Kazakhstan, and parts of southern Siberia and western China.
This region is influenced by its distinct climatic patterns influenced by the Siberian High and the Westerlies, with a landscape that has historically supported the nomadic lifestyles of its inhabitants, influencing their interactions with neighbouring civilisations through trade and invasion.
Methods to study past climates
The authors used proxy data, including tree ring indices (provide annual records that can be precisely dated, and variations in ring width are indicators of climatic conditions, especially precipitation) and speleothem records (secondary mineral deposits formed in caves, such as stalactites and stalagmites, are valuable for climate studies), which provided insights into past monsoon behaviours and precipitation patterns. Further, the isotopic composition of speleothems, particularly oxygen isotopes, can indicate changes in rainfall and vegetation, which are influenced by monsoon patterns. The study references speleothem data from several caves across India and Central Asia, covering extensive periods. For instance, the Dandak Cave records span from 625 to 2007 CE, and the Kadapa Cave records extend from 214 to 2014 CE.
“We chose a tree ring record from 1484 CE to 2003 CE from Kerala, which provides annual resolution records of monsoon precipitation. Beyond 1484 CE, we used oxygen isotope records (2014 CE to 1293 BCE) from cave deposits (speleothems) in central and peninsular India. Similarly, we employed oxygen isotope records covering the last 10,000 years from cave deposits in central Asia,” Dr Gandhi said.
“Past climate data were selected based on the criteria that the record should represent variations in Indian monsoon precipitation, and the time resolution should be very high to resolve short historical events lasting 5-10 years,” said Gandhi.
This extensive data allowed the researchers to construct a detailed timeline of climatic conditions corresponding to major historical invasions noted in historical texts.
Beyond new insights into the historical dynamics of the Indian subcontinent, the research offers a lens through which to view current and future geopolitical strategies in the context of climatic change. "Understanding how climate influenced historical events helps us appreciate the complex interplay between environment and human actions," said Mukhopadhyay. "It's a reminder of the profound impact climate can have on the fate of civilisations."
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