Monsoon rains. Image: McKay Savage (Flickr, used under a CC BY 3.0)
Over the last few millennia there has been a fundamental shift of the Indian Monsoon, from a steady humid monsoon that favoured lush vegetation to extended periods of drought. A new and controversial study led by researchers at the Woods Hole Oceanographic Institution (WHOI) is trying to understand the implications of the monsoon’s response to climate change.
Understanding the Monsoon
The Indian subcontinent sustains over a billion people, yet lies at the same latitude as the Sahara Desert. Without a monsoon, most of India would be dry and uninhabitable and so the ability to predict the timing and amount of the next year’s rainy season is vital, yet our knowledge of its past variability remains incomplete.
One key to this understanding lies in the core monsoon zone (CMZ) – a region in the central part of India that is a very sensitive indicator of the monsoon throughout the India peninsula.
“If you know what’s happening there, you know more or less what’s happening in the rest of India,” said Camilo Ponton, a student in the MIT-WHOI Joint Program in Oceanography and lead author of the study recently published in Geophysical Research Letters, Holocene Aridification of India. “Our biggest problem has been a lack of evidence from this region to extend the short, existing records.”
Map of the Indian peninsula, showing where the monsoon winds blow (white arrows) and how the salinity (white lines) is lower in Bay of Bengal due to monsoon rain over the Bay and rivers draining into the it. (The black arrow represents non-monsoon wind.) The study's sediment core (red dot) was extracted from a “sweet spot” in the Bay of Bengal where the Godavari River drains the central Indian peninsula and over which monsoon winds carry the most precipitation. (Courtesy of C. Ponton and L. Giosan)
Reconstructing 10,000 years of climate
The study made use of a sediment core collected by the National Gas Hydrate Program of India in 2006. Sailing around India aboard the drilling vessel JOIDES Resolution for several months, Giosan enlisted colleagues from India and US to help with the project. Extracted from a “sweet spot” in the Bay of Bengal where the Godavari River drains the central Indian peninsula and over which monsoon winds carry most of the precipitation, the core has provided the basis for a 10,000-year reconstruction of climate in the Indian peninsula’s CMZ .
Every centimetre of sediment from this river contains 10 to 20 years’ worth of information, allowing for a high temporal resolution.
When put together, the research tells the story of growing aridity in India, enables valuable insights into the impact of the monsoon on past cultures, and points scientists toward a way to model future monsoons.
Camilo Ponton, examines a sediment core from the Bay of Bengal with geologist Liviu Giosan. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)
To assemble the 10,000-year record, the team looked to both what the land and the ocean could tell them. Contained within the sediment core’s layers are microscopic compounds from the trees, grasses, and shrubs that lived in the region and remnants of plankton fossils from the ocean.
A simple story
“The geochemical analyses of the leaf waxes tell a simple story,” said Giosan. “About 10,000 years ago to about 4500 ago, the Godavari River drained mostly terrain that had humidity-loving plants. Stepwise changes starting at around 4,000 years ago and again after 1,700 years ago changed the flora toward aridity-adapted plants. That tells us that central India – the core monsoon zone – became drier.”
Analyses of the plankton fossils support the story reconstructed from plant remains and reveal a record of unprecedented spikes and troughs in the Bay of Bengal’s salinity – becoming saltier during drought periods and fresher when water from the monsoon filled the river and rained into the Bay. Similar drought periods have been documented in shorter records from tree rings and cave stalagmites within India lending further support to this interpretation.
Climate, cultivation and culture
Archaeobotanist Dorian Fuller of the Institute of Archaeology, University College London was interested in what this information reflected, and noted the new paleo-climatic information showed a shift towards more arid conditions around 4,000 years ago which corresponded with the expansion of agricultural populations.
Arid-adapted food production is an old cultural tradition in the region, with cultivation of drought-tolerant millets and soil-restoring bean species. Importantly there may be lessons to learn from this, as these drought-tolerant agricultural traditions have been eroding over the past century, with shift towards more water and chemical intensive forms of modern agriculture.
Mohenjo Daro. Image: {link url="http://www.flickr.com/photos/benny_lin/" target="_blank" rel="nofollow"}Benny Linn{/link} (Flickr, used under a {link url="http://creativecommons.org/licenses/by-nd/3.0/" target="_blank" rel="nofollow"}CC BY-ND 3.0{/link})
The geological record and archaeological evidence may now tell a story of the possible fate of India’s earliest civilizations. Cultural changes occurred across the Indian subcontinent as the climate became more arid after ~4,000 years. In the already dry Indus basin, the urban Harappan civilization failed to adapt to even harsher conditions and slowly collapsed. But aridity favoured an increase in sophistication in the central and south India where tropical forest decreased in extent and people began to settle and carry out more agriculture. Human resourcefulness proved crucial in the rapid proliferation of rain-collecting water tanks across the Indian peninsula, just as the long series of droughts settled in over the last 1,700 years.
What can this record tell us about future Indian monsoons?
According to Ponton, “How the monsoon will behave in the future is highly controversial. Our research provides clues for modelling and that could help determine whether the monsoon will increase or decrease with global warming.”
The study found that the type of monsoon and its droughts are a function of the Northern Hemisphere’s incoming solar radiation – or “insolation.” Every year, the band of heavy rain known as the Inter-Tropical Convergence Zone, or ITCZ, moves north over India.
“We found that when the Asian continent is least heated by the sun, the northward movement of the rain appears to hesitate between the Equator and Asia, bringing less rain to the north,” said Giosan. “The fact that long droughts have not occurred over the last 100 years or so, as humans started to heat up the planet, but did occur earlier, suggest that we changed the entire monsoon game, and may have inadvertently made it more stable!”
Source: Woods Hole Oceanographic Institution
More information:
- Ponton, C., L. Giosan, T. I. Eglinton, D. Q. Fuller, J. E. Johnson, P. Kumar, and T. S. Collett (2012), Holocene aridification of India, Geophys. Res. Lett., 39, L03704, doi:10.1029/2011GL050722.
- Liviu Giosan
- Camilo Ponton
- Woods Hole Oceanographic Institution]
- Harappan Culture: 1,200 illustrated pages by leading scholars around the world of the ancient Indus Civilization.