Natural variations in the strength of monsoon rainfall of the distant past are inferred from speleothems (mineral deposits in caves). The ratio of the heavy to the light isotope of oxygen in these deposits is an indirect measure of rainfall in the past. These data showed that the Indian monsoon rainfall was very low during the peak of the last ice age about 21,000 years ago. The simulation of the variation in global climate during the past 22,000 years (using a coupled ocean-atmosphere model developed by the National Centre for Atmospheric Research at Boulder, Colorado, USA) has been documented recently. In this simulation, the Indian summer monsoon rainfall increased from around 3.3 mm/day around 22,000 years ago to 6.6 mm/day 14,000 years ago and then decreased to 6 mm/day in the 20th century. This change in monsoon rainfall was attributed to changes in the amount of solar radiation incident on India during the summer monsoon. The amount of solar radiation incident over India has not changed as much (only 6% between 22,000 years ago and 10,000 years ago).
Schematic showing the evolution of Indian monsoon rainfall
We have proposed a simple model to unravel the factors that impact monsoon rainfall. Using this model we have shown for the first time the role of water vapor and clouds in amplifying the effect of solar radiation on monsoon rainfall. The concentration of carbon dioxide in the atmosphere increased from 180 ppm to 280 ppm over the last 22,000 years. This is one of the main causes of the increase in water vapour during this period. We have shown that this rise in water vapour led to the doubling of monsoon rainfall. Due to anthropogenic activities, carbon dioxide has increased by about 100 ppm in the last 100 years. It will increase further in the near future, and the diagnostic model proposed can be used to understand how monsoon rainfall will change.
The diagnostic model used in this study
Picture of our group : Chetankumar Jalihal, Prof. J. Srinivasan, Prof. Arindam Chakraborty
Jalihal, C., Srinivasan, J. & Chakraborty, A. Modulation of Indian monsoon by water vapor and cloud feedback over the past 22,000 years. Nat Commun 10, 5701 (2019). https://doi.org/10.1038/s41467-019-13754-6