‘Over millions of years, mountains, ice sheets and oceans shaped the Indian monsoon’

Steven Clemensis associate professor of earth, environmental and planetary sciences at Brown University. Speaking to Srijana Mitra Das at Times Evoke, he discusses the factors that make the Indian monsoon:
What is the core of your current research?
My work right now focuses on the East China Sea — it looks at the East Asian monsoon where we are seeking to reconstruct changes in the isotopic composition of the rainfall and the runoff from river valleys into the East China Sea. This is similar to the work we’ve done in the Indian region, trying to differentiate changes in the isotopic composition of rainfall from how much rain was occurring locally.

How do you study a transient phenomenon like the monsoon over the millions of years you survey — what sources and traces does this involve?
We mainly study sediments which accumulate on the sea floor — these integrate vast amounts of time. For example, the sedimentation rates we typically look for are in the order of 8 to 20 centimetres per thousand years. A given sample can integrate several hundred years of time. So, although the monsoon is a transient feature on the inter-annual and decadal timescale, we are looking at large-scale changes over long periods of time through these deep-sea sediments.

What are some natural factors which shape the Indian monsoon?
Many different aspects drive the monsoon which has inter-annual and multi-decadal changes and whose timescales range all the way to tectonic variability, associated with the growth and decay of mountain belts. At the interannual timescale, the El Nino Southern Oscillation (ENSO) Index is a big factor.
In the multi-decadal timescale, changes in ocean temperatures influence evaporation from sea water. In the tectonic timescale, the rise of the Himalayan and Tibetan plateau are big players in the evolution of the Indian monsoon because they determine largely where the locus of heating is in summer, which drives low pressure over India and pulls in moisture from the ocean.

Among other factors, when Earth’s ice volume is low and solar radiation is very high, the monsoon grows strong. So, there’s also a linkage to high-latitude ice volume — when this is high, that tends to weaken the monsoon. When this is low, it tends to strengthen it. The combination of very strong solar radiation as a function of change in the orbital geometry of Earth around the sun and low ice volume are when you’ll find very strong monsoons — this happened 1,25,000 years and 11,000 years ago.


Can you tell us about your recent research on how the monsoon responds to rising carbon dioxide (CO2) concentrations?
The modern monsoon in India has decreased in strength over the past many decades — this is largely attributed to aerosol forcing. But over the much long er term, like a 100-year-timescale, that factor is going to be in competition with CO2 forcing because the latter is a global scale phenomenon. It causes a large-scale global increase in atmospheric temperature which drives more evaporation from ocean waters — hence, the winds falling from the oceans onto the continent during the summer monsoon will be carrying more moisture. So, there will be a competition between the monsoon-weakening aerosol forcing over India and this longer-term increase in global temperatures associated with CO2 which will increase monsoon precipitation. At this point in time, most models predict the CO2 increase factor will win out — the Indian monsoon will become stronger in terms of rainfall.

HOW IT PEAKS: The Himalayas and the Tibetan Plateau influence heat and rain. Picture courtesy: iStock
Does studying the monsoon’s past help us learn more about its future, particularly in an era of climatic change?
Yes, absolutely. The IPCC models seeking to predict future changes in monsoons are fairly confident the impacts of the greenhouse gas forcing can be larger than the impacts of aerosols in the long run — so, part of our work involves going out to research to what extent the Indian monsoon is sensitive to this greenhouse gas forcing. A wonderful aspect to this search is that we know what the greenhouse gas forcing was over the last 8,00,000 years — we understand that as it’s been recorded by ice cores. If we reconstruct the monsoon and statistically compare the runoff from the monsoon to the greenhouse gas forcing, we can derive those linkages quantitatively. That shows, for instance, how there were times when the greenhouse gas forcings were high — and these were followed by strong monsoons.