The summer monsoon is known for its torrential rains. It usually occurs between the months of April and September. Warm, moist air from the southwest Indian Ocean drifts toward India, Sri Lanka, Bangladesh, and Myanmar as winter draws to a close. These places experience a humid environment and heavy rainfall during the summer monsoon.
The South Asian monsoon, also known as the Indian Summer Monsoon (ISM), is critical for 40 percent of the world’s population’s food security and socioeconomic well-being. Monsoon rainfall patterns have been linked to the emergence and fall of civilizations in the Indian subcontinent throughout history.
Researchers are becoming increasingly concerned that global warming may jeopardize the monsoon system’s stability, but reliable forecasting has been impeded by a lack of long-term climate data on the Indian subcontinent.
A team of researchers from the Max Planck Institute for the Science of Human History, Kiel University, and the Alfred Wegener Institute of the Helmholtz Centre for Polar and Marine Research published a new study in the Proceedings of the National Academy of Sciences that aims to improve climate predictions by reconstructing Indian Summer Monsoon rainfall changes over the past 130,000 years.
The study shows for the first time that the Indian Summer Monsoon was reduced by continuous high sea surface temperatures in the equatorial and tropical Indian Ocean during the Last Interglacial, implying that present sea-level rises could exacerbate droughts in South Asia.
The economy of the region suffers when the summer monsoon is late or poor. Fewer people are able to raise their own food, and major agribusinesses are unable to market their products. Food must be imported by governments. Electricity prices rise, restricting development to major corporations and rich individuals.
Changes in the hydrological cycle will affect agricultural land, natural ecosystems, and consequently the livelihoods of billions of people. We, therefore, need to improve our understanding of the control mechanisms of summer monsoon rainfall to better predict weather extremes such as droughts and floods and devise adaptation measures. Time is of the essence, especially if ocean warming continues at the rate it is.Dr. Yiming Wang
Sedimentary biomarkers in paleoclimate archives: a window into the past
The severity of the Indian Summer Monsoon is typically attributed to solar radiation, with increased solar radiation increasing humidity, wind circulation, and, eventually, precipitation.
Increased monsoon intensity should have resulted from higher levels of solar radiation during the Last Interglacial, however, this impact has never been proven using paleo-proxy evidence.
The researchers used a 10-meter-long marine sediment core obtained from the northern Bay of Bengal, roughly 200 kilometers south of the mouth of the Ganges-Brahmaputra-Meghna rivers, to reconstruct previous Indian Summer Monsoon rainfall.
Researchers were able to track changes in rainfall during the planet’s last two warmer climate states, the Last Interglacial (130,000-115,000 years ago) and the current warm period, the Holocene, which began 11,600 years ago, by analyzing stable hydrogen and carbon isotopes in leaf wax biomarkers preserved in the sediment.
Despite the fact that solar insolation was higher during the Last Interglacial, isotopic analysis of a leaf wax biomarker revealed that the Indian Summer Monsoon was less powerful than it was during the Holocene.
“This unexpected finding not only contrasts with paleoclimate model simulations,” says lead author Dr. Yiming Wang, paleo-climatologist at the Max Planck Institute for the Science of Human History, “but also challenges common assumptions that incoming solar insolation is the biggest factor in monsoon variability in a warm climate state.”
Sea surface temperature plays a dominant role
The researchers compared available reconstructions of past sea surface temperature from the Indian Ocean to find that the equatorial and tropical regions were 1.5-2.5°C warmer during the Last Interglacial period than during the Holocene, which helped them identify the main driver of the monsoon’s rainfall during warm climate states.
Furthermore, the researchers employ paleoclimate model simulations to show that as the surface temperature of the Indian Ocean rose in the past, monsoon rainfall decreased on land while increasing in the water above the Bay of Bengal.
“Our work strongly suggests that sea surface temperature plays a dominant role in shaping the Indian Summer Monsoon’s variability in South Asia,” says Dr. Wang, “and that higher surface temperatures in the Indian Ocean during the Last Interglacial period could have dampened the ISM intensity.”
An urgent need for understanding monsoon response in warm climate
According to the team’s findings, Indian Summer Monsoon failures are anticipated to increase as sea surface temperatures in the Indian Ocean rise. The extent to which sea surface temperature influences monsoon intensity in other tropical locations is yet unknown.
“The obvious discrepancy between our data and prevailing climate model simulations underscores the importance of hydroclimate proxy records for understanding the range and rate of climate change in the past,” says Prof. Ralph Schneider, senior author of the study, paleoclimate researcher at the Institute of Geosciences and the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research at Kiel University.
“Our results show that, in addition to the effect of solar radiation on continents, the effect of ocean warming on precipitation intensity needs to be re-evaluated in climate models.”
“Changes in the hydrological cycle will affect agricultural land, natural ecosystems, and consequently the livelihoods of billions of people,” Dr. Wang emphasizes. “We, therefore, need to improve our understanding of the control mechanisms of summer monsoon rainfall to better predict weather extremes such as droughts and floods and devise adaptation measures. Time is of the essence, especially if ocean warming continues at the rate it is.”