A study by an international research team including scientists from The Open University (OU) has had its findings published in the journal Science Advances. It found that increases in monsoon rainfall over the past million years were linked with increases in atmospheric CO2, continental ice volume and the import of moisture from the southern hemisphere, which suggests stronger rains in the future as CO2 levels rise and continental ice volume decline.
The group from the OU led by Dr Pallavi Anand, an ocean biogeochemist in the School of Environment Earth and Ecosystem Sciences in the STEM faculty, have been working to better understand the major drivers of Indian Monsoon rainfall. In November 2014, the research team sailed aboard the research vessel JOIDES Resolution to the Bay of Bengal, off the coast of India, to recover sediment core samples from beneath the sea floor. Those core samples preserve a record of monsoon activity spanning millions of years.
The findings show that:
- fluctuations in atmospheric carbon dioxide (CO2), continental ice volume, and moisture import from the southern hemisphere Indian Ocean over the past 900,000 years were associated with changes in the intensity of monsoon rainfall
- precipitation maxima occur well after northern hemisphere summer insolation at both orbital bands (precession minima and obliquity maxima), indirectly suggesting that other drivers are at play
- consistent timing and strong link between monsoon precipitation and large-scale winds in cross-equatorial moisture transport
Dr Pallavi Anand said:
“We show that monsoon rainfall intensity is sensitive to CO2 levels and amount of polar ice volume, and both of these internal climate drivers are changing due to anthropogenic activities which will impact monsoon rainfall patterns in future. OU researchers measured chemical make-up of plankton shells, preserved in the deep-sea mud, to infer changes in salinity in response to changing rainfall. The river derived rainfall were brought to the study site which changes the saltiness of the ocean water – making seawater fresher when the monsoon rainfall and runoff increases.”
Capturing the history of the monsoon
The rainwater produced by the Indian Summer Monsoon feeds the Indian river system which eventually drains off into the Bay of Bengal. This river runoff, capturing continental rainfall, creates a layer of dilute seawater in the bay that rides atop the denser, more saline water below. The surface seawater is a habitat for microorganisms such as planktonic foraminifera, which are made of calcium carbonate (CaCO3) shell made by the organism using dissolved seawater components. When the creatures die, the shells sink to the bottom and become preserved along with continental sediments brought by runoff.
Layer by layer these sediments capture history of monsoon signal. By taking core samples of sediment and analysing the chemical composition in those fossils, scientists can derive the salinity of the water in which the creatures lived. That salinity signal can be used as an indicator of changing rainfall amounts over time.
The researchers found that periods of more intense monsoon winds and rainfall tended to follow peaks in atmospheric CO2 and low points in global ice volume. Cyclical changes in Earth’s orbit that alter the amount of sunlight each hemisphere receives played a role in monsoon intensity as well, but on their own could not explain monsoon variability. Taken together, the findings suggest that monsoons are indeed sensitive to CO2-related warming, which validates climate model predictions of strengthening monsoons in relation to higher CO2.