A new study of monsoon rainfall in the Indian subcontinent over the past million years provides vital clues about how they will respond to future climate change.
The study, published in Science Advances, found that periodic changes in monsoon intensity over the past 900,000 years were linked to fluctuations in atmospheric carbon dioxide (CO2), the volume of continental ice and the import of moisture from India. circumference of the Southern Hemisphere.
The findings reinforce climate model predictions that rising carbon dioxide and global temperatures will lead to stronger monsoon seasons.
“We have shown that over the past 900,000 years, elevated carbon dioxide levels along with concomitant changes in ice volume and moisture transport have been associated with more intense monsoon rains,” says Stephen Clemens, Professor of Geosciences at De Brown University in the US. States, lead author of the study.
“This tells us that CO2 levels and associated warming have been drivers of monsoon intensity in the past, which supports what models have predicted about future monsoons: that precipitation will increase as CO2 increases and global temperature rises,” he adds.
According to Clemens, the monsoons in South Asia are the strongest expression of the watery climate on Earth, and in some places it rains several meters each summer. Rain is vital to the region’s agriculture and economy, but it can also cause flooding and disrupt crops in particularly heavy years. With monsoons playing an important role in the lives of nearly 1.4 billion people, understanding how climate change affects them is critical.
For several years, Clemens has worked with an international team of researchers to better understand the main factors driving monsoon activity. In November 2014, a research team sailed aboard the research vessel JOIDES Resolution to the Bay of Bengal, off the coast of India, to retrieve sediment samples from the sea floor. These samples keep a record of monsoon activity spanning millions of years.
The rainwater produced by the monsoon winds every summer ends up from the Indian subcontinent to the Bay of Bengal. Runoff creates a layer of dilute seawater in the bay, which lies over the denser and saltier waters below.
Surface waters are home to microorganisms called planktonic foraminifera, which use nutrients from the water to build their shells, which are made up of calcium carbonate (CaCO3). When the creatures die, the shells sink to the bottom and are trapped in the sediment.
By taking sediment samples and analyzing oxygen isotopes from those fossils, scientists can guess the salinity of the waters in which the creatures lived. This salinity signal can be used as an indicator of changes in precipitation amounts over time.
Data from foraminifera are supplemented with data from other samples. River run-off in the bay brings sediment from the mainland, providing another indication of the intensity of rainfall.
The isotopic composition of plant matter carbon transported to the ocean and buried in sediments provides another rainfall signal that reflects changes in vegetation type. The hydrogen isotopic composition of plant leaf wax varies with different precipitation environments, and this signature can also be reconstructed from sediment cores.
“The idea is that we can reconstruct precipitation over time using these indices and then look at other paleoclimate data to see what might be the main driver of monsoon activity,” Clemens explains.
He adds: “This helps us answer important questions about the factors that drive the monsoons – he adds -. Are they primarily driven by external factors such as changes in the Earth’s orbit, which change the amount of solar radiation coming from the sun? Or are they internal factors? From the climate system Like carbon dioxide, the volume of ice and wind that carries moisture is more important?” (Sunday Vision)
