Science.-South Asian monsoons will be strengthened with more CO2


A new study on monsoon rainfall in the Indian subcontinent over the past million years provides vital clues as to how they will respond to future climate change.

The study, published in the journal Science Advances, found that periodic changes in the intensity of monsoon rains over the past 900,000 years were associated with fluctuations in atmospheric carbon dioxide (CO2), the volume of continental ice and the import of moisture from the Indian Ocean of the southern hemisphere.

The results reinforce climate model predictions that rising CO2 and global temperatures will lead to stronger monsoon seasons.

“We show that over the past 900,000 years, higher CO2 levels along with associated changes in ice volume and moisture transport were associated with more intense monsoon rains,” says Steven Clemens, professor of geological sciences at the University. de Brown, in the United States, and lead author of the study.

“That tells us that CO2 levels and associated warming were the drivers of monsoon intensity in the past, which supports what the models predict about future monsoons: that rainfall will intensify with increasing CO2 and global temperature warming, “he adds.

According to Clemens, the South Asian monsoon is the most powerful expression of the hydroclimate on Earth, and in some places it rains several meters each summer. Rains are vital to the region’s agriculture and economy, but can also cause flooding and crop disruption in particularly heavy years. With monsoons playing such an important role in the lives of nearly 1.4 billion people, understanding how climate change can affect them is critical.

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For several years, Clemens has worked with an international team of researchers to better understand the main factors driving monsoon activity. In November 2014, the research team sailed aboard the JOIDES Resolution research vessel to the Bay of Bengal, off the coast of India, to retrieve sediment core samples from the seabed. These samples preserve a record of monsoon activity spanning millions of years.

The rainwater produced by the monsoons each summer ends up draining from the Indian subcontinent into the Bay of Bengal. Runoff creates a layer of diluted seawater in the bay, which sits on top of the denser, more saline water below.

Surface water is a habitat for microorganisms called planktonic foraminifera, which use nutrients from the water to build their shells, which are made 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 the oxygen isotopes from those fossils, scientists can guess the salinity of the water in which the creatures lived. This salinity signal can be used as an indicator of changes in rainfall amounts over time.

The data from the foraminifera are complemented by other data from the samples. River runoff into the bay brings with it sediment from the mainland, which provides another indicator of the intensity of the rains.

The isotopic composition of the carbon of the vegetal matter carried to the ocean and buried in the sediments offers another signal related to the rains that reflects the changes in the type of vegetation. The hydrogen isotopic composition of plant leaf waxes varies with different rainfall environments, and that signature can also be reconstructed from sediment cores.


“The idea is that we can reconstruct rainfall over time using these indicators and then look at other paleoclimatic data to see what might be the main drivers of monsoon activity,” Clemens explains.

“This helps us answer important questions about the factors that drive monsoons,” he adds. “Are they mainly driven by external factors such as changes in the Earth’s orbit, which alter the amount of solar radiation coming from the sun? Or are the internal factors of the climate system such as CO2, the volume of ice and the winds that carry moisture more important? “

The researchers found that periods of heavier monsoon winds and rains tended to follow peaks in atmospheric CO2 and lows in global ice volume. Cyclical changes in Earth’s orbit, which alter the amount of sunlight each hemisphere receives, also influenced the intensity of the monsoons, but alone could not explain their variability.

Taken together, the results suggest that monsoons are indeed sensitive to CO2-related warming, validating climate model predictions of monsoon strengthening relative to CO2 rise.

“The models tell us that in a warming world, there will be more water vapor in the atmosphere,” says Clemens. “In general, regions that get a lot of rain now are going to get more rain in the future. monsoons in South Asia, that’s totally consistent with what we see in this study. ”

Myrtle Frost

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