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Monsoon likely to get more extreme & unreliable due to global warming, predicts new study

Study looks at past geological periods when temperatures were higher than today to predict what the future has in store.

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New Delhi: It’s well established today that the global climate is getting more erratic. A new study has now predicted the same future for the Indian monsoon, saying the weather system crucial to most aspects of life on the subcontinent is likely to become more extreme and unreliable in the decades to come.

In the latest study, researchers from the UK, New Zealand, China and the US looked at past geological periods when temperatures were higher than today to get an idea of what to expect when similar temperatures are reached in the near future.

According to their findings, the Indian summer monsoon season was longer, more extreme and less reliable about 125,000 years also — the last interglacial period — when the climate was 1-2 degrees warmer on an average than today.

A similar behaviour on the part of the Indian monsoon can be expected in the decades to come as the global average temperature rises, says the study published in Geophysical Research Letters last month.

“The last interglacial period is often considered an analogy to the expected climate changes [in the future],” Ola Kwiecien, from Germany’s RUB Institute of Geology, Mineralogy and Geophysics, and one of the authors on the study, said in a statement. “Even though the factors that led to the warming were different than they are today, of course.”

Due to the nature of summer monsoon, agriculture in India has historically been established around its reliability with summer crops comprising over half of the total annual food output. India receives 70 per cent of its annual rainfall during the monsoon season.

However, recent observations indicate that monsoons are becoming increasingly more erratic. Data suggests that the region-wise distribution of rainfall is changing, with the number of extreme rainfall days increasing along with the number of dry spells.

Predicting how the monsoon patterns will evolve in the future amid a rapidly changing climate is crucial for the Indian agriculture, and in turn, the economy.

Also read: Scientists have found East Asian Monsoon is over 145 million yrs old, predates Himalayas

How the study was conducted

For the latest study, the authors performed isotope analysis on a dripstone — a rock formed from precipitation of dripping water from rainfall, commonly known as stalactite and stalagmite — from the Mawmluh Cave in the north-eastern state of Meghalaya.

Dripstones are formed over thousands of years and their vertical composition changes over time with changing environmental and geographical conditions. In other words, they act as small time capsules containing thousands of years of climate history.

Researchers calculated the ratio of heavy to light isotopes of oxygen in the dripstone, denoted by delta-18-O, which changes with moisture content.

High atmospheric moisture and freshwater run-offs from rains typically lead to low values of delta-18-O. Consequently, delta-18-O values are the lowest during monsoon months. It effectively serves as an indicator of overall monsoon strength.

However, the delta-18-O signal is contaminated by more sources than just the monsoon rains. The region around these caves also receives moisture through air circulation from the Indian Ocean, Bay of Bengal, fresh water run-off from the Asian continent and through storms. This makes it difficult to isolate the regional and local monsoon trends based on delta-18-O alone.

“The delta-18-O value tells us something about the strength of the monsoon, but not how much precipitation falls and how the rain spreads over time,” RUB Institute’s Sebastian Breitenbach said in the statement.

To deal with this, the scientists turned to calcium. Rainwater collects into puddles on the ground above the cave which then seeps through the soil and reaches the cave through underground channels, carrying dissolved calcium from the soil. Upon reaching the cave, the calcium in the run-off combines with carbon dioxide to form calcium carbonate salt which ultimately deposits on the dripstone.

The researchers calculated the ratio of the heavy to light isotopes of calcium, denoted as delta-44-Ca, in the dripstone. Fresh deposits of calcium from rainwater run-off lower the delta-44-Ca value, hence, indicating the changes in rainfall amount in the vicinity of the cave providing local level information.

The team was able to assess the overall strength of monsoons, the regional distribution of rainfall and the intensity of dry spells, before, during and after the interglacial period by combining the results from these different types of measurements in the dripstone.

The data showed monsoons were more erratic and riddled with more extreme weather events during the last interglacial period.

Also read: India’s greenhouse gas emissions highest in the world in 2018

Future of Indian monsoon

In the statement, Ola Kwiecien said, “On the whole, our data show that the Indian monsoon was less reliable in the last interglacial period than it is today, which suggests that global warming today might be having the same effect.

“This tallies with the tendency for weather extremes to become more frequent,” said Kwiecien.

According to the researchers, human impact on the climate in the Indian summer monsoon has not yet fully manifested itself. The UN Intergovernmental Panel on Climate Change projects that if business continues as usual, then the 1.5 degrees Celsius above pre-industrial levels mark will be crossed by 2050 and the 2 degrees Celsius mark by 2080. In the coming decades, the impact of climate change on monsoon can be expected to get worse.

This year’s monsoon in India was the worst recorded since 1994, with 10 per cent more rainfall than the long period average (LPA) overall and 52 per cent more than the LPA in the month of September, resulting in a deluge in many cities.

Over 25 lakh people were affected in 22 states and 2,100 lost their lives. Such extreme events are likely to become more common in the decades to come.

Also read: Over 11,000 scientists from 153 countries declare global climate emergency


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