Climate Change

Humans have a hand in weakening Asian monsoons in the last 80 years

Researchers studied tree rings from the Loess Plateau in China to conclude that increasing aerosol levels due to emissions are responsible

By Akshit Sangomla
Published: Friday 17 May 2019
The Loess Plateau in China, where the scientists carried out their experiments. Photo: Getty Images

There is a decreasing trend in the strength of monsoon rains over Asia in the past 80 years, a new study has found.

As part of the study, published in the journal Geophysical Research Letters on April 9, 2019, the researchers studied data from the last 448 years and found that the major reason could be an increase in the aerosol levels of the atmosphere due to human-made emissions.

Scientists studied tree rings of 584 cores from 310 trees from the western Loess plateau in north central China, a region where the correlation between tree growth and monsoon rainfall is very high.

With the collected data, they were able to reconstruct monsoon patterns beginning from 1566 with much better resolution than done before. The current data “surpasses previous dendrochronology (the dating of tree rings and study of ancient climate through them) studies in terms of the time span covered and the number of trees involved,” said Steve Leavitt, co-author of the study and dendrochronologist at the University of Arizona, United States.

Dendrochronology is one of the methods researchers used to reconstruct past climates, also known as paleoclimatology, as actual observational data from instruments only date back to a little over a century. This is done by dating tree rings and then studying the thickness of tree rings to know about their health and growth in particular climatic conditions.

Understanding past climates and closely analysing patterns through time helps scientists gain a perspective on the current climate. This also aids in knowing exactly how much of current changes in climatic phenomenon can be attributed to global warming induced by greenhouse gas emissions and how much of it is natural cycles that take place over decades, centuries or even millennia.

The study also lists a series of historical events that validate the strong and weak phases of monsoons in the Asian region. For instance, during 1586/87, there is evidence of half of the population of the eastern Gansu region of China fleeing their homelands and there are reports of them even indulging in cannibalism. This fits in perfectly with a lean monsoon period during the time as informed by data from the tree rings.

The scientists also looked at various natural cycles like the Pacific Decadal Oscillation and North Atlantic Oscillation impacting the enhanced decreasing trend observed in the last years 80 years. They found that this was not the case, hence they considered human pollutant emissions as a possible cause. They postulate that sulphate (SO42-) emissions from industries and vehicles could be responsible for the declining trend.  

They ran two separate climate models based on the monsoon data and found that the one where sulphate emissions were considered, clearly showed a decreasing trend in regions of China where there should have been an increasing trend since the 1940s. From this, the researchers concluded that sulphate aerosols might be responsible for the observed trend hence humans might be responsible for the decrease in monsoon rains over Asia.

Monsoons form an essential component of annual rainfall in most Asian countries. In India, the south west monsoon brings almost 70 per cent of the annual rainfall. In China, the East Asian monsoon is responsible for up to 60 per cent of the country’s rainfall.

A long-term decline in the monsoons will mean more frequent droughts, which has been the case in India in the past decade. It will also mean impacts on the economy. For example, 60 per cent of Indian agriculture is still rain-fed. Fewer rains mean less agricultural output which will increase distress among the farmers.

There has been evidence of decreasing monsoons in India from other sources as well. Monsoon winds coming to India have become weaker in the last few decades, leading to a decrease in overall rainfall on the Indian Subcontinent, says a research paper published by scientists from Harvard University, Peking University and Chinese Meteorological Administration in the journal Science Advances in December 2018.

The study says that significant warming in the Indian Ocean has decreased the temperature difference between the land and sea, which is the main trigger for the flow of monsoon winds.

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