Air pollution behind weakening Asian summer monsoon, shows study.

• Researchers point out that the decline in the quantum of rainfall, which is contrary to what is expected from the greenhouse warming, has been ‘unprecedented’ over the last 450 years

• The Asian summer monsoon accounts for the bulk of rainfall over Asia in the summer months

Highlighting the hazards of air pollution, a new study has found that Asian summer monsoon rainfall has been decreasing over the past 80 years due to human-caused air pollution causing regional droughts.

The researchers pointed out that the decline in the quantum of rainfall, which is contrary to what is expected from the greenhouse warming, has been ‘unprecedented’ over the last 450 years.

The Asian summer monsoon affects nearly half of the world’s population and accounts for the bulk of rainfall over Asia in the summer months.

One of its dominant sub-systems is the South Asian monsoon which makes up for over 75% of India’s annual rainfall and is extremely crucial as it has an impact on the livelihoods of half of the population and impacts the country's agricultural GDP.

The other sub-system is East Asian monsoon which brings rainfall over mainland China, Japan, and the Korean peninsula.

Both South-Asian monsoon and East-Asian monsoon are jointly referred to as Asian summer monsoon, even though they differ in dates of their onset, decay and amount of precipitation. Studies suggest that the East Asian monsoon even manifests unique characteristics as compared to its South Asian counterpart.

According to the study, published the AGU journal Geophysical Research Letters, the decline in Asian summer monsoon rainfall would not only impact water availability, but ecosystems, food security, and agriculture in the Asian countries from India to Siberia.

Researchers used an ensemble of ten tree-ring chronologies including 584 cores from 310 trees from north-west China to track precipitation trends over the last 448 years and used that information to reconstruct the Asian summer monsoon back to 1566.

In wetter years, trees grow thicker rings and precipitation records can be obtained by measuring the thickness and density of individual layers.

"We were able to gather nearly 450 years worth of tree-ring data with clear annual resolution from an area where tree ring growth correlates very strongly with rainfall," said co-author Steve Leavitt, a dendrochronologist at the University of Arizona, US.

Several factors are thought to affect the strength of the monsoon, including solar variability, volcanic eruptions and anthropogenic aerosols.

However, according to researchers, increasing sulfate aerosols emissions, types of atmospheric pollutants that cause haze are likely the dominant forcing agent controlling the decline over the past eight decades, as indicated in the coupled climate models.

"The decreasing trend was likely due to increasing human-made atmospheric pollutants, because the decline in rainfall coincided with the ongoing boom in industrial development and aerosol emissions in China and the northern hemisphere that began after 1945," the study said.

The tree rings also captured the major drought periods during the last eight decades, including the ones that struck in 1928 leading to widespread famine where more than 500,000 people died in China alone, said the researchers from Chinese Academy of Sciences who led the study. (Source: livemint)