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[NEW DELHI] Indian summer monsoon rains have been decreasing steadily over the past three decades, a trend not seen in the ninteenth century, says a new study.

The findings confirm the weakening of India's summer monsoon (see Indian summer monsoon weakening, say scientists).

A team from Andhra University in Andhra Pradesh analysed records of rainfall across 30 meteorological subdivisions in India during the four monsoon months from June to September.

They compared monsoon rainfall trends in these four months over two time scales: from 1871–2005 and from 1970–2005, which marks the "global warming era". Previously, no one had analysed the long-term changes in India's weather to ascertain whether or not recent monsoon phenomena can be attributed to global warming.

"This is the first time recent rainfall patterns after the 1970s have been compared with long-term tendencies," C. V. Naidu, assistant professor at the university's department of meteorology and oceanography told SciDev.Net.

Unlike the late ninteenth century, when almost all of the subdivisions recorded active monsoon rains, during the global warming period 19 out of the 30 subdivisions showed decreased rainfall.

The research was published in the Journal of Geophysical Research last month (31 December)

"This pattern of rainfall tendencies suggests a weakening of monsoon activity over India," the authors wrote. They said this decline has persisted over three decades, unlike in the previous centuries when rainfall rose and fell between, and even within, decades.

The team detected a general trend in the shifting rainfall pattern. Most of the regions lying below 20 degrees north latitude (India lies between 8.7 and 37.6 degrees north) are experiencing fewer rains, they said.

Information on such shifts needs to be understood for proper planning in agriculture, hydroelectric power, and economy, said the report. The Indian economy depends strongly on agriculture that in turn is influenced by the summer monsoon.

There has also been stronger warming of the tropical northern Indian ocean during the global warming decades, which has resulted in intensified cyclones, the report said.

Link to abstract in the Journal of Geophysical Research