India's monsoon rain depends on where air gets cleaner

Cleaning up pollution around the world could bring significantly more rainfall to India’s monsoon than regional action alone, research from the University of Reading shows, highlighting the value of international cooperation on air quality

Air pollution blocks some of the sun's energy from reaching land and sea, which weakens rainfall. When a region cuts its own pollution, more sunlight reaches the ground, warming the surface and strengthening the monsoon circulation that delivers rain.

This is exactly what happens in China and the rest of East Asia. Cutting pollution there lets more sunlight through, warms the surface up to one degree Celsius, and brings 0.20 millimetres more rain each day across the region during the summer monsoon season.

The research, published this month in Environmental Research: Climate, found the opposite for India. The same pollution cuts may reduce rainfall by 0.2 to 0.6 millimetres a day over parts of west-central and eastern India, through wind patterns linking the two regions across thousands of miles.

Ankit Bhandekar, lead author from the University of Reading, said: “Cleaning up pollution is good news almost everywhere it happens, but our findings show one country's clean air plan can quietly cost another country's rain. 

“Millions of farmers across India depend on monsoon rain that starts with decisions made thousands of miles away. If China and India coordinate their air quality plans, rather than acting in isolation, they have a better chance of avoiding these unintended tradeoffs."

Coordinated cleanup needed

Scientists ran ten climate models to test what happens to the Indian monsoon when different countries cut air pollution. This work, called RAMIP, involved thousands of computer runs carried out by teams around the world.

The research found cleaning up pollution everywhere increases rainfall across India by about fifty percent more than cleaning up pollution in South Asia alone does. All India rainfall rises by 0.28 millimetres a day under worldwide cleanup, compared with 0.19 millimetres a day when only South Asia cleans up its air.

Rain increases most over the northern Bay of Bengal, the Western Ghats and the Indo Gangetic Plains, the land stretching from Pakistan through northern India to Bangladesh.

Researchers say the next step is working out how these changes affect not just how much rain falls, but when it falls and how intense individual storms become. That detail will matter most for farmers and water planners trying to prepare for what comes next.