The monsoon creates thunderstorms and rain erratically. A storm can dump an inch or more of rain in one location and leave another area, just a few miles away, dry.

Raghu Murtugudde is a professor at the University of Maryland's Earth System Science Interdisciplinary Center (ESSIC) and the Department of Atmospheric and Oceanic Science. Murtugudde contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

The World Health Organization (WHO) has warned for more than a decade that rising air pollution levels pose a serious threat to human health worldwide, especially in developing countries, and high levels of pollution in the urban centers of China and India are now responsible for the premature deaths of more than 2 million people every year.

As if this news were not bad enough, my colleagues and I have found that pollution and dust particles blanketing that region are responsible for a 20-percent decline in South Asian monsoon rainfall over the past century — findings published June 16 in the journal Nature Communications.

Predictable monsoon?

The word "monsoon" is derived from the Arabic word mausam, which means season. The South Asian summer monsoon is a dramatic phenomenon that has inspired prose and poetry for millennia. The onset occurs like clockwork around June 1, varying by only a few days from year to year.

Much of the southwestern corner of India gets a gully-washing downpour that can bring normal life to a complete halt, yet is a much welcome gift from Mother Nature. Nearly 80 percent of the subcontinent's annual rainfall occurs during the summer monsoon, providing a lifeline to more than 2 billion denizens of the region. The monsoon's end is nearly as predictable as its onset, occurring within several days of Oct. 3. Thus, the rainy season typically lasts a solid four months, or about one-third of the year.

The monsoon's predictability is governed by the interplay of several atmospheric and geophysical phenomena. The joining of the northern and southern trade winds, combined with maximum solar heating near the equator, creates the Inter Tropical Convergence Zone (ITCZ), a broad band of clouds and storms that circles the globe. The ITCZ tends to fluctuate seasonally from north to south, trending northward during the Northern Hemisphere's summer and vice versa.

Because land heats faster than the ocean does, and there is more land sitting north of the equator, the ITCZ becomes more active during the Northern Hemisphere's summer. Although there are monsoonal circulations associated with nearly all of the continents, the South Asian summer monsoon is the most powerful due to the significantly large landmass that extends deep in the tropics. The majestic Himalayan mountain chain and the Tibetan plateau amplify this effect even further, with their high altitude contributing to more intense heating overall. The warm Indian Ocean is a perfect partner for the monsoon's elegant seasonal dance, providing the ideal temperature gradient next to the scorching-hot South Asian landmass.

As the seas warm

With global warming, it was expected that the South Asian landmass would warm faster than the Indian Ocean and thus enhance this land-sea temperature gradient, delivering more rainfall to South Asia. A majority of the climate models used for making future climate change projections have indeed predicted that the monsoon would strengthen into the 21st century as global mean temperatures continue to climb. But rainfall data over the Indian subcontinent has shown a consistent decline for the entire 20th century, especially over Pakistan and the floodplain of the Ganges River. The exact cause of the downward trend in monsoon rainfall had remained a mystery. [Ocean Warming: Probing a Blue Abyss ]

Our new study in Nature Communications unravels this mystery by demonstrating that the increase in pollution has, in fact, slowed down the warming of the South Asian landmass. Meanwhile, the Indian Ocean has continued to warm rapidly, thus reducing the overall magnitude of the land-ocean temperature gradient. This weakening of the land-ocean contrast has weakened the monsoon.

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Indeed, the Indian Ocean is evaporating more due to its warmer temperatures, but this moisture is not being carried to South Asia. Instead of traveling to the South Asian landmass, this water is being dropped back into the ocean.

In addition, the South Asian monsoon has been further affected by El Niño events and decades of climate variability due to factors like the Pacific Decadal Oscillation (PDO). The latter refers to the tendency of El Niño to manifest as either stronger or weaker events for decades at a time.

In a separate study with colleagues from India, we found that the inversion of the PDO in the mid-1970s is also associated with a delay in the monsoon's arrival. Since that time, which marked a transition toward more intense and frequent El Niño events, the monsoon's onset has averaged closer to June 5, rather than June 1. Another collaboration with Indian students and scientists detected that the average withdrawal date has moved up by several days since the mid-1970s as well. This late onset and early withdrawal in recent decades has shortened the rainy season overall, robbing the region of even more life-giving rain.

This reduction in critical freshwater has put a huge strain on South Asian communities. A new NASA report released in June reaffirmed that groundwater continues to be depleted at unsustainable rates by most countries, with India and China being among the top groundwater miners. [Earth's Groundwater Basins Are Running Out of Water]

With ever-increasing populations and some of the fastest-growing economies in the world, a continued decrease in monsoon rainfall is a double whammy for the most populous region of the planet. Air pollution continues to be a major concern for respiratory health, but this new understanding of the role that particulates play in the monsoon cycle should be a real wake-up call for action on reducing air pollution.

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