The monsoon rains that batter India each summer, unleashing 80% of the country's yearly rainfall in four months crucial for its farmers, are at the whim of forces far beyond its borders.

Summer dust storms in the Arabian Peninsula and fossil fuels burned in countries across the world are causing heavier seasonal rains in India, according to two separate studies of a precarious climate system upon which more than 1 billion people rely on for food.

The first study, a paper published in the journal Earth-Science Reviews in April, found that dust particles swept into the atmosphere from deserts in the Middle East grow so hot under sunlight that they change the air pressure over the Arabian Sea. This creates a kind of heat pump in the sky, which drives moisture from above the ocean to the Indian subcontinent, leading to a wetter monsoon season that then strengthens winds and could whip up even more dust particles.

The second study, published in the journal Earth Systems Dynamics on Wednesday, found that human-wrought climate change is making the Indian summer monsoon wetter and more erratic. Using the latest climate models, researchers from Germany's Potsdam Institute for Climate Impact Research (PIK) found that every additional degree of warming is likely to increase monsoon rainfall by 5%.

The Earth has already warmed by more than 1 degree Celsius (3.6 degrees Fahrenheit) since the Industrial Revolution, and a November United Nations report found world leaders' plans to keep it to 1.5 C this century "woefully inadequate," adding that global heating is on track to more than double that. The countries least responsible — such as India, Pakistan and Bangladesh — are typically poorer and more reliant on agriculture than historical emitters, and already suffer from worsening weather extremes.

"The summer monsoon is even more sensitive to global warming than previously thought," said lead author Anja Katzenberger from PIK. "We have the power to shape the intensity of these changes via [our] greenhouse gas emissions."

Heavier Indian summer monsoon rains

The English word monsoon comes from the Arabic mawsim, meaning season. It refers to the twice-yearly shifts in prevailing wind direction that bring warm rains to land in the summer, and send cold, dry air to the sea in the winter. In parts of India like the Western Ghats, the coming and going of the summer monsoon is strong enough to turn semi-arid mountains into lush green landscapes.

For thousands of years, farmers have timed the planting and harvesting of staple foods like rice and wheat to the beat of the monsoon, which varies naturally from year to year. But as greenhouse gases clog up the atmosphere, trapping sunlight and warming the planet, scientists expect the monsoon to become increasingly chaotic.

"More erratic rainfall in the future poses a challenge for farmers to cope with a broader range of potential rainfall amounts," said Katzenberger. At first glance, an increase in rainfall might seem good for crops — but too much can significantly lower yields for some plants during the growing season, she added.

Most Indians rely on agriculture for their livelihood, and their crops are highly sensitive to rainfall variability. Climate scientists from three separate institutions told DW via email that the PIK study is in line with previous climate modeling that projects Indian summer monsoons will get wetter and more chaotic as greenhouse gas levels rise.

"This new paper, based on the most recent models, supports earlier research," said Andrew Turner, an associate professor in monsoon systems at the University of Reading in the United Kingdom. 

The study finds that "even with modest warming projected under the low-emission trajectories, the monsoons are likely to intensify," said Deepti Singh, an assistant professor in the School of the Environment at Washington State University Vancouver. "One of the key findings is that these latest climate models project even more pronounced intensification of the monsoon."

But not all of the new climate models used in the analysis simulate monsoon circulation well, and that "reduces confidence in the results," said Roxy Matthew Koll from the Center for Climate Change Research at the Indian Institute of Tropical Meteorology. "There is, however, one factor that all climate model projections agree with: That the extreme rainfall events are going to increase. In fact, this is already visible in observations."

Local cooling from aerosols

Scientists struggle to assess monsoon patterns because they depend on competing factors. Although climate change is warming the planet, changes in land use along with aerosol emissions — from vehicle exhaust fumes and crop burning, for instance — are cooling factors.

Since 1950, Indian summer monsoon rainfall has actually declined. Scientists think this is because of a solar dimming effect from those energy-absorbing aerosols. The effect could "still partly offset greenhouse gas-induced increases in monsoon rainfall in the next 10 or 20 years or so," said Turner.

But even though most aerosols lead to less rainfall, some can have the opposite effect.

The heavy layers of black carbon and sulfate hanging over South Asia — home to the most chokingly polluted cities on the planet — cool the surface and reduce monsoon rains. Yet mineral dust blown over from the Middle East heats the atmosphere and can instead increase rainfall.

While most studies agree that these dust aerosols strengthen the Indian summer monsoon, their estimates of how and where rain is likely to fall vary widely, according to the new Earth-Science Reviews paper on dust in the Mideast.

Understanding these processes better could help modelers trying to predict rainfall, said Qinjian Jin, lead author of that study and a lecturer at the University of Kansas in the United States. "Our understanding [of] the monsoon is very limited, even though we have made promising progress during the past several decades," he wrote in an email

Since the beginning of the century, global warming has overtaken other human activities like aerosol emissions in driving Indian summer monsoon rains, Katzenberger concluded. "This is projected to continue to do so for the rest of the century."

Correction: This article has been updated to clarify that some climate models do not simulate monsoon dynamics well, rather than not simulating them at all.