Aerosol boundary confirmed: Increased air pollution could tip monsoon

08/01/2026

Earth Commission scientists have quantitatively validated the regional “safe limit” for air pollution over South Asia, confirming that exceeding certain levels of airborne particles increases the risk of severe drought in India.

Aerosols are tiny particles floating in the air, like black carbon, sulfates, and dust. These particles pollute the atmosphere worldwide, with severe health implications. Millions die every year and many more suffer from ill health because of them. South Asia is one of the global hotspots of air pollution.

Beyond their health effects, aerosols also impact our climate and the water cycle, which is why scientists are concerned about what is a safe and just limit for air pollution. The Earth Commission has long highlighted the risks of aerosols, emphasizing that rising levels not only harm human health but can also destabilize crucial life-support systems on Earth.

“It has long been believed that the heavy aerosol loading in South Asia could disrupt the regional water cycle and the summer monsoon rainfall over India”, explains Earth Commissioner Govindasamy Bala, a professor at the Indian Institute of Science.

Models confirm expert judgement

Together with Earth Commission contributor Thejna Tharammal, and lead author Neethu C, he carried out a study that used climate modelling to verify how much aerosols South Asia can endure and what concentrations could significantly disrupt the South Asian monsoon.

The researchers looked at different levels of what is known as aerosol optical depth (AOD). It is a measure of how much sunlight is blocked or scattered by particles in the atmosphere. A higher AOD means hazier skies and more pollution. 

Published in the journal Environmental Research Letters, their findings essentially confirm earlier expert judgment. The study shows that an AOD of 0.25 would lead to drought, defined as 10 percent reduction in summer monsoon rainfall. An AOD of 0.50 would lead to a 20 percent reduction in rainfall. Today, the annual mean AOD is 0.14. Going from today’s level to 0.25 would mean an increase in aerosol loading over South Asia by a factor of 2.7.  

Nearly two billion would be impacted

“This could lead to major and persistent droughts in South Asia and affect nearly two billion people in the region”, says Thejna Tharammal.

These findings have important implications for policy. Some future high-emission climate scenarios that lack stringent air quality controls project aerosol levels in South Asia that could reach an AOD of 0.69 by 2050, far exceeding the safe boundary proposed by the Earth Commission. Under this scenario, known as SSP3, the world could see a global mean temperature increase of 3.6 degrees Celsius. On top of that, the increase in aerosols could reduce the mean summer monsoon rainfall by 30 percent.

“For the water security and health of nearly two billion people in the region, the aerosol emissions in the region should be drastically reduced. The answer to the question of how much aerosols are too much and risky to the hydrological cycle in South Asia provides valuable guidance for policymakers on the upper limit to air pollution”, says Govindasamy Bala.

A boundary for aerosols was first proposed in 2015 by Professor Will Steffen and colleagues in Science.  They proposed a limit of 0.25 for regional AOD in South Asia with a zone of uncertainty of 0.25 to 0.50 to avoid major disruptions to regional hydrology. The boundary limit was based on expert judgement. The Earth Commission adopted this limit in Safe and Just Earth System Boundaries published in Nature in 2023. The Commission also determined a safe boundary for aerosols at the global level. It is ​​defined by the difference between AOD in the northern and southern hemispheres, as a large difference between the two could disrupt the tropical rain belt and cause serious adverse effects on tropical monsoon rain patterns.

How Aerosols Cause a Decline in  Rainfall

• Aerosols come in two major categories: Reflective and absorbing aerosols. Reflective aerosols, such as sulfates, reflect solar radiation back to space and cool the climate. Absorbing aerosols, like black carbon, absorb sunlight and heat the surrounding air. However, both types cause a reduction in rainfall. 
• Aerosols weaken the South Asian monsoon by triggering two key climate mechanisms:
  • Fast Adjustments: These are immediate effects where the aerosols block sunlight (the dimming effect), causing the land surface to cool while simultaneously warming the upper atmosphere. This process enhances atmospheric stability and suppresses cloud formation and rainfall.
  • Slow Responses: Over time, aerosols cause sea surface temperatures to change, leading to temperature differences between the North Indian Ocean (cooling) and the Western Pacific (warming) in the case of an increase in South Asian aerosols. This temperature difference drives a shift in large-scale atmospheric circulation, creating a circulation that causes sinking air (subsidence) over South Asia, which suppresses convective activity and reduces monsoon precipitation.
• Since anthropogenic, or human made, aerosols over South Asia are primarily composed of sulfates, simulations where sulfates dominate show a similar reduction in precipitation, but the overall total response leads to reduced monsoon rainfall regardless of whether the aerosols are primarily reflective or absorbing.