IIT Madras-led study redefines the role of human activity in shaping clouds and climate

MUMBAI: The pandemic lockdown of 2020 silenced traffic, stilled factories, and briefly cleared India’s skies. But what seemed like a pause in human activity became, for scientists, a once-in-a-lifetime laboratory.

An international team led by IIT Madras has now turned that natural experiment into breakthrough insights that challenge long-held assumptions about how tiny airborne particles—known as aerosols—shape our climate.Their findings, published in the ES&T Air Journal of the American Chemical Society, show how human emissions along India’s coasts drive dramatic shifts in cloud-forming aerosols. These particles, called cloud condensation nuclei (CCN), are vital for precipitation and climate regulation.

The study found that when lockdowns curtailed emissions, and then activity resumed, the atmosphere responded with startling speed—revealing just how deeply human behaviour is entwined with atmospheric processes.Challenging old beliefs

Aerosols are among the most vexing uncertainties in climate science. Models have long struggled to predict how they form clouds, and conventional wisdom held that organic aerosols might hinder droplet growth by forming surface films.

The IIT Madras-led team found otherwise. Anthropogenic organic matter, they discovered, can in fact accelerate cloud formation when present in large numbers.In coastal India, this meant that the surge of organic particles after lockdown directly raised CCN concentrations by 80–250%. Despite being less efficient at attracting water compared to inorganic particles, their sheer abundance enabled them to fuel cloud development.

In other words, the very particles once thought to inhibit cloud growth were, under certain conditions, catalysing it.“Our research reveals that anthropogenic emissions strongly influence aerosol behaviour, particularly in how they form clouds,” said Prof. Sachin S. Gunthe, lead author and atmospheric scientist at IIT Madras. “These findings challenge existing models and propose new avenues for understanding how human activities shape climate patterns.”When emissions stopped and restartedThe study, conducted from March to July 2020, documented what happened as emissions plummeted during lockdown and then rebounded. The researchers observed more frequent new particle formation—a process where aerosols emerge from gases through complex chemistry—as human-caused emissions crept back. This surge was directly responsible for the spike in CCN.“We witnessed firsthand how the drastic reduction in air pollutants during the lockdown provided a unique ‘natural experiment’,” recalled Aishwarya Singh, co-author and now a postdoctoral researcher at the Max Planck Institute for Chemistry, Germany.

“Our observations show that a cleaner atmosphere can be highly sensitive to new emissions, altering the aerosol-cloud interactions significantly, which can have a profound impact on climate predictions.

”From models to measurementsClimate forecasts depend heavily on computer simulations, but this research underscores the need for grounding them in field data. “Measurements like ours, drawn from real-world conditions, offer clarity that computer-based models alone cannot achieve,” Prof.

Gunthe explained. “This new science provides the foundation for more accurate climate projections, which is essential for informing policy and environmental management.

”That sentiment was echoed by Prof. R. Ravikrishna, co-author and faculty at IIT Madras. “We cannot predict the future of our climate without rigorously understanding the current state of our environment,” he noted. “Measurements provide an essential context to refine and enhance climate models.”External experts agree. Dr. M. Ravichandran, Secretary at the Ministry of Earth Sciences, observed: “Aerosol-cloud interactions are intrinsically complicated, and these findings underscore that human activities can dramatically influence underlying processes. This is critical information for negotiating future atmospheric dynamics.”Implications for climate policyFor IIT Madras, which coordinated the study with national and international collaborators, the significance lies not just in the science but in its global resonance.

If climate models are to guide policies and commitments, they must reduce uncertainty. This research provides that missing empirical link—evidence that human activity alters atmospheric chemistry in ways both subtle and profound.As the world debates net-zero commitments and nations prepare adaptation plans, such insights become essential. By showing that organic aerosols can promote cloud formation, the study challenges researchers to rethink long-standing assumptions.

It also reinforces the urgency of collecting robust field measurements across diverse geographies.The team hopes these findings will catalyse further research and raise public awareness of how everyday choices ripple into planetary systems. The message is as simple as it is sobering: our actions do not just leave a footprint on the ground—they leave fingerprints in the clouds.