In the months of March, April and May in 2020, at the peak of the Covid lockdowns across Asia, Indian scientists began to notice a startling change in the atmosphere above parts of the country. Not long after, these changes reverberated through the massive weather systems that dominate much of Asia – the monsoons.
The changes to the monsoons were an unexpected side effect of the lockdowns that restricted human activity. With no vehicles plying the streets and with industrial activity slowing considerably, the reduced emissions led to a significant decrease in atmospheric aerosols. These are tiny solid and liquid droplets suspended in the atmosphere, and they include microscopic particulate matter (PM 10 and PM 2.5) harmful to human health. Aerosols include sulphur dioxide, from burning coal, and black carbon or soot. The latter is a byproduct of agricultural burning, which in India is often from wood fires commonly used in cooking.
We have long known the ill-effects of aerosols. Our airways suck up them up like a vacuum cleaner would dust – they coat our lungs and the toxins enter our bloodstream. But 2020 was the first time researchers were able to study the effects of a sharp decline in aerosols in our atmosphere, and at a broader level, map the impacts on the Indian and East Asian summer monsoons.
"During the lockdown period, when anthropogenic (human) activity was negligible, we saw as much as a 30% reduction in aerosols in the atmosphere over India," says Suvarna Fadnavis, who has studied monsoon patterns at the Indian Institute of Tropical Meteorology for the past 25 years. The effects were seen widely and chronicled in news reports: especially of how residents of Saharanpur in the Central Indian city of Uttar Pradesh could catch a breathtaking glimpse of the Himalayas for the first time in 30 years.
Crucially for the monsoons, these aerosols usually form a layer in the atmosphere which reflects solar radiation. The thinner aerosol layer over North India during lockdown meant that the land was now heating up rapidly, says Fadnavis.
In general, monsoons work on a temperature gradient – the difference in temperature between a warming land (with lower atmospheric pressure) and a relatively cooler ocean (with higher pressure). Winds blow from high pressure to low pressure areas, driving moisture-laden winds from ocean to land. When the land warms up faster – as it did without its usual aerosol shade – the monsoon rains are stronger, and that's what researchers observed when the aerosol layer thinned.
"We observed that it increased annual monsoon rainfall by as much as 5-15%, around 3mm a day," says Fadnavis.
To put this in context, the increase in monsoon rainfall was the equivalent of an hour's moderate shower of rain per day. In drought-prone India, highly dependent on seasonal monsoon rainfall for its agriculture, economy, food security and overall health, this is significant, she says.
The thick smog in Beijing, China, contributes large quantities of aerosol particles to the atmosphere, with the potential to alter rainfall (Credit: Getty Images)
But the impact wasn't just observed in India. The climate of Asia as a whole is largely dictated by its monsoons. Large swathes of East Asia saw an abrupt reduction in aerosols and its effects were felt over China, Korea and Japan.
"Our research shows that the sudden reduction in aerosol concentration significantly increased the total monsoon rainfall over East Asia as well," Chao He, associate professor at the Institute for Environmental and Climate Research, Jinan University in Guangzhou, China, tells BBC Future in an email. This was proven by observational and modelling studies in the summers of 2020 and 2021. "Previous studies confirmed that the emission reduction during Covid-19 was not strong enough to hamper global warming," he adds. "We agree that Covid-19 has no substantial global-scale climate effect, but its impact on regional climate may have been ignored – and East Asia is a hot spot."
As India and China have developed rapidly over the past few decades, there have been more and more aerosol emissions to contend with, says Andrew Turner, a UK-based scientist studying monsoons and a lead author of the sixth assessment report of the Inter-Governmental Panel on Climate Change (IPCC) released in August 2021. And the increase in aerosols has reduced the quantity of rainfall in the region over the years.
Observational data from 1901-2011 collected by Indian researchers show that south-west monsoon rainfalls have declined over the years. Some researchers believe that the decline has been aided by other factors as well – and one of these is rapidly warming waters over oceans.
"Our research shows that the Indian Ocean is the fastest warming ocean in the world," says Roxy Mathew Koll, a climate scientist in the Indian Institute of Tropical Meteorology in Pune who is studying marine heat waves and the changing patterns of monsoons across the Indian Ocean. "We've noted a rise in (surface) temperature of 1.4C from the 1950s onwards, especially in the western region including the Arabian Sea. This is higher than the global average of 0.7C for ocean warming over the same period," he says.
These rising ocean temperatures, he says, weakens the monsoon winds and the temperature gradient too – the difference in temperatures over land and sea which actually drives the monsoons. "This is causing a decline in monsoonal rains," says Koll.
While this may compound the pressures on the monsoons, researchers such as Turner believe that aerosols are the key factor in driving their decline so far. And in the long run, a warming world may in fact have the opposite effect on the monsoons.
Much of Asia's climate is governed by the monsoon rains, but human activity is profoundly changing the way these weather systems operate (Credit: Getty Images)
Historical studies of monsoon intensity millions of years ago have shown that when CO2 levels are higher, the monsoons are stronger. Higher CO2 levels mean that land warms more rapidly, creating a larger temperature difference between land and sea. In addition, a warmer atmosphere can hold more water – creating a greater potential for rainfall.
Global carbon dioxide emissions began a sharp, steep rise from the 1950s. The 10 warmest years on global record have been noted since 2005. With rising CO2, Turner says that the monsoon rains will reach a tipping point, when the monsoons will return to their previous strength and then possibly surpass it.
In the long run, CO2 will play the dominant role in the fate of the monsoons. "The big difference between carbon dioxide and aerosols is that carbon dioxide is well mixed in the atmosphere," says Turner. "We call it a well-mixed greenhouse gas, whereas the aerosol emissions tend to exist very close to their region of origin."
Another difference is that aerosols tend to have a much shorter lifespan than long-lived greenhouse gases such as carbon dioxide, says Wen Zhou, a professor at Fudan University, Shanghai, and a co-author of the study on Covid's effect on the East Asian summer monsoon. The aerosol effect on the climate can change rapidly, just as it did when the Covid lockdowns began and ended, she says.
What happens to the monsoons when both carbon dioxide emissions and aerosol emissions increase in the future? We could reach a tipping point, says Turner.
"At some point the effects of carbon dioxide will become dominant over the monsoon, if they have not done so already," says Turner. "Our findings from the IPCC report suggest that aerosol emissions have been the dominant factor driving the weakened Indian monsoon since the 1950s. And we know that in our future climate experiments with higher CO2 emissions (for instance, in the year 2100), we will have stronger monsoons."
When that tipping point will come, Turner says, is so far impossible to say.
Although the monsoons have been drying for decades, this trend may not last much longer (Credit: Getty Images)
When it does arrive, the outcome is likely to contribute to the rise of more extreme weather events like cyclones and floods.
"One of the many issues that has come out of the sixth IPCC report is that the more you increase the levels of global warming, the worse the impact gets," says Turner. "For every degree of global warming, heat waves when they happen get hotter, heavy rains get heavier and thus as a result, mountainous regions will see landslides, and it can lead to inundation of agricultural regions and crop damage."
In the future, this may leave the Asian region more vulnerable to extreme weather events. While the aerosol effect over the monsoons is easily reversible, (at least in theory, if governments take steps to control industrial emissions and clean up air pollution), the warming effects of long-lived atmospheric CO2 are harder to mitigate. Preparing for these extreme events now, says Zhou, is all the more critical.
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