What is polluting cities?

It is now a part of record that several cities in India are among the most polluted in the world. The villain, in most cases, is aerosols and particulate matter. It is a catch-all term for particles of a certain size that are suspended in the lower reaches of the atmosphere. Aerosols emerge from a range of sources including dust, half-burnt carbon particles from vehicle exhaust and crop residues. Natural sources of aerosol include fog and haze. Studies, most of them in Europe, have drawn a link between particulate matter-levels and increased incidence of cardiovascular disease and respiratory problems. Now, it turns out, aerosols may be a grave threat to the Indian monsoon and maybe a bigger worry than greenhouse gases, like carbon dioxide.

What about greenhouse gases?

There’s an ongoing debate on the relative role of greenhouse gases, such as water vapour and carbon dioxide, and aerosols in their influence over the South Asian monsoon. To step back a bit, monsoon clouds gust into the mainland due to a pronounced difference in temperature between the land and the sea. Greenhouse gases trap heat and, over time, cause temperatures to rise over the land and the sea. This affects the temperature gradient between them and, over the decades, leads to a rise in the frequency of extreme rainfall or long, rainless spells. The IMD last year recorded 2016 to be the hottest year in India for over a century, and India has seen at least five drought years since 2002.

How are aerosols different?

Aerosols, while responsible for air pollution, smog and asthma, are known to shield the land from solar radiation. Though short-lived in the atmosphere compared to carbon dioxide, their absence would have made the earth hotter. However, the enormity of these dust clouds means that they depress land and sea temperatures. Consequently, this reduces the strength of the monsoon circulation. More than the quantity, it makes rain spells more erratic and because much of agriculture in India is still dependent on monsoon rains between June-September, they pose an additional threat to farmer livelihoods.

Why are aerosols a threat?

A study at the Indian Institute of Tropical Meteorology, Pune, led by climatologist R. Krishnan made us aware of aerosols. It is based on the institute’s ongoing work to forecast the effect of greenhouse gases — responsible for global warming and climate change — on Indian monsoon, over the next century. In 2015, Mr. Krishnan reported in the peer-reviewed Climate Dynamics that a combination of greenhouse gases, aerosols and changes in forest-and-agricultural cover was weakening the monsoon over the last 50-odd years. They derived this from mathematical modelling and simulating the climate on supercomputers. Then, the relative contribution of each of these factors were not known. “New simulations however suggest that aerosols may be a far more important factor than GHGs,” said Mr. Krishnan while discussing preliminary findings at a meeting of the Indian Academy of Sciences in Bengaluru last week.

What does this mean for India?

The relative role of these climate-meddlers has consequences for India’s plans to mitigate the effects of climate change. Measures to reduce aerosol emissions without curbing greenhouse gas emissions could mean a hotter land mass and more instances of untimely, extreme rainfall events. Persistent aerosol emissions might lead to more instances of moderate rainfall but could mean anomalous weather and health hazards over large parts of north India. The aerosol-greenhouse gas relationship in exacerbating climate change is an old area of research but teasing out the relative contribution of each is challenging and influences the costs countries must incur to address them.

India has generally maintained that man-made carbon dioxide pollution is largely due to the years of pollution by the developed West. However, such an argument might weaken if aerosols were brought into the picture because this is a largely South Asian concern. Were carbon dioxide and aerosol interactions proven to be strongly linked, India could be under pressure to adopt more stringent climate-proofing policies.