Over the second half of the 20^th century the monsoon rains in the Ganges Valley in north and northeast India decreased by 10 percent. Meanwhile monsoon rains in the south and the Indus Valley [remember 2010 Pakistan floods] increased.

Burning of fossil fuels and wood fires is causing the Indian monsoon disturbance, but not in the way most people think, show Princeton researchers.

Droughts hurt agriculture in the northeast and people in the south and west suffer from floodings. Often this is blamed to climate change – and although climatic chance is what is in fact happening – under question is a local phenomenon, which has little* connection to the rise in average temperatures that is caused by greenhouse gas emissions.

[*) ‘Little’ does not mean no connection. A main consequence of the current global climate change is an increase in the force of the general circulation. Theoretically this should result in stronger precipitation over the Inter Tropical Convergence Zone (ITCZ) – thus a generally stronger monsoon – and an increase in droughts during the dry season. So from theory we learn a drying of the Ganges Valley monsoon is hard to link to global warming – but there could be a contribution to the damaging monsoon rains increase in the southwest of the country.]

Normal monsoon

The monsoon is formed when during northern hemisphere summer the Sun warms Eurasia more than it warms the surrounding oceans. Over India air rises and low pressure systems are formed. This sucks in a compensating air flow from the Indian Ocean and Arabian Sea, which carries a lot of moist.

As over India in turn this air rises too – it cools, and the water condensates, raining down: monsoon.

Halfway monsoon

The general movement of this process is from the southwest to the northeast. If the northeast becomes drier and the southwest wetter, it means somehow the monsoon is blocked and the ITCZ gets stuck halfway over the subcontinent.

Perhaps the culprit isn’t warming, but cooling instead, relative cooling that is – of the northeast of the country – in turn sucking in less of the moist air from the southwest.

‘It is local aerosol pollution’

The Princeton group write in Science they think they know what is going on. Or at least, their climate model offers a strong indication: aerosols.

The cumulative effect of 1.2 billion living, working, acting people, consuming electricity from Soviet era coal-fired power plants, burning wood stoves, driving motors cycles and cars, mass-producing goods at prices the West cannot resist.

Although all these combustion and oxidation processes create a lot of climate-warming soot, apparently that does not way up to the large amounts of climate-cooling sulfur – light-coloured aerosols, which the wind accumulates in the northeast and which increase the atmosphere’s local albedo, thereby reflecting solar radiation back to space, slightly, but noticeably cooling the air.

Climate consequence of environmentalism

Because aerosol pollution in China, India and other parts of Asia seriously affects public health, the authors think in decades to come India may implement a range of aerosol emissions reducing measures.

If so the summer droughts were just a temporary phenomenon, and the monsoons could recover.

For the global climate perhaps that wouldn’t be such good news. Replacing old-fashioned coal plants with state of the art ones does little to reduce CO2 emissions, but filtering sulfur aerosols does take away that one ‘benefit’ of coal many of us have silently and unknowingly enjoyed for years: the rising Asian coal industry is one of the factors [apart from solar variation, ENSO and ocean storage] that has contributed to a temporary slow-down in the global temperature rise over the last decade or so.

As solar activity is picking up, it is unlikely La Niñas will keep dominating another decade, specialists say upper ocean warming will resume, greenhouse gas concentrations simply keep rising, and sulfur aerosols would decrease – the world definitely seems in for some 2010-breaking climate records.

500 million extra aerosol emitters

Abatement of the Indian aerosol emissions is however only an assumption. Who could ever be so sure of successful future environmental policy in light of possibly overwhelming trends, economic, political… or demographic? One other thing that is forecast to happen in this half of the 21^st century is for the Indian population to rise close to 1.7 billion people. If cumulative human activity is apparently this important, simply counting individual numbers of humans is that one assessment step we so often – so strangely – tend to ignore.

© Rolf Schuttenhelm | www.bitsofscience.org