Asian brown clouds, climate change and reduced rice harvests in India

By: Dr Wolfgang Cramer
Apprehensions about climate continue to rise, despite our limited ability to make precise assessments of its impact. This is partially due to the complexity of chemical and physical processes at various scales and also due to confounding factors from other changes such as air pollution.
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The combined effects from atmospheric brown clouds (ABC), and of the global increase in greenhouse gas (GHG) concentrations are good examples of complex chemical and physical processes at various scales. ABC block a part of the incoming solar radiation and thereby reduce direct warming effects from GHG. Climate change is an ever more serious issue for South Asia, particularly for the agricultural sector where both warming and reduced water availability may severely harm crop productivity. It is crucial to note that even a small change in climate may result in high social vulnerability of those dependent upon agriculture for their survival because many crops rely on the regular return of monsoon. Moreover, the economic potential to adapt is very low for most Indian farmers. The warming of recent decades (≈ 0.44°C since 1930) has likely affected crop yields through several mechanisms associated with direct temperature as well as changes in water availability.

A paper by Maximilian Auffhammer, V. Ramanathan, and Jeffrey R. Vincent in the Proceedings of the US National Academy of Sciences (December 2006), ‘Integrated model shows that atmospheric brown clouds and greenhouse gases have reduced rice harvests in India’, highlights that pollution and warming both have negative impacts on Indian agriculture. They concluded that June-September rainfall and October-November minimum temperatures had a significant influence on rice yield. Although higher rainfall could have allowed both larger areas to be cultivated and higher yields to be achieved, the higher night-time temperatures damaged crop development and therefore reduced yield. Hence, despite the ‘masking’ of global warming due to ABC (which occurs only during the day), the higher night temperatures outweighed the possible gain from this. V. Ramanathan has shown in another study (On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead, July 2008) that replacement of sootless cooking fuels could reduce the black carbon levels over the South Asian region by 60 per cent. This would not only clean the air and reduce damaging impacts on agriculture, but have additional positive effects on Indian water resources. This is because Himalayan glaciers would retain their white colour and therefore be less subjected to mass loss due to warming.

Auffhammer et al. do not claim that their model can be applied directly to other crops or to other regions. However, they demonstrate that rice cultivation in India has already seen considerable damage from air pollution and global warming. Importantly, they refute the argument that ABC might be ‘helpful’ for checking the impacts of global climate change in Asia. Current trends in ABC and warming continue to be negative, implying risks of nonlinear and drastic failures of the all-important monsoon system. These failures cannot be predicted with precision, but they become more likely, given the steady weakening of the temperature gradient above the Indian Ocean – which in turn has been linked to ABC and climate change. In conclusion, while efforts to reduce GHG emissions must continue,
it is also necessary that implementation of available technology that reduces aerosol emissions must be underway in India and indeed all over the world.

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