Concerns about climate change continue to rise, despite our limited ability to make precise assessments of its impact on the economic and/or social environment. This is partly due to the complexity of chemical and physical processes at various scales, but also due to confounding factors from other changes in the environment such as air pollution. The combined effects from atmospheric brown clouds (ABC), and of the global increase in greenhouse gas (GHG) concentrations are good examples of such processes.
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 rainfall. 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, titled ‘Integrated model shows that atmospheric brown clouds and greenhouse gases have reduced rice harvests in India’ proposes that pollution and warming both have negative impacts on Indian agriculture. They developed a statistical model linking rice harvests (as a function of cultivated area and weather) and the area cultivated (as a function of the previous years’ area, prices, and weather) to climate indicators. Auffhammer et al. first 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.
They then tested for the potential effects of (hypothetical) removal of ABC and its effects on yields and the extent of the cultivated area. If only the yield effect is considered, harvests would benefit from cleaning the atmosphere, due to the higher amounts of rainfall. Although removal of ABC also increases temperature, this effect was less than the advantage obtained from rainfall gains. This was even stronger when the influence on cultivated area was considered, indicating that farmers have already reduced cultivations due to the negative effects of ABC. Avoiding global GHG emissions and therefore global warming resulted in even further increased harvests, making a clear case of combined negative impacts of haze and climate change.
Such assessments have profound implications for ongoing and future efforts to improve both climate and air quality. While reducing GHG emissions requires international agreements, it is more straightforward to reduce ABC at the scale of the Indian nation. V. Ramanathan has shown in another recent study 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.
The ultimate conclusion is that, while efforts to reduce GHG emissions must continue, it is also necessary at the technological, economical and social viability of wide implementation of available technology that reduces aerosol emissions in India but indeed all over the world.