The Hindu Business Line : `Check aerosols, retain green cover for better monsoon performance'

AFFORESTATION programmes in India are at best a stopgap arrangement and certainly not enough to compensate for human activities that threaten to force a `man-made reduction in monsoon precipitation' in the country, says a leading climate researcher.

The only way to make sure that the Indian monsoon does not undergo significant changes is to limit human interference with the climate system, according to Dr Kirsten Zickfeld of the Potsdam Institute for Climate Impact Research, Germany, who lead-authored a recent paper on the stability of the Indian summer monsoon against global changes.

The monsoon could run dry either due to stronger aerosol load or forest clearance; alternately, it can become more intense due to global warming caused by rising levels of atmospheric carbon dioxide.

Dr Zickfeld and her team described these as the possible two `stable monsoon regimes'. The worst-case scenario would be a "roller-coaster", in which the drying up would be followed by an even more intense wet monsoon.

It can be seen that an increasing concentration of greenhouse gases (GHGs) would have an opposite effect on the Indian monsoon as distinct from that of a stronger aerosol load.

The reason is that more GHGs in the atmosphere lead to a temperature increase over land and thus to stronger precipitation.

Currently, however, it is not possible to determine which one of these two effects — the drying due to aerosols and forest clearance or the intensification due to GHGs — will dominate.

The monsoon is driven by an air-pressure difference between the land and the Indian Ocean. Usually, the hot season creates low-pressure zones over the warm continent. Air rushes in from the higher-pressure zone over the water, bringing rain to the land.

Anything that reduces this pressure difference, such as cooler land temperatures, can weaken the monsoon. Once the weakening exceeds a certain threshold, the climate switches into a new state in which moist air over the ocean is no longer carried inland.

Several factors are causing less sunlight to warm the ground in India. There are more aerosols because of industrial growth and greater vehicle use, which reflect light back into space.

And clearing forests for farmland is replacing dark, light-absorbing treetops with lighter, more reflective soil.

India will do well to reduce emissions of greenhouse gases and airborne particles as well as stop clearing of forests, two human interventions of the earth system that threaten to weaken the summer monsoon circulation.

The research paper stops short of making any kind of `predictive statement' about the fate of the Indian monsoon.

"The reason is twofold: first, the model on which our analysis is based is rather crude. Second, in order to assess the probability and timing of significant changes in the monsoon, we would need to take into consideration realistic scenarios of land use change and air pollution in South Asia," says Dr Zickfeld.

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