India is heading for a drought, in meteorological terms, for the fourth time in the past 11 years. The previous droughts during this period were in 2002, 2004 and 2009.

A meteorological drought, in the sense that atmospheric scientists typically use the term, occurs when a monsoon ends with nationwide rainfall during the season falling below 90 per cent of the long-term average.

Such a string of droughts is not unprecedented. However, scientists have noticed some worrisome features in the monsoon in recent years. The question, inevitably, is whether human-induced factors driving climate change are involved, and whether they could affect rain in the coming years too.

There were five droughts between 1965 and 1975, and five more between 1979 and 1989, points out J. Srinivasan, who heads the Divecha Centre for Climate Change at the Indian Institute of Science in Bangalore. However, during the periods of 1965-1975 and 1979-1989, there were two ‘excess’ monsoon years when the nationwide rainfall quantum exceeded 110 per cent of the long-term average.

But there has not been a single excess-monsoon year since 1994. The balance between instances of drought and excess monsoon seems to have become skewed, he remarks.

“We are passing through a phase where the monsoon is tending to be on the negative side,” notes K. Krishna Kumar of the Indian Institute of Tropical Meteorology in Pune.

La Nina and El Nino

The monsoon has traditionally been sensitive to below-average temperatures in the tropical Pacific Ocean, known as La Nina, he points out. Such a cooling in the Pacific would usually produce copious rain over India and the monsoon would often end on the excess side. However, despite a La Nina that persisted for two years, the monsoons of 2010 and 2011 were only close to average. On the other hand, even a moderate El Nino can greatly retard the monsoon. “We need to understand what is holding the monsoon back and often pushing it to the negative side,” Dr. Krishna Kumar says.

Downward trend

There has been a downward trend in all-India monsoon rainfall since the late-1990s, observes Raghuram Murtugudde of the University of Maryland in the U.S. However, it is also important to bear in mind that the monsoon has cycles of high and low rainfall, and “it could be that we are in the middle of a low-rainfall cycle.” Even so, the severity of the last few droughts is unusual. “My worry is whether climate change is modulating and extending this long-term cycle [of low rainfall],” he told The Hindu.

Professor Murtugudde points to the steady and rapid warming of the Indian Ocean over at least the last 50 years. By reducing the land-ocean contrast, such warming could affect the monsoon. “We’ve to figure out what that means for the monsoon.”

In recent years, some high-profile research papers have pointed to fine particles in the atmosphere generated by human activity as having a major impact on the monsoon. These particles are borne in soot and in emissions by vehicles, thermal power stations and industrial plants. A paper published in Science last year by Massimo Bollasina of Princeton University, along with Yi Ming and V. Ramaswamy of the Geophysical Fluid Dynamics Laboratory in the U.S., came to the conclusion that such aerosols were weakening the monsoon over South Asia (see “Do aerosols have an impact on Indian monsoon?”, The Hindu, September 30, 2011).

However, S.K. Satheesh of the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, who works in this field, argues that more needs to be understood about the characteristics of aerosols found over India and their interaction with clouds before conclusions can be drawn about how these particles influence the monsoon.

Climate models

Climate models, which simulate the complex interactions over land, in the oceans and atmosphere, have grown in sophistication and become powerful tools. These models let scientists examine how factors like carbon dioxide build-up in the atmosphere, which produce global warming, and how aerosols can, individually and collectively, change climate.

Although these climate models predict global mean temperature well, they do poorly when it comes to the Indian monsoon and its variability, observes Professor Srinivasan. Some models show the monsoon increasing in strength as a result of climate change and others show the opposite.

“We know that climate change is going to affect the monsoon,” he says. “But there is still considerable uncertainty about just what’s going to happen, when and where.”

More In: National | News