Today, any natural calamity relating to weather often raises the concern of ‘climate change’. The floods that have wreaked havoc in Chennai have also raised this concern. Given the recurring pattern of such calamities in the recent past, one can say that climate change is possibly one of the culprits. The other reason is natural variability in climate, which is quite a different phenomena.
The Chennai disaster is a result of a set of meteorological and climatic events happening at the same time. It is not a freak event, but also not a predicable one that can be solely attributed to a single phenomenon such as climate change.
Madhavan Nair Rajeevan, Director, Indian Institute of Tropical Meteorology (IITM), Pune, sheds light on the causes, “Due to El Nino, we were expecting prolonged rainfall spells during the northeastern monsoon season. It could be attributed to natural climate variability. However, the heavy rains on 2-3 December may not be. We have documented that in both the southwestern and northeastern monsoon seasons, the frequency of heavy rainfall spells is increasing partly due to global warming.”
Northwestern monsoon this year appears to be influenced by a remote, but globally-felt, phenomenon called El Nino- Southern Oscillation (ENSO) comprising fluctuations in ocean temperatures in the equatorial Pacific. El Nino involves unusually warm ocean temperatures and its counterpart La Nina unusually cool temperatures. These fluctuations affect weather patterns across the world. El Nino literally means ‘little boy’, referring to Infant Jesus as it occurs around Christmas. La Nina means the little girl.
J Srinivasan, chairman of the Divecha Centre for Climate Change at the Indian Institute of Science (IISC), Bangalore shares the view: “People have attributed the floods in Chennai to El Nino because the last time the November rainfall exceeded 1000 mm in Madras was in 1918 which was also an El Nino year. However, there have been many El Nino years when the November rainfall in Chennai was not high (1972, 1982, 1987, 2009, 2012). Hence additional factors may have played a role. The other candidate is warming in the Indian Ocean.”
Srinivasan remembers the similarly heavy rains that Chennai received on 25 November 1976 (450 mm in one day) but he does not recall such devastation. “That year, Kanchipuram received 3 times the normal rainfall in the same period but the flooding was not so bad because the urbanisation and illegal construction was much less.”
According to the US National Oceanic Atmospheric Administration (NOAA) Climate Prediction Centre, strong El Nino continued this season as indicated by the well-above-average sea surface temperatures across the central and eastern equatorial Pacific Ocean. The World Meteorological Organisation (WMO) acknowledges that a strong and mature El Nino continues in the tropical Pacific Ocean. Climate models indicate that it will strengthen before the end of the year, pushing the peak three-month average surface water temperatures in the tropical Pacific Ocean, exceeding 2 degrees Celsius above average. That makes this event among the three strongest since 1950.