Extreme rainfall events may be due to greenhouse gases
Greenhouse emissions from human activities play a significant role in extreme rainfall events across India, says a study by the Indian Institute of Technology – Gandhinagar (IITGn) and US-based Lawrence Berkeley National Laboratory.
The study has found that greenhouse gases emitted by industries, vehicles, biomass burning, and deforestation – also called anthropogenic, or man made, emissions – cause an increase of 10% to 30% to extreme rainfall events in India. Greenhouse gases include carbon dioxide, methane, nitrous oxide, and the gases are blamed for trapping heat in the earth’s atmosphere, leading to global warning. Extreme rainfall events are described as periods of intense rain between one and five days.
The influence of these gases on heavy rainfall events was more pronounced in south and central India as compared to other parts of the country. These regions are also likely to witness a rise in the frequency of precipitation extremes by mid and end of 21st century. This means, extreme rainfall events that occur once or twice over 30 years will take place once every two years in the south and central India.
The study, which has implications for infrastructure, agriculture, and water resources, is important because India has seen a rise in extreme rainfall events over the last decade. The July 2005 deluge in Mumbai that claimed more than 1,000 lives, floods in Uttarakhand in 2013 killing around 6,000 people, and the more recent floods in Tamil Nadu (2015) and Bangalore (2016).
“In general, warmer climate due to anthropogenic forcing increases the moisture holding capacity of the atmosphere. Increased moisture in the atmosphere results in more intense extreme rainfall events,” said Vimal Mishra, associate professor, civil engineering department at IITGn, and lead author.
Previous studies have looked at trends in observed rainfall extremes, but did not quantify the contribution of anthropogenic warming on one to five day extreme rainfall totals.
The study by the four-member team used two data sets – from detection and attribution projects and CMIP5 – to quantify the role of anthropogenic warming on extreme rainfall over India. Two scenarios fed into the model simulations comprised one that factored in carbon dioxide emissions from natural factors such as volcanic eruptions, and the other that took into account emissions due to manmade activities.
Most research till now has focused on the role of surface air temperature during extreme rainfall events to find a negative relationship. The IITGn led study used Dew Point Temperature (DPT) – temperature at which saturation of water vapour takes place – since it is considered as a better predictor of precipitation extremes, and found an increase in both extreme precipitation events and dew point temperature. As maximum rainfall increased in the range of 5-15% between 1979 and 2015 in western, central, and peninsular India, so did dew point temperature by 0.25-0.50 degrees Celsius during the same period.
“Since surface air temperature is affected by rainfall during monsoon, it does not provide a robust relationship between rainfall extremes and temperature over the tropics,” said Mishra.
Annual maximum precipitation (AMP) did not increase over some regions including the Gangetic Plain, north eastern India, and Jammu and Kashmir. “The decline in AMP in the Gangetic Plain region can be attributed to a significant reduction in the monsoon season rainfall driven by the increased atmospheric aerosols, and warming of the Indian Ocean,” stated the study.
The study ‘Increase in extreme precipitation events under anthropogenic warming in India’ was published in Weather and Climate Extremes, an international peer-reviewed journal, on March 22.
MUMBAI: