The Intergovernmental Panel on Climate Change (IPCC) has predicted that due to climate change, there is likely to be a “lengthening of the monsoon” along with an increase in monsoon extremes such as heavy rainfall, floods and droughts. The recent 2021 monsoons show that these expected increases in the variability of rainfall patterns and extreme rainfall events have already begun. Over the past decade, India has seen measurable, significant changes in the climate, especially the rainfall patterns, when compared to the historical mean, including but not limited to:
- Erratic rainfall – Below/above average rainfall in different parts of the country. Change in annual rainfall seasonality patterns, the months of high rainfall historically receiving lower rainfall and vice-versa.
- The summer monsoon in 2021, though 'normal', was marked by a number of localised extreme rainfall events and displayed variability in patterns of rainfall dispersal. There has been unseasonal rainfall across the country, with August being lower than typical years and September being exceptionally higher.
- The monsoon started off at a healthy 110% of normal levels in June, dipped to 93% in July, dipped further to drought levels in August at 76%, and then bounced back to a rare 135% of LPA in September. The incidence of localised extremely heavy rainfall events in July and September, especially on the western coast, was among the highest in the past five years.
- October and November this year saw very heavy rainfall, 133% and 186% of LPA at a country level. Monsoon 2021 missed even the revised, extended dates for monsoon withdrawal, adding to the October uncertainty.
- Regional and sub-regional variation: The erratic rainfall patterns are not just observed at a country level but also at a regional and sub-regional level. The countrywide total summer monsoon rainfall in 2021 was unevenly spread over both time and across four broad regions. September saw exceptionally high rainfall in Gujarat and Rajasthan, with a 268% variation over LPA in Gujarat and 175% in Rajasthan. October and November saw exceptionally high rainfall in Andhra Pradesh, Tamil Nadu, Puducherry, Kerala and the south interior of Karnataka.
- Increasing frequency: An increase in the frequency of such events (erratic rainfall, changing rainfall seasonality pattern in a year, and regional variation) is witnessed. The Ministry of Earth Sciences climate change assessment for the Indian region highlighted that India was seeing more frequent dry spells and more intense wet spells during the summer monsoon season, a pattern witnessed in August and September 2021 as well. Over central India, the frequency of localised extreme single-day rain events exceeding 150 mm per day increased by about 75% between 1950 and 2015.
As per the IPCC's Assessment report, published ahead of the COP26 "There has been a noticeable declining trend in rainfall with monsoon deficits occurring with higher frequency in different regions in South Asia. Concurrently the frequency of heavy precipitation events has increased over India while the frequency of moderate rain events has decreased since 1950."
