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Climate change has been the talk of the town for the last two months as the milestone UN climate summit, COP26, took place in Glasgow, UK, in November. Back home, winters have set in, and the weather has turned pleasant in most places in India. Come summer, as the mercury levels start to breach the 40°C-mark, a fear of deadly heat waves and extremely hot days grips many parts of the subcontinent.
The most critical consequence of increasing carbon dioxide in the atmosphere is the rise in the number of hot days. Many wonder how an increase in global mean temperature around 1°C can have such a large impact on the number of hot days.
Hot days on the rise
To understand this better, let’s look at the distribution of maximum temperature in Bengaluru during two 20 year periods. During 1969-1988, the number of days on which the maximum temperature was 34°C was around ten days. However, during 1994-2013, such days were 60 in number. This six-fold increase in the number of hot days is due to a combination of factors like the increase in carbon dioxide and the urban heat island effect.
Large portions of our urban landscapes are composed of buildings and roads. These regions absorb solar radiation and retain it for a longer period. The lack of greenery in many urban areas prevents the cooling of these regions by evaporation. The hot days become severe when certain weather conditions lead to more clear days, and the air descends from above.
Hot days become even more tantalising when the cities are located in coastal areas. During a hot day, we maintain our internal body temperature at around 37°C by evaporating sweat from our skin. This becomes more difficult if the air outside is humid. This humid air prevents the evaporation of sweat from our skin. Moreover, the higher water vapour in the coastal areas increases the amount of earth’s radiation trapped by water vapour. Many people do not realise that water vapour in the atmosphere traps more heat than carbon dioxide because there is more water vapour in the atmosphere than carbon dioxide.
During the past 50 years, the number, severity and duration of heatwaves have increased, and this causes more and more deaths during heatwaves in summer. Heatwaves have an adverse impact on crops also as they need more soil moisture during the heat waves, which may not be available in areas that are not irrigated.
Unpredictable extreme rainfall events
The unpleasant summer heatwaves are followed by spells of extremely heavy rainfall during the monsoon season. These extreme rainfall events typically span a few hours to a couple of days. India has long-term observations of ground-based rainfall measurements, and they show that the number of extreme rainfall events has been rising in the past few decades.
The spatial aggregation tendency of these extreme rainfall events in recent decades is even more concerning. Such co-location of heavy rainfall increases the risk of floods and related damage. The spatially large extreme rainfall events with sizes larger than about 70,000 square km have been rising at a rate of about one per cent per year since the 1950s. Several recent floods in India are associated with such events. These sporadic extremes also disrupt the quasi-rhythmic nature of the active-break cycle of monsoon rainfall, impacting the seasonal total.
It is less evident how increased atmospheric CO2 can increase extreme rainfall events. The maximum water vapour holding capacity of the air, which varies with temperature, may be used to explain the observed changes in rainfall extremes. A differential increase of the water vapour holding capacity with height in the troposphere can change the possibility of the occurrence of extreme rainfall events. In addition, the large-scale rainfall events of the Indian monsoon require an anti-clockwise air circulation at about 3 to 7 km above the surface.
The sporadic nature of the extreme rainfall events makes them poorly predictable. Although recent advances in data assimilation and numerical modelling have improved the prediction skill of such extreme events, much more extensive research on their nature and origin will help forecast them better in the future, saving life and property.
J Srinivasan is presently a Distinguished Scientist at the Divecha Centre for Climate Change of the Indian Institute of Science (IISc), and formerly Professor at the Centre for Atmospheric and Oceanic Sciences, IISc. He specializes in the areas of fundamentals of global monsoon and thermal sciences.
Arindam Chakraborty is a Professor at the Centre for Atmospheric and Oceanic Sciences of IISc. He specializes in the areas of the theory and prediction of monsoon onset, and global teleconnection.
This article is a guest column reflecting the author’s opinions and does not necessarily represent the official views of The Weather Channel.