India is heating up

The report of Intergovernmental Panel on Climate Change (IPCC) predicts that there will be a  3–6°C rise in temperature by the 21st century at a rate of 0.2°C per decade under higher emission conditions triggered further due to  urbanisation, industrialisation, deforestation, transportation, utilisation of fossil fuels thus increasing the atmospheric load of greenhouse gases (GHGs).  

This increase in the regional and local temperatures has been found to lead to irregularity of the Indian Summer Monsoon (ISM) circulation, which contributes almost 80 percent of the annual rainfall to the country informs this paper titled 'Will warming  climate affect  the  characteristics  of  summer monsoon rainfall and associated  extremes  over  the  Gangetic Plains in India?' published in Earth and Space Science.

The annual mean, maximum and minimum temperatures averaged over India show a warming trend of 0.15 °C, 0.15 °C and 0.13 °C per decade respectively since 1986. The maximum warming trend is seen during the pre-monsoon season for the recent 30-year period 1986–2015. 

All India mean surface air temperature is projected to increase in the future (2070–2099) and the semi-arid north-west and north India will warm more rapidly than the all India mean. The changes will be more pronounced for cold nights and warm nights. The frequency, duration, intensity and areal coverage of pre-monsoon heatwaves over India are  projected to increase substantially during the twenty-first century. And the effects are already being felt this year says this recent report by Climate trends published in March 2023.

India receives more than 70 percent of the rains in summer monsoon season (June through September). The intensity, length and timing of monsoon is influenced by the moisture content in the atmosphere, land–sea temperature contrast, land surface feedbacks, atmospheric aerosol loading, regional distribution of land and ocean, topography as well as other factors.

The monsoon is changing due to this heating effect

The monsoonal rainfall is projected to become more intense in the future and affect larger areas and this will mainly be due to increase in atmospheric moisture content due to increase in temperatures. Studies project an increase in extreme rainfall over the Indian region and a decrease in the mean rainfall. There has been a decreasing trend in summer mean rainfall over northern India post-1950, while there has been a threefold rise in extreme precipitation over the Central Indian region.

The decrease in mean India Summer Monsoon rainfall (ISMR) has been mainly because of increased warming over the Indian Ocean that leads to reduction in land–sea thermal gradient and a weakened monsoon circulation. However, ocean warming also increases the atmospheric temperature above the South Asian landmass leading to more evaporation and increasing the local atmospheric moisture. This leads to enhanced moisture convergence, which further results in increased extreme rainfall. This contrasting response further increases the complexities associated with future projections of ISMR.

Thus, frequency of extreme precipitation events may increase all over India, and more prominently so over the central and southern parts as a response to enhanced warming. Monsoon onset dates are likely to be early or not to change much, and the monsoon retreat dates are likely to be delayed, resulting in lengthening of the monsoon season.

Theses changes are already showing effects with extreme rainfall events resulting in floods and landslides  in Mumbai (July 2005), Kedarnath (June 2013), Chennai flood (2015) and Kerala (in August 2018) respectively, which caused huge loss of lives and property.

Studies show that there has been a three times enhancement in the frequency of extreme rainfall events accompanied by about a 20 percent decline in ISMR over the monsoon core region of India. The decline has happened due to a range of atmospheric and oceanic factors such as frequency of El Nino and La Nina southern oscillations (ENSOs), Indian Ocean warming, land use, convective available potential energy (CAPE), aerosol concentration etc.

The Indo Gangetic Plains (IGP) are experiencing monsoon extremes

IGPs form one of the most fertile lands of the world, covering 13 percent of the geographic area of the country and providing food for around 40 percent Indian population. Agriculture in the IGP is totally dependent on the monsoons and any changes in the monsoon can adversely affect the food supplies, health and economy of the region. However, little is known about the changes experienced in the Indian Summer Monsoon in the  region. 

This study provides an overall assessment of ISMR characteristics and associated extremes over IGP in present as well as its possible future aspects under the high emission scenarios. 

The analysis reveals:

• A substantial decline in mean ISMR while negligible changes in annual rainfall have been projected which shows that there has been a shift in the monsoon regime toward the later months of the year under warming climate.
• Future projections show slight variations in yearly mean rainfall while a huge decline in mean ISMR over IGP especially during far future.
• A decline is seen in the number of wet days (WD) and Simple/mean daily intensity of rainfall on wet days (SDII) in the range of 7 – 21 percent over the entire IGP except few western regions where increase ranging from ∼14 percent has been projected. The increasing WD and SDII over the western region may show a shift from an arid to a humid regime.
• Maximum 1-day and 5-day rainfall will also intensify by 21 to 35 percent over most of the IGP except lower IGP regions during the future under intense warming conditions. 
• There will be a massive increase in extremes over Himalayan foothills, i.e. the northern belt of the entire IGP region which may be due to enhanced anthropogenic aerosols.
•  Thus, there may be a weakening of monsoon circulations and a possible shift in monsoon season (other than JJAS months) over IGP in a warming climate. 

The fertile land of IGP is the main source of food-grain and water supply for the huge Indian population and the outcomes of this study can be used as a reference by the policymakers to implement location-specific adaptation and mitigation measures, argues the paper.

This is an open access paper under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License