Mumbai has been experiencing significant temperature fluctuation over the past two weeks, with an average difference of about 10 degrees Celsius between maximum and minimum temperatures. At times, particularly during the last week of January and first week of February, this gap has widened to as much as 16 degrees Celsius. While afternoons brought warmth, early mornings and nights carried a noticeable chill in the air. According to the Indian Meteorological Department's (IMD's) weekly forecast, this significant gap in temperatures is expected to persist until at least February 15.
Some variance in day and night temperatures is typical during seasonal transitions (Mumbai is currently transitioning out of winter). The IMD also reported a shift in wind patterns due to an anticyclonic phenomenon. The resulting winds contributed to rising mercury levels. This change in wind direction remained prevalent until early February, maintaining maximum temperatures between 34 and 35 degrees Celsius.
However, this temperature variability has brought focus to diurnal temperature range (DTR) and its implications.
What is diurnal temperature range (DTR)?
DTR refers to the difference between the maximum and minimum temperatures experienced within a single day, and varies significantly across India. Regions like the peninsular area exhibit lower DTR due to the moderating influence of nearby oceans, whereas the northwestern part experiences higher DTR due to factors such as lower rainfall and distance from the sea. But there can be variations.
“We know that global warming is causing an overall increase in average surface temperature, but the rise is not uniform across day and night temperatures,” says Tanmay Kumar, an independent climate change researcher based in Chennai. For instance, a 2023 study indicated a decline in DTR by approximately 1.5 degrees Celsius in Delhi and 0.2–0.4 degrees Celsius in Mumbai during the period from 1991 to 2018.
It’s not the first such a change has come to light. In 2020, a research team from Banaras Hindu University published long-term temperature trends from 1951 to 2016, and recent changes from 1991 to 2016 across various agro-climatic zones of India. The study revealed a decline in DTR over the past 30 years in the northwest, parts of the Gangetic plain, and central India. However, they also observed an overall increasing trend in DTR, maximum, and minimum temperatures over the long term, and an increase in the temperature range along the East and West coasts and in the Southern part of India.
A recent study conducted by researchers from Chalmers University of Technology in Sweden has shed more light on long-term DTR variations. According to it, daytime temperatures have risen more rapidly globally since the 1990s, while nighttime temperatures have remained relatively stable. This alteration in the pattern has resulted in an increased gap between day and night temperatures.
What’s the cause?
One likely explanation for the rise in daytime temperatures, according to experts, is a phenomenon called “global brightening”, observed since the late 1980s. It results from reduced cloud cover, allowing more sunlight to reach the Earth's surface, leading to higher daytime temperatures.
Presently, there is considerable uncertainty about the factors influencing changes in cloud cover. A complex interplay between cloud-free and cloudy atmospheres, along with the impact of atmospheric aerosols generated by natural processes such as sea spray and wildfires, as well as human activities like fossil fuel burning, may be contributing to this phenomenon. Another probable cause for DTR variations could be the increase in regional drought events and heatwaves.
Brace for impact
The impact is potentially profound. Changes in DTR can lead to an increase in the risk of droughts and heat stress, crop failure, and an increase in morbidity rate.
Dr Manoj Kamath, a general practitioner based in Mumbai, explains that when the temperature difference exceeds 10-degrees Celsius, it significantly increases the likelihood of colds, coughs, viral, and respiratory infections. Experts also recognise huge temperature variations as an environmental stressor. “It could lead to elevated heart rate and blood pressure, subsequently increasing the workload on the heart and the risk of cardiovascular diseases,” says Kamath.
Furthermore, it affects the ecosystem at large. “Studies have shown that temperature variability has a significant impact on population viability of endothermic species. It could also affect the growth and survival of aquatic species. Similarly, the increased temperature difference between daytime and nighttime might prove disadvantageous for trees in dry regions, as higher daytime temperatures may increase evaporation, leading to soil water deficiency and unfavourable conditions for tree growth,” says Kumar.
Such fluctuations could also have severe consequences for agriculture and crop yields. Plants are highly responsive to temperature variations. Studies show that climate stress could lead to an imbalance in plant hormones, impairing plant growth, photosynthesis, pollen development, and reproduction.
Addressing these challenges, however, will require greater understanding of DTR, and concerted efforts to mitigate climate stressors and foster resilience across ecosystems and human communities alike.
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