The Ocean as a Giant Heat Battery
At the heart of this phenomenon is a simple scientific principle: water has a very high heat capacity. This means it can absorb and store a massive amount of energy without its own temperature increasing dramatically. Throughout the day, the ocean soaks
up solar radiation like a colossal, heat-retaining solar panel. While land heats up quickly under the sun and cools down just as fast at night, the ocean holds onto its warmth far more effectively. This property, known as thermal inertia, makes the ocean a huge reservoir of heat, absorbing over 90% of the excess warmth trapped by greenhouse gases in our climate system.
How Nighttime Warming Works
During the day, land is typically warmer than the sea, creating a pressure difference that pulls cooler, denser air from the ocean towards the land—the classic sea breeze. At night, this process can reverse. The land cools down rapidly, but the nearby ocean remains warm from the heat it absorbed all day. This temperature difference means the sea surface continues to radiate heat. This warmth, combined with high humidity from evaporation, gets carried over to adjacent coastal areas by gentle onshore winds, preventing land temperatures from dropping as much as they would further inland. Instead of providing a cooling effect, the warm sea acts like a blanket, keeping coastal cities warmer through the night.
The Indian Context: More Frequent and Intense Heat
For India, with its extensive 7,500-kilometre coastline, this effect has significant consequences. The Arabian Sea, in particular, has been warming at an alarming rate, faster than the global average. Studies show that this warming is leading to more frequent and intense marine heatwaves. These prolonged periods of unusually high sea surface temperatures are not just an underwater issue. They directly influence the weather on land, especially in bustling coastal metropolises like Mumbai, Chennai, and Kolkata. The increased heat and moisture from the sea can exacerbate heatwaves on land, making nights particularly oppressive and dangerous.
Broader Implications for Health and Weather
The impact goes beyond just uncomfortable nights. Sustained high temperatures, especially when combined with high humidity, pose a serious risk to public health, increasing the chances of heat exhaustion and heatstroke. This humid heat makes it harder for the human body to cool itself through sweating. Furthermore, warmer oceans fuel more powerful and rapidly intensifying cyclones in the Arabian Sea and Bay of Bengal, posing a greater threat to coastal communities. The added moisture in the atmosphere can also lead to more extreme rainfall events. This complex interplay shows that the warming ocean is fundamentally changing weather patterns, from daily temperatures to the intensity of major storms.
