Thanks to climate change, nights are warming faster than days

In 1895, while studying past Ice Ages, Swedish chemist Svante Arrhenius showed that if carbon dioxide levels in the atmosphere were halved, temperatures could decrease by about five degrees Celsius. He also showed that temperatures would increase by the same amount if the amount of carbon dioxide was doubled. His work demonstrates that the right concentration of carbon dioxide is essential for sustaining life on Earth.

Arrhenius' study may have been purely of theoretical interest, but decades later, modern climate models have confirmed that his numbers are not far off the mark. Today, we know that too much carbon dioxide in the atmosphere has made our planet hotter, triggering widespread changes in the Earth's weather patterns. In fact, this summer kicked off with searingly hot temperatures for many of us in the northern hemisphere. New all-time highs were set across the United States, in western Canada and in towns near and above the Arctic Circle in Europe. The end is not yet in sight. The flashing red spots on weather maps are signalling another heat wave, this time covering most of the US.

One of the most "perverse" effects of climate change is balmy nights. During the last 50 years or so, nights are warming at a much faster rate than days. There are several plausible scientific explanations for this asymmetrical warming. First, the greenhouse effect responsible for global warming operates 24/7. Second, nighttime temperatures are inherently more sensitive to climate forcing.

More importantly, Earth's "energy imbalance," which is the difference between the amount of solar energy absorbed and the amount of energy emitted, almost doubled in the last 15 years. Obviously, any increase in the energy imbalance means the overall climate system of the planet is gaining energy, thereby raising both the nighttime and daytime temperatures. However, during daytime, the extra energy is spread through a thick layer of the atmosphere, up to a few kilometres from the ground. So the air warms slowly. At night, the extra energy is trapped in a thin layer, just a few hundred metres near the ground. Thus air at night warms quickly.

Warmer nights are more pronounced in big cities than rural areas due to the heat island effect, a term used to describe higher air temperatures in an urban setting as opposed to the lower temperatures found in rural areas. Cities and their suburbs have a lot of asphalt roads, pavements and concrete structures whose surfaces absorb copious amount of solar radiation during the day but does not dissipate it as quickly at night. Hence, warmer nights.

One of the effective strategies to make nights relatively comfortable is by painting roofs with white reflective paint to reduce heat absorption. As a longer-term plan, some large cities are using lighter colour and water-retaining paving options for roads. Furthermore, planting trees and other vegetation, preserving lakes and rivers, and creating artificial water surfaces and large-scale heat retention expanses, can keep the night as well as day temperatures under control.

Hot summer nights can lead to significant health-related problems, because they take away our ability to cool down from the overly high temperature. One of the main mechanisms for our body to cool down to a core temperature of about 37 degrees is through sweating. When temperatures fail to drop at night, sweat does not evaporate as efficiently. Less evaporation means the body has trouble cooling itself off. As a result, the sweat clings to our body and body temperature can, in effect, rise. The body nevertheless works harder and harder to lower its temperature, putting more stress on organs like the heart. At the same time, too much sweating leads to the loss of fluids and electrolytes in our body.

In addition, there is a threshold temperature­—35 degrees—beyond which the human body cannot sweat enough to cool itself down. Known as the "wet bulb" temperature, the threshold is determined by wrapping a wet wick around the bulb of a thermometer. The ambient temperature, which is always greater than the wet bulb temperature, is called the dry bulb temperature. The difference between these two temperatures is a measure of the humidity. When wet bulb temperatures are very high, the difference is close to zero and the humidity of the air is well-nigh 100 percent. Consequently, sweating becomes ineffective at removing the body's excess heat.

Steamy nights spell trouble for people with underlying health conditions, all at a time when the body is supposed to be resting. Age can also be a factor in how we respond to heat while we are asleep. Young children and elderly people are more likely to experience heat-related illness than older children and adults.

While air conditioning can provide a respite from intense heat, it is not a panacea. Air conditioners work by dumping hot air from inside a house to an even hotter air outside, thus adding more heat to the atmosphere. Besides, fossil fuels that provide power for air conditioners exacerbate climate change. Also, increased use of air conditioners tax electrical grids, making power failures more likely. Clearly, air conditioners make a sizeable contribution to global warming.If air conditioners are climate unfriendly, then how about fans? Contrary to popular belief, fans do not cool air, they just move it around. Having a fan blow air that is hotter than our body temperature can actually make it more difficult for our body to shed heat by sweating. But if the indoor air temperature is in the low 30s or below, turning on a fan can create an artificial breeze that will help evaporate sweat from our skin, making us feel cooler. Moreover, if the blades of ceiling fans rotate counterclockwise, they will push cooler air down towards the floor, producing a somewhat summer breeze effect. Clockwise rotation will raise the temperature as it will create an updraft that will send the warmer air near the ceiling back into the living space.

Finally, as we try to cope with precedent-shattering extreme temperatures, one overwhelming conclusion is: Thanks to human-induced climate change, temperatures are increasing rapidly during nights. And this trend is likely to continue in the coming decades unless we are serious about arresting global warming.

Quamrul Haider is a Professor of Physics at Fordham University, New York.