Why Bangladesh is seeing such a cold winter
It may seem counterintuitive, but the extreme cold spell currently sweeping through Bangladesh and elsewhere seems like a good time to remind us that climate change means much more than an increase in dangerous heat waves during the summer months. Nevertheless, like clockwork, every time the temperature in winter drops precipitously, deniers of climate change like to remind us, "It is bitterly cold out there! Where is all that global warming and climate change that you guys talk about?"
Everyone who understands the science of climate change knows that the arguments presented by the deniers are bogus and irrational, and are often presented in bad faith. In reality, winters across the contiguous US have warmed by an average of nearly 1.5 degrees Celsius over the last half century. But "less cold" does not necessarily mean "never cold." So even as winters on average have been getting shorter and warmer, many places should still expect to see bouts of very cold weather—at least for the foreseeable future.
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Paradoxically, there is growing scientific evidence that extreme cold temperatures are an outcome of human-induced climate change. Indeed, a study published in 2017 in the journal WIREs Climate Change lays out how the warming Arctic and melting ice appear to be linked to cold weather being driven farther south.
As a matter of fact, extreme cold spells in the Northern Hemisphere are caused by distortion of the polar vortex―a cold air mass which, under normal circumstances, sits above the North Pole in a large, low-pressure zone. It exists at two levels of the atmosphere: one in the troposphere, where most of the weather phenomena occur, and the other a bit higher up, in the stratosphere, home of the ozone layer that protects us from the harmful effects of ultraviolet radiation. Like a spinning top, the vortex rotates counter-clockwise, keeping blisteringly cold air locked in the Arctic region.
However, scientists believe that global warming distorts the vortex, thereby resulting in a sudden plunge in temperature south of the Arctic Circle. That is because increases in global temperature are not evenly spread around the world. They are greater on land and at higher latitudes. Consequently, on average, Arctic temperatures have increased in recent decades at about twice the global average.
As a result of warming, more ice of the Arctic Ocean is melting during the summer months. As the ice melts, the Arctic ice sheets reflect less sunlight, causing the Arctic Ocean to absorb more heat, which it then releases into the atmosphere, adding to global warming. This process, and other Arctic feedback loops, are known as Arctic Amplification. Eventually, the amplification has a ripple effect extending well into the stratosphere, weakening and distorting the polar vortex, thereupon allowing the air to escape south. In other words, instead of staying where it belongs in winter, closer to the Arctic Circle, the air moves down south into continental United States, Europe, and Asia. Clearly, even a tropical country like Bangladesh cannot escape the wrath of the distorted polar vortex.
There is another factor that affects winter temperatures. It is the jet stream―a narrow band of strong wind in the lower atmosphere that generally blows from west to east all across the globe and acts as a divider between warm and cold air. Evolving research suggests that the loss of Arctic sea ice and rise in average global temperature, along with distortion of the polar vortex, trigger the jet stream to slow down and become increasingly erratic. In the winter months, this allows bone-chilling cold Arctic air held in place by the once stronger jet stream to spill much further south, effecting a sharp drop in temperatures.
While climate scientists are predicting that our planet could warm, on average, roughly two degrees Celsius by the end of the century, we should not interpret that to mean an end to bitter cold waves during winter―at least not yet. Cold blasts will still occur; but depending on how much greenhouse gases we dump into the atmosphere, they will become rarer over time. It is thus unlikely that we will see more cold snaps in the future, but the ones we are experiencing now are more likely to be persistent, or at least last a longer amount of time. In other words, in the future, in addition to extreme summer temperatures, winter temperatures will gradually warm across the globe.
As for the fog, it is due to temperature inversion―a phenomenon where ground-level pollution causes the ambient air temperature to increase with altitude instead of decreasing, giving rise to colder air near the ground and warmer air above it. This creates a warm air lid over cooler air. Temperature inversions can occur anywhere from the ground level up to a few thousand feet into the atmosphere, and they generally occur during winter months, when nights are long and cold.
In an area experiencing temperature inversion, the warm air lid prevents ground-level air from rising. Hence, the cool, dense ground air cannot mix vertically. In fact, the air is so stable that it is quiescent and pollutants become trapped below the warm layer of air, creating dirty air with dangerous concentrations of noxious pollutants. In areas with heavy pollution, and high humidity in the cooler layer of the atmosphere, thick ground-level fog will form.
Temperature inversion begins to form a few hours before the Sun sets. The situation is reversed in the morning when sunlight strikes the Earth and vertical mixing of the air begins. Nonetheless, depending on the severity of pollution, the duration of inversion can vary from a few hours to several days.
In conclusion, the message from the recent cold waves is loud and clear. Our romance with fossil fuels has fundamentally changed the global weather systems to the point where we have to do something drastic if we want to live on the only habitable planet in the solar system beyond the 21st century. Therefore, we should not take the bait from climate change deniers that global warming is a hoax, because they are the ones with an interest in protecting the fossil fuel industry.
Dr Quamrul Haider is professor emeritus at Fordham University in New York, US.
Views expressed in this article are the author's own.
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