Global warming, local cooling: Winter will bite Bangladesh harder this year

A common misconception about climate change and global warming is the belief that these phenomena can solely be reflected in the form of extreme heat waves during the summer months, with winter seasons becoming consistently milder. 

This led many in the country to be genuinely caught off guard when, after the delayed arrival of winter, a severe cold wave swept across the country in January this year. It was also accompanied by chilly winds and dense fog. 

This, however, was not a one-off occurrence. It happened before, and it is very likely to happen many times in the future. 

According to the Bangladesh Meteorological Department's (BMD) weather outlook for November 2024 to January 2025, the country may experience eight to 10 mild to moderate cold waves during the upcoming winter. Of these, two to three could escalate into severe cold waves, with temperatures dropping as low as 4 degrees Celsius in the northwestern, northeastern, and central regions. 

BMD classifies a severe cold wave as temperatures falling below 6 degrees Celsius in extensive areas, often affecting daily life and posing health risks. Temperatures ranging from 6-8 degrees Celsius are defined as moderate cold waves, which can also cause discomfort and impact agriculture. Meanwhile, those between 8-10 degrees Celsius are categorised as mild cold waves, typically resulting in chilly but manageable weather conditions. 

"The primary reasons, aside from global warming, are twofold: the increase in extreme events and local-level warming. Together with global warming, these factors are contributing to a shift in the stable climate, which is likely causing both colder and warmer conditions across the country."

Dr Ashraf Dewan, associate professor, School of Earth and Planetary Sciences, Curtin University

These fluctuations in temperature are typical of the winter season but may be more pronounced this year due to changing climatic patterns. Hence, some argue that this is yet another example of climate change being overstated and the global environmental situation is not as dire as scientists suggest. 

That could be far from the truth. 

Paradoxically, the extreme cold spells seen during Bangladeshi winters are a manifestation of climate change. It is a timely reminder that global warming disrupts atmospheric and oceanic systems, contributing to these temperature extremes.

A recent study published in Nature Communications Earth and Environment, titled 'Anomalous Arctic Warming Linked with Severe Winter Weather in Northern Hemisphere Continents,' found that unusually warm temperatures in the Arctic are linked to an increased likelihood of severe winter weather across Northern Hemisphere continents, which Bangladesh is a part of. 

The Arctic is warming faster than other parts of the world, a phenomenon known as Arctic amplification. This is primarily due to the rapid melting of ice, which is occurring at a rate of about 13% per decade in the region.

Ice reflects more sunlight than land or ocean, which are darker and absorb more heat. As the ice melts, it exposes darker surfaces like ocean water or land, which absorb more sunlight and increase warming. This creates a feedback loop, where melting ice leads to more absorption of heat and further warming.

Arctic amplification also disrupts the jet stream, a fast-moving air current high in the atmosphere. Normally, the jet stream helps to keep the polar vortex — an area of cold air that surrounds the Earth's poles during winter — contained. 

However, as the jet stream weakens and becomes more wavy due to warming in the Arctic, the cold air from the polar vortex can spill southward. This allows colder temperatures to reach regions like South Asia, leading to colder weather in those areas.

There are also some other factors at play. 

"The primary reasons, aside from global warming, are twofold: the increase in extreme events and local-level warming. Together with global warming, these factors are contributing to a shift in the stable climate, which is likely causing both colder and warmer conditions across the country," said Dr Ashraf Dewan, associate professor at the School of Earth and Planetary Sciences at Curtin University. 

He explained that global warming caused by human activities is intensifying and making these extreme events, such as heavy rainfall, heat waves, cold spells, floods and droughts, more recurrent. Rising sea temperatures, faster than previously anticipated, further exacerbate these conditions.

Bangladesh's vulnerability is heightened due to its dense population and over-reliance on natural resources, making it particularly susceptible to climate-related disruptions.

Regional climate phenomena also play a pivotal role in the severity of Bangladeshi winters. 

Dr Rashed Chowdhury, an applications scientist in water, climate, and society and an adjunct faculty member at Arizona State University, points to La Niña as a key factor. During La Niña years, sea temperatures in the central and eastern Pacific Ocean drop significantly, disrupting global weather patterns. In South Asia, including Bangladesh, this often results in colder-than-usual winters. 

He noted that according to current projections, La Niña is expected to emerge from October to December 2024 and persist through January to March 2025. 

"There is another reason, I believe, La Niña is particularly important for this year's colder winter — as during La Niña years, the jet stream shifts southward and brings cold air into the region, making the winter feel colder in Bangladesh. So, the combined impact of La Niña and the jet stream may cause the winter to be colder than normal in Bangladesh this year," said Dr Chowdhury. 

According to him, the next important reason is the Indian Ocean Dipole (IOD). If the IOD turns negative, it can bring cooler air to the region, including Bangladesh. 

The IOD will transition to a negative phase shortly in November–January, which generally means cooler waters in the western Indian Ocean and warmer waters in the east, potentially affecting weather patterns in the region, including Bangladesh.

"Lastly, another big factor is cold air moving south from the Himalayas. High-pressure systems over northern India and Nepal push this cold, dry air into Bangladesh, leading to a significant drop in temperatures, especially during the winter months," Dr Chowdhury concluded.