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The BMD defines a heatwave based on actual air temperature, counting it when the daily maximum reaches at least 36°C for two or more consecutive days. On the other hand, the Heat Index or apparent temperature used by the Climate Change Knowledge Portal combines air temperature and humidity to estimate how hot it actually feels.

RISING FASTER, AND SPREADING

Bangladesh is already one of the most heat-exposed countries in the world.

In 2024, it ranked fourth globally in the number of days with a Heat Index above 35°C, behind only Bahrain, Qatar, and the United Arab Emirates. Unlike these desert countries, however, Bangladesh’s heat is intensified by high humidity, making it more dangerous for human health.

Due to its high humidity, Bangladesh has historically ranked among the world’s top 10 countries for the number of days each year with a “feels-like” temperature above 35°C.

During the monsoon season (June-September), relative humidity across Bangladesh regularly reaches 80–85%, peaking at 90%, compared to much lower levels in the dry winter and pre-monsoon months.

In this moisture-heavy air, sweat does not evaporate easily, making it harder for the body to cool off. As a result, the “feels-like” temperature can be much higher and more dangerous than the actual temperature.

Although the country has long been a global hotspot for humid heat, such conditions were manageable in the past. However, the situation has worsened over the past 15 years, with extreme heat days becoming increasingly frequent and hazardous.

During the 2010s, Bangladesh on average experienced 91 such days a year when the apparent temperature exceeded 35°C. Between 2015 and 2024, the country recorded more than 100 such days each year on average, according to the WB-compiled data.

A long-term review of climate data also shows an alarming upward trend.

In the 1950s, Bangladesh experienced fewer than 30 days per year on average when the heat index crossed 35°C. That number has steadily increased each decade, nearly quadrupling over time.

It accelerated sharply in the 2010s, which saw nearly 300 additional days above this threshold compared to the previous decade. This spike continued in the 2020s.

Regional patterns also show how widespread the change has become. Khulna has consistently been the hottest division, with heat days rising from around 51 days per year in the 1950s to about 132 days in the 2020s.

Chattogram, historically the least affected, still records fewer heat days than other divisions, but it now experiences more extreme heat than the hottest regions did in earlier decades.

Md Rabiul Awal, chairman of the Department of Meteorology at Dhaka University, attributed this rise in extreme heat to global warming.

“According to most climate projection models, this trend is expected to continue in the coming decades,” he told The Daily Star yesterday.

HEALTH, ECONOMY HURTING

According to Lancet Countdown data, more than 4 out of every 1,000 deaths in Bangladesh between 2010 and 2021 were linked to heat exposure, an increase of 33 percent from the previous 12-year period.

Older people are particularly vulnerable. Around 1,430 people aged 65 and above died from heat-related causes in Bangladesh between 2017 and 2021, according to a 2023 study by the Grantham Research Institute on Climate Change of London School of Economics.

Extreme heat causes heatstroke and exhaustion and worsens existing conditions such as heart disease, diabetes, and asthma. It also limits outdoor activities. In 2024, an average person in Bangladesh would have been exposed to extreme heat stress for about 5.2 hours per day while outdoors. Only Cambodia fared worse in this indicator.

A nationally representative World Bank survey in 2024 also found that the likelihood of depression increased by 23.8 percent and anxiety by 37.1 percent on days when temperatures exceeded 35°C. Women were more affected.

Extreme heat also hurts the economy. In 2024 alone, heat-related physical and mental health impacts resulted in an estimated loss of up to 25 million workdays in Bangladesh. The economic cost is estimated at between $1.33 billion and $1.78 billion, or roughly 0.3 to 0.4 percent of the country’s GDP, the same WB study found.

Much of this loss comes from reduced labour productivity. According to Lancet Countdown estimates, an average worker in Bangladesh lost about 391 working hours in 2024 due to heat stress. Between 2015 and 2024, only 10 countries recorded higher annual work-hour losses.

Agricultural workers were the most affected, losing an average of 732 working hours in 2024 due to extreme heat, the equivalent of around 92 full working days, based on an eight-hour workday. Construction workers lost about 285 hours, while service-sector workers lost around 111 hours.

For many workers who depend on daily wages, this translates directly into lost income. For example, assuming a daily wage of Tk 500 for a farm worker, losing 92 working days would mean a loss of around Tk 46,000 in a single year or about Tk 4,000 per month.

In a country where millions rely on daily earnings to survive, such losses can have serious consequences for household income and food security.

The Daily Star spoke to four rickshaw pullers aged 50-65 in Dhaka’s Farmgate. All four said they lose working hours during extreme heat, as the conditions make it difficult to carry passengers.

Md Azad, one of the rickshaw pullers interviewed, said that due to the heat, he has to rest a while after every trip. “On very hot days, I earn Tk 200-300 less.”

A GRIM PROJECTION

Climate projections show Bangladesh will face even more days with heat index above 35°C in the coming decades.

Between 2026 and 2050, the country could see an additional 32 such days each year under a moderate, “middle-of-the-road” climate scenario. This scenario is based on the assumption that socio-economic development and greenhouse gas emissions will continue along historical trends until around mid-century before a gradual decline.

Even in the best-case scenario, where the world reaches net-zero emissions by 2050, Bangladesh is likely to face at least 23 extra days annually.

In the worst-case scenario, where fossil fuel use remains high, the increase could reach 41 additional days.

Coastal regions are expected to be hit the hardest. For example, Khulna is projected to remain the most heat-affected area until around mid-century, after which Barishal could overtake it. By the 2080s, both divisions may face nearly 200 days of extreme heat each year even in the middle-of-the-road scenario.

This growing intensity is driven not just by rising temperatures, but by humidity. In Khulna and Barishal, increasing moisture levels significantly raise the “apparent” temperature, making heat more dangerous.

By the end of the century, humidity is expected to become the region’s dominant climate stressor, increasing discomfort and risk in these already waterlogged, riverine landscapes.

WHAT’S CAUSING IT

According to Lancet Countdown, climate change was responsible for 51 out of 86 heatwaves recorded in Bangladesh between 2020 and 2024. While climate change is identified as the primary driver of extreme heat in Bangladesh, experts and a growing body of research point out that local factors also play a role.

For example, rapid urbanisation is increasing heat exposure, especially in cities like Dhaka. Concrete buildings, roads, and infrastructure trap heat, create what is known as the Urban Heat Island effect. This causes cities to become significantly hotter than surrounding rural areas.

Bangladesh is also losing tree cover at a faster rate than before.

Between 2001 and 2024, the country lost an average of 10,906 hectares of forest each year, according to satellite data from Global Forest Watch, which tracks canopy cover at a 30% density threshold. The annual loss has increased over time -- from about 4,394 hectares in the 2000s to over 17,000 hectares in the 2020s.

While deforestation does not immediately raise temperatures in a single year, long-term loss of tree cover reduces a country’s natural cooling capacity, which makes heat more intense over time.

Golam Rabbani, director of the Centre for Climate Change and Environmental Research at BRAC University, said that temperature levels tend to rise with increasing human and industrial activity.

“Urbanisation undoubtedly influences local temperatures. Expanding industrial operations, growing vehicle use, dense construction, and rising population concentrations all contribute additional heat to the urban environment,” he told this newspaper yesterday.

As the intensity of these activities varies across different parts of the city, temperature readings can also differ between areas such as Sher-e-Bangla Nagar, Farmgate, and Jatrabari, he said.

“Deforestation also contributes to extreme heat by reducing evapotranspiration -- the natural process through which trees and vegetation release moisture into the atmosphere, helping cool the surrounding environment. As green cover declines, this cooling effect weakens, increasing the risk of higher temperatures,” he said.