Projected Compound Hydroclimatic Extremes in Khulna, Bangladesh Using Cmip6 Based Climate Projections

In the low-lying coastal districts of Bangladesh, the weather does not merely change; it shifts between two opposing extremes that define daily life. For decades, scientists have understood that a warming planet disrupts the water cycle, making some places wetter and others drier. But a more complex reality is emerging: the danger often lies not in a single event, but in the combination of conditions happening at once. When high heat arrives without rain, the soil dries out and crops fail. When heavy rain falls on already saturated ground, the land floods. These paired events, where two hazards strike together or in quick succession, can cause damage far greater than the sum of their parts. For a region like Khulna, situated in the delta of three great rivers and bordered by the Bay of Bengal, understanding how these paired threats will evolve is a matter of survival. The question is no longer just whether the climate will change, but how the specific dance of heat and water will rearrange itself in the coming decades.

Researchers from Ahsanullah University of Science and Technology have turned to the most advanced climate models available to answer this question for Khulna. They utilized data from the Sixth Coupled Model Intercomparison Project, a global effort that runs massive computer simulations to predict future climate conditions under different scenarios of human activity. The team focused on two specific pathways: one where the world takes moderate action to reduce emissions, and another where fossil fuel use continues unchecked. To ensure their predictions were reliable for this specific coastal district, they first tested thirteen different global climate models against historical weather records from 1951 to 2005. By comparing how well each model recreated the past, they selected the three that performed best: MPI-ESM1-2-LR, EC-Earth3, and BCC-CSM2-MR. These three models were then used to project what Khulna's climate might look like during two future windows: the near future, from 2021 to 2060, and the late century, from 2061 to 2100.

The simulations reveal a future defined by a persistent and intensifying warmth. Across all scenarios, the region is projected to get significantly hotter. The rise in temperature is not limited to the daytime; nights are warming even faster than days, meaning the relief of cooler evenings will become less common. This warming trend accelerates over time, becoming most severe in the late century under the high-emission scenario. As the mercury rises, the frequency of extreme heat events increases. The simulations show a clear decline in cold days and cold nights, while the number of warm days and warm nights climbs steadily. Heatwaves, defined as periods of at least six consecutive days of intense heat, are expected to become longer and more frequent, particularly in the latter half of the century.

While the air gets hotter, the rain behaves in a more erratic and dangerous manner. The total amount of rain falling in a year is projected to increase, but this extra water will not arrive evenly. Instead, the simulations suggest a sharpening of the seasonal divide. During the monsoon months, from June to September, the models predict heavier downpours and longer periods of continuous wet weather. This intensification of rainfall during the already wet season increases the risk of severe waterlogging and flooding, threatening homes and infrastructure in the low-lying delta. Conversely, the dry seasons and the period before the monsoon are projected to experience longer stretches of dry weather. The simulations indicate that the gaps between rain events will widen, leading to extended periods where the ground remains parched.

The most critical finding of the study, however, comes from looking at how temperature and rainfall interact. The researchers used a statistical method to examine the relationship between heat and rain, treating them not as separate variables but as partners in a single system. They discovered a strong, persistent link: as temperatures rise, rainfall tends to drop. This negative connection means that the hottest months are increasingly likely to be the driest. In the future, the simulations show a growing probability of "warm-dry" events, where high heat coincides with a lack of rain. This combination is particularly damaging because high temperatures increase evaporation, drying out the soil and depleting freshwater supplies just when they are needed most. This dynamic exacerbates water scarcity and increases soil salinity, a major threat to agriculture in the region.

The study also highlights a stark contrast in the timing of these threats. The pre-monsoon season is projected to become a time of intense heat and water shortage, driving up evaporation and salinity intrusion. Meanwhile, the monsoon season will bring a surge of heavy rain that overwhelms drainage systems, leading to prolonged flooding. The region faces a future caught between these two opposing stresses: drought and flood. The simulations suggest that under the high-emission scenario, this dual pressure will become more severe and more frequent by the end of the century. The findings do not present a single disaster, but rather a climate that swings violently between extremes, making it difficult to plan for either water storage or flood control.

These projections offer a clear, if sobering, picture for the people of Khulna. The climate is shifting toward a state where heat and water scarcity are linked, and where heavy rains are more likely to cause flooding. The study concludes that managing water in this future will require a new approach that addresses both drought and flood risks simultaneously. The data suggests that relying on strategies designed for a single type of weather will no longer be sufficient. Instead, adaptation efforts must be integrated, focusing on better freshwater storage to survive the dry, hot spells, and improved drainage to handle the intense monsoon downpours. The simulations provide a scientific basis for these urgent decisions, showing that the window to prepare for a climate of compound extremes is narrowing.