Warm Night Causing Rice Yield Fall: Can We Afford It?

In 1966, when the International Rice Research Institute (IRRI) released IR8, the famous “miracle rice” that drove Asia’s Green Revolution, Bangladeshi plant breeders and planners created an entire food system based on a simple way of thinking about climate risks: daytime heat, monsoon floods and winter cold snaps. Boro rice, which is now the country’s main source of daily calories, was deliberately moved to the dry and sunny months from December to May. The goal was to avoid monsoon season problems and take advantage of cool, clear nights to help the grain fill properly. That plan worked well for half a century. However, it is now quietly failing, and almost no one is talking about the reasons why.
Weather records from 35 meteorological stations across Bangladesh, collected from 1949 to 2020, show something surprising: nighttime (minimum) temperatures have risen faster than daytime (maximum) temperatures in most parts of the country. This trend is especially clear during the monsoon and post-monsoon months. National assessments using statistical trend tests show that average, minimum and maximum temperatures are rising by about 0.13°C per decade across the country, with some seasons and southwestern districts warming even faster. That rate might seem small compared to major news reports predicting a 1.4°C temperature increase by 2050. But it is actually very important, because rice crops do not respond to average daily temperatures; they react specifically to how warm it gets at night. This part of the climate story is almost completely buried under news coverage of cyclones, saltwater intrusion and sudden flash floods. Nighttime warming gets none of this attention because it produces no dramatic pictures or videos. A rice field does not visibly wither overnight. The harm remains hidden until farmers weigh their harvest at the end of the season.
Rice plants use sunlight during the day to make sugar and starch through photosynthesis. At night, when there is no sunlight, plants breathe and burn a portion of that stored energy just to stay alive. Warmer nights speed up this energy burning. Long-term field studies from IRRI in the Philippines showed this over twenty years ago: rice grain yields dropped by about 10% for every 1°C rise in nighttime temperatures during the growing season, even though slightly warmer days increased yields a bit. The daytime gains do not make up for the nighttime losses; night losses are consistently much larger. Later research proved that when warm nights happen during key stages, especially flowering and grain filling, the crop damage is most severe. For Bangladesh, where Boro rice alone accounts for more than half of all rice produced, it is a direct threat to the primary food crop the nation relies on, caused by a process that the country has spent almost no effort studying or explaining to the public.

Local field studies and regional research in Bangladesh show that this problem has already started happening. The daily temperature gap (difference between a day’s highest and lowest temperature) is getting smaller across the country’s dry, high-land and coastal rice regions. This shrinking gap is mainly caused by nights staying unusually warm. Models studying the effects of high night temperatures on Aman rice found that warm nights explain nearly one-third of crop yield changes in some areas. This impact is so big that it equals rainfall as a main reason for a good or bad harvest. One national study using historical climate data warned that Boro rice harvests could drop by about 20% by 2050, and by as much as 50% by 2070, if minimum and maximum temperature trends continue without being addressed.
There is a second and related problem getting even less attention: as farmers try to adapt to heat and saltwater problems, crop choices are narrowing around a small group of tough rice varieties, such as BR10 and BR23. Meanwhile, the planting of traditional Aus rice, Boro rice and jute is shrinking. While switching to short-growing vegetables is often praised as a smart move, it actually reduces the variety of crops and seeds. This happens at the exact moment when having more crop options would protect against unpredictable warm nights, which do not affect every crop variety, district or season in the exact same way. A single set of “climate-smart” crops is still a single set of crops, and it brings its own dangers if climate patterns change again.
Against such a backdrop, three key steps need much more attention from the public and policymakers. Firstly, Bangladesh’s farm support programmes and plant breeding efforts must focus specifically on creating rice varieties that handle warm nights, rather than just heat tolerance during the daytime. These two types of heat tolerance require completely different research and testing methods. Secondly, weather reports given to farmers are currently focused on daytime highs and rain forecasts. However, they should start including nighttime low-temperature warnings during the critical flowering times of each growing season. And thirdly, research money and news reports must stop treating warm nights as a minor part of overall rising temperatures, and start reporting it as a major threat with its own distinct numbers.
None of these changes require waiting for new technological inventions. Long-term weather data already exists across 35 weather stations in the country. The tools to measure how much crop loss comes from warm nights versus warm days are already available in published studies. The existing networks of the Bangladesh Rice Research Institute (BRRI) and the Department of Agricultural Extension (DAE) already cover most farming districts. The missing piece is not ability, but focus: the choice by leaders and reporters to treat warming nights as a major headline risk rather than a minor detail. Bangladesh built its food system on the simple idea that nights would always remain cool and safe for crops. That idea is no longer true, and we cannot afford another decade of ignoring this reality.
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The writer is an Assistant Professor at the Institute of Development Studies and Sustainability (IDSS), United International University (UIU) and is an Adjunct Faculty member in the Department of Agriculture at Daffodil International University