On April 4, 2021, a Nor'wester blew through Bangladesh between 6PM and 12PM from the northwest part to southwest part of the country.
As per the Department of Agricultural Extensions (DAE) field survey report, the hot air induced by the Nor'wester caused damage to Boro rice of more than 68,123 hectares. The panicles of Boro rice dried up due to the heat shock. Most of the damages occurred in Gopalganj, Kishoreganj, and Netrokona district, as reported in the newspapers.
Spikelet sterility in the rice panicle
Temperature requirements for rice growth and development vary according to its growth stages. In general, the optimum temperature of rice growth is 20°C to 30°C. However, rice can grow beyond this range, but injury/damage can happen in different degrees depending on the stages of growth.
Rice growth and development is classified into three phases, i.e., vegetative, reproductive, and ripening phases. High temperature can cause very little effect by damaging the leaves at the vegetative phase. At ripening, high temperature may hamper grain filling and increases chalkiness which can lower yield and grain quality.
But a high temperature is most damaging at the reproductive phase. A high percentage of spikelet sterility occurs when the temperature remains above 35°C at anthesis (during spikelet opening, pollination, and closing in the morning, about 10.00 AM to 12.00 PM), causing more than 80% sterility if it lasts for 1-2 hours.
Heatwave induced spikelet sterility in the rice panicle
The pre-monsoon seasonal storms cause huge damage due to severe wind speed with hail-storm in small areas. But this year, the storm hit countrywide simultaneously characterized by rain-free hot air (35°C to 37°C) and very low humidity (<40%) which gusted for almost two to three hours.
We referred to this as a 'heatwave.' This high-velocity hot air caused 'forced transpiration' (transpiration is the process of water movement through a plant and its evaporation from aerial parts) from active panicles (bloomed within 4-5 days) and in some cases from flag leaves above panicles.
Transpiration is the cooling mechanism of plants for surviving active and sensitive organs against very hot and dry conditions. In general, rice canopy (active leaves) and flowering panicles can maintain 4°C to 6°C lower temperature than the air through transpirative cooling system.
Therefore, during a heatwave, rice plants reduce the temperature from their flowering panicles, and flag leaves cause quick drying of the spikelets and leaves. It was reported that increasing the vapour pressure deficit around rice panicles, leaves and difficulty in quick water transport might lead to the drying of panicles and leaves. This type of spikelet sterility was reported in Japan and the Korean peninsula, mainly caused due to the high wind velocity for 2-3 days. In that case, wind velocity and low humidity is the major cause of drying rather than temperature.
Climate change induced extreme events
NASA Earth Observatory has recently reported that the global average surface temperature rose 0.6°C to 0.9°C between 1906 and 2005, and the rate of temperature increase has nearly doubled in the last 50 years. But if emissions continue to increase, Earth's temperature would rise beyond 5°C by the end of this century.
National climate assessment reports of many countries already mentioned that the number of heatwaves and other extreme events has increased globally, and the strength of these events has increased, too. Heat stress considering both temperature and humidity, will determine how such kinds of events will occur in the future.
Under global warming, it is expected that both the magnitude and frequency of extreme heatwave events will be increased in the future. Combined heatwave, low humidity, and high wind from Nor'wester are expected to increase the likelihood of occurring such types of unprecedented events in the future.
Right after this event, a group of Bangladesh scientists and experts met virtually on 11th April 2021 to discuss the agrometeorological perspectives of panicle drying due to Nor'westers mediated heat shocks. The meeting was moderated by Dr. Md. Golam Mahboob, Senior Scientific Officer of Bangladesh Agricultural Research Institute.
More than thirty renowned climatologists, meteorologists, and agricultural scientists, including Professor Dr. AKM Saiful Islam of Bangladesh University of Engineering and Technology, Professor Dr. M Tofazzal Islam of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Professor Dr. Towhida Rashid of Dhaka University, Professor Dr. Dewan Abdul Qadir of NPI University of Bangladesh, Dr. Md. Sazzadur Rahman of Bangladesh Rice Research Institute, Dr. Md. Shah Kamal of the Department of Agricultural Extension, Mr. SM Quamrul Hassan of Bangladesh Meteorological Department, participated in the meeting.
The group discussed the event that took place on 4th April 2021 and decided to initiate collaborative research to further investigate and understand the nature of the event from multiple dimensions, including meteorological, climatological, disaster science, crop science, and socio-economic perspectives, and find out ways to overcome such problems in future. An open-source knowledge platform for sharing and exchanging ideas and data would assist in achieving faster results for the greater interests of science and society.
Unfortunately, none of our currently available modern varieties can withstand such type of hot air-induced panicle sterility. Therefore, we must be better preparing and developing the possible adaptation strategies to reduce the loss and damage caused by such kinds of extreme events in the future. Based on initial discussion with the experts, a few recommendations can be made:
- Crop standing water (about 2 to 3 cm) should be maintained till 1/3rd of the grain filling stage.
- Innovative research should be conducted for improving heat-tolerant varieties by a multi-disciplinary research team consisting of national and international experts.
- Adjustment of sowing and planting time to avoid the heading/flowering can escape such extreme heatwave conditions.
- Excavation of ponds and natural canals (khals) to conserve more water during the monsoon, which can be later used during the dry period.
- Provide compensation to the farmers who are badly affected by such extreme climatic events.
- Crop insurance with subsidised insurance premium can be introduced as an instrument for climate change adaptation.
- Early warning systems should be improved for providing necessary information well ahead of such extreme events.
- Necessary assessment of the damage and actions should be taken by the Government immediately after such events for ensuring food security of the country and lessen the adverse impacts.
- Paris climate agreement should be implemented by the countries that agreed to cut greenhouse gases to keep global warming well below 1.5°C.
- Awareness should be built among the farmers and stakeholders about such events and about the emergency responses.
Collective efforts from all major stakeholders, including relevant government agencies, academic and research institutes, NGOs, financial institutes, farmers, and media are needed to meet the future challenges under the warming world. A good strategy will help us better manage these types of hazards in the future.
Professor A.K.M. Saiful Islam works at the Institute of Water and Flood Management (IWFM) of the Bangladesh University of Engineering and Technology.
M. Golam Mahboob is the Senior Scientific Officer of Bangladesh Agricultural Research Institute (BARI), Gazipur.
Md. Sazzadur Rahman is the Principal Scientific Officer of Bangladesh Rice Research Institute (BRRI), Gazipur.