Drought in rainy season
But this year the monsoon did not receive normal rains. There was no flooding, even in the most flood-prone low-lying areas of the country. Rather, most of the rivers, haors, and water bodies have dried up. Cultivation of aman paddy and fish farming are being seriously hampered. Many transplanted aman fields also dried up, were fragmented into pieces due to want of water. In many areas of the country, the farmers could not immerse their jute stalks in water due to want of water in the water bodies.
The monsoon usually receives 2,000-2,500 mm rainfall in Bangladesh. But this year some northern districts so far have received less than 100 mm rainfall. On an average, 800 mm rainfall was recorded across the country. Moreover, there was no onrush of waters from upstream across the borders where similar drought-like situation has been prevailing.
For all latest news, follow The Daily Star's Google News channel. If we analyse the characteristics of rainfalls in the sub-continent during the last few years, we would find that the climate has been unusual. Last year, the Indian cities of Mumbai experienced over 1,200 mm rainfall while Chennai 1,000 mm in only 24 hours (the average annual rainfall in Chennai is 1,000 mm). Some of the northern and southern Indian cities which normally receive scanty rainfall, are also experiencing excessive precipitation causing flooding this year. On the other hand, the late September deluge triggered by excessive rains wrought havoc in aman paddy production in northern Bangladesh in 2004.
This year Bangladesh has been suffering from a draught-like situation during the full-monsoon. What are the reasons of this unusual behaviour of the nature? Why is there no rain in the rainy season?
Why does it rain?
To understand why it rains two things should be kept in mind: water vapour condenses as air cools and air cools as it rises. Whenever air is rising, clouds, rain or snow might form. Cooling occurs because of changes in pressure at different altitudes: air cools as it rises, i.e. pressure decreases; air warms as it sinks, i.e. pressure increases. Air rises in convection currents where solar heating is high, such as over the equator. Air also rises when it encounters mountains.
If the air is moist, condensation and rainfall are likely as the air is lifted. Regions with intense solar heating, frequent cooling air masses, or mountains tend to receive a great deal of precipitation.
On the other hand, where air is sinking, it tends to warm because of increasing pressure. As it warms, available moisture evaporates. Rainfall occurs relatively rarely in areas of high pressure. High pressure and clear, dry conditions occur where convection currents are sinking. High pressure also occurs where air sinks after flowing over mountains. Another ingredient is usually necessary to initiate condensation of water vapor - condensation nuclei. Tiny particles of smoke, dust, sea salt, spores and volcanic ash all act as condensation nuclei. These particles form a surface on which water vapor can begin to coalesce.
Why does the monsoon occur?
A large part of the world, especially the tropics, receives seasonal winds and rainy seasons that are essential for sustaining ecosystem and human life. The most regular seasonal winds and rains are known as monsoon.
In India and Bangladesh monsoon rains come when seasonal wind blows hot, and humid air flows from the Indian Ocean. Strong convection currents lift this air, causing heavy rain across the sub-continent. When the rising air reaches the Himalayas, it rises further, creating some of the heaviest rainfall in the world.
As the earth orbits the sun, different regions fall bellow the sun, and different regions receive seasonal rain and winds. The seasonal rains support seasonal tropical forests, and they fill some of the world's greatest rivers, including the Ganges, the Jamuna, and the Amazon. As the year shifts from summer to winter, solar heating weakens and the rainy season ends, and little rain may fall for months.
Is climate change responsible?
Climates have always changed, often in cycles and on many different time scales. A sudden cooling, 65 million years back, is thought to have ended the age of dinosaurs, along with 75 per cent of the species existing at that time. There may have been a dozen such mass extinctions. On a shorter time scale, several ice ages, each lasting hundreds of thousands of years, have come and gone in the past two million years. Even shorter climate shifts occur, such as the "little ice age," which began in the 1300s and caused crop failure throughout Europe.
There are two important shorter-term climate changes -- El Nino and Anthropogenic global warming.
El Nino, La Nina and the Southern Oscillation (ENSO) are the terms referring to a major ocean-current connection that affects weather throughout the Pacific and possibly throughout the world. The core of this system is a huge pool of warm surface water in the Pacific Ocean that sloshes slowly back and forth between Indonesia and South America like water in a giant bathtub. Most years, this pool is held in the western Pacific by steady equatorial trade winds that push ocean surface currents westward. From South-East Asia to Australia, the concentration of warm, equatorial water provides latent heat that drives strong upward convection in the atmosphere. As a result heavy rains occur.
On the other side of the Pacific, the westward-moving surface waters are replaced by cold water welling up along the South American coast. While the trade winds blow westward on the ocean's surface, returning winds high in the troposphere flow back form Indonesia to Chile and to Mexico and southern California. There the returning air sinks, creating dry, desert conditions.
Every three to five years the Indonesian low-pressure system collapses, and the mass of warm surface water surges back east across the Pacific. This phenomenon is called El Nino and the opposite is La Nino. Fishermen of Peru were the first to notice the irregular cycles of rising ocean temperatures. The reasons behind the system collapse are yet to be understood.
El Nino may cause drought in South Asia and Australia, causing crop failures.
Some scientists believe that ESNO events are becoming stronger or more irregular because of the global climate change. There are signs that warm ocean-surface temperatures are spreading, which could contribute to El Nino's strength or frequency.
Anthropogenic (human-caused) global warming has been increasingly becoming prominent in the news in recent years. Global mean-surface air temperatures have increased about 0.5oC over the past hundred years. The 20th century was the warmest since at least AD 1400, and 12 of the warmest years were recorded during the last 20 years. Much of this change is now understood to result from a number of human activities that release energy-absorbing "greenhouse" gases into the atmosphere. Chief among these activities is burning oil, gas, and coal.
Steps required
Heavy rains during the monsoon may sometimes cause flooding in the country, but optimum flooding is essential for survival of our agriculture, rural livelihood and ecosystem. Many agriculturists believe that our land would have turned fallow if recurrent rains and flooding did not replenish our land every year. The floodwater leaches out and dissolves harmful soluble substances, which accumulate in the soil through excessive use of chemical fertilisers, insecticides and over-cropping.
The ongoing sustained draught ultimately may cause failure of aman paddy, the main source of our food grains, if steps are not taken immediately to irrigate the land artificially. Such crop failure may be a disaster for the country's economy. Although the transplantation period was running out fast, the concerned departments of the government did not take any step to face the situation. Now, it is the duty of the government to protect the seedlings already planted in the fields by making arrangement of artificial irrigation. They should come forward to offset the impending disaster by ensuring adequate supply of fuel-oil and fertiliser with subsidised prices to help the farmers irrigate their lands artificially. Moreover, various agriculture departments should immediately launch special counseling programs to educate the farmers on how to conserve water and cultivate crops with minimum water.