Now people, all over the world, do not suffer from any illusion anymore.

As an authoritative report released in the early part of 2000 by the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC) makes plain, the trend toward a warmer world has unquestionably begun. Worldwide temperatures have climbed more than 0.5 degree centigrade over the past century, and the 1990s were the hottest decade on record. After analyzing data going back at least two decades on everything from air and ocean temperatures to the spread and retreat of wildlife, the IPCC asserts that this slow but steady warming has had an impact on no fewer than 420 physical processes and animal and plant species on all countries.

Global warming can't be blamed on any particular heat wave, drought or deluge, but scientists say a hotter world will make such extreme weather more frequent and deadly. From melting glaciers to rising oceans signs are everywhere. Take the case of glaciers. Since they are typically formed as a response to cold climate, glaciers always reflect any change in climate.

Antarctica, home to Adelie penguins is heating up. The annual melt season has decreased by up to three weeks in 20 years. Mount Kilimanjaro has lost 75 percent of its ice cap since 1912. The ice on Africa's tallest peak could vanish entirely within 15 years. Lake Baikal in eastern Siberia now freezes for the winter 11 days later than it did a century ago. Montana will lose all the glaciers in Glacier National Park by 2070 if their retreat continues at the current rate. Venezuelan mountaintops had six glaciers in 1972. Today only two remain. Nearer home, Indian glaciologists have also observed that even after freezing winter in north India some years back, Himalayan glacier, contrary to usual expectation outran its annual average of 16m and retreated by an all-time high of 20m.As the glaciers melt India, Bangladesh and other adjoining areas continue to experience hottest summers of all time.

Faced with these hard facts, scientists no longer doubt that global warming is happening and almost nobody questions the fact that humans are at least partly responsible. There is no reason to feel complacent that the changes are over. Already humans have increased the concentration of carbon dioxide, the most abundant heat-trapping gas in the atmosphere, to 30 percent above pre-industrial levels, and each year the rate of increase gets faster. The most obvious conclusion : temperatures will keep going up. By 2100, says the IPCC, average temperature will increase between 1.4 degree centigrade and 5.8 degree centigrade -- more than 50 percent higher than predictions of just a half decade ago. That may not seem like much but what if we consider that it took only a 5 degree centigrade shift to end the last ice age? Even at the low end, the changes could be problematic enough with storms getting more frequent and intense, droughts more pronounced and coastal areas even more severely eroded by rising seas, rainfall scarcer on agricultural land and ecosystems thrown out of balance.

But if the rise is significantly larger, the result could be disastrous. With seas rising as much as 1 m, enormous areas of densely populated -- land coastal Florida, much of Louisiana, the Nile Delta, the Maldives, Bangladesh -- would become uninhabitable. Entire climatic zones might shift dramatically. Agriculture would be thrown into turmoil. Hundreds of millions of people would have to migrate out of unlivable regions. Many environmental scientists say that the polar ice caps will melt causing sea levels to rise and swamping coastal communities. Increased precipitation due to higher rates of evaporation may produce stronger and more frequent storms.. The same increased evaporation could suck the land dry in other regions of the world causing widespread drought and famine.

Impact on water and environment
Warmer climate means decreased river flows. Consequently higher temperatures could harm the water quality of the country's rivers, and lakes. In areas where the river flows decrease, pollution concentration will rise because there will be less water to dilute the pollutants. But in order to keep pollution concentrations from increasing, sewage treatment plants and other water pollution controls will have to be upgraded which entails hefty expenses that Bangladesh's fragile economy can hardly bear. Bangladesh has already started experiencing the brunt of global warming trend through increased frequency of storms, floods and degradation of water quality due to decrease in the flow of water in the rivers, especially in the lean period.

Most of the rivers in the country are either dead or dying because they have been dammed indiscriminately by constructing polders and embankments over their course in the most haphazard and unplanned manner either for irrigation purposes or for halting salinity intrusion. These rivers flowing through the big cities and towns are now narrow streams of urban and industrial waste water that's pumped continuously into these glorified sewers. Precisely true, these rivers have long had the capacity of self-purification -- pollutants are diluted and slowly absorbed -- but with the rivers drying up and pollution load increasing by the day, the death of the rivers is now a reality.

Most worrying, toxic effluents from the industrial units as well as human wastes pumped into these rivers have poisoned the water. As reports from WASA sources suggest coliform bacteria and chromium concentration in the river Buriganga and Shitalakhya in the Dhaka city and adjoining Narayanganj district have reached near danger level. Experts say that toxic metallic element chromium is harmful for health and can cause serious diseases like cancer. As the rivers wither away, millions who depend on them for their livelihood, either fishing or farming, are finding their way of life changing. Worse, they find their very lives in peril. The country's major river basins -- making up 80 per cent of the total surface water and home to nearly 60 per cent of the population-- is so polluted, mainly in stretches near towns and industrial belts, that bacteria feeding on the waste are the only things that have proliferated -- their counts are anywhere between 10 times to 100 times over safe levels.

Changing climate could also increase the salinity of water bodies. Public health could suffer. Rising sea level and reduced precipitation would increase the salinity of estuaries. Moreover, decreased river flows could lead to excessive concentration of minerals that run off from farms in various rivers. Rising seas would contaminate water supplies with salt. Higher levels of urban ozone, the result of stronger sunlight and warmer temperatures could worsen respiratory illnesses. More frequent hot spells could lead to a rise in heat-related deaths. Warmer temperatures could widen the range of disease carrying rodents and bugs, such as mosquitoes and flies, increasing the incidence of dengue fever, malaria, encephalitis and other afflictions.

Worst of all, this increase in temperatures is happening at a pace that outstrips anything that the earth has seen in the past 100 million years. Humans will have a hard enough time adjusting, especially in poorer countries like Bangladesh, but for wildlife , the changes could be devastating. Study conducted by Nash and Gleick, water experts in the US, with various climate model scenarios revealed that salinity from run off in the Columbia River could rise 15 to 20 per cent. Such increase in salinity, as well as decrease in flow could impair compliance with the US obligations to the Third World.

As it is seen these days, global climate change could also increase the frequency and severity of inland flooding, particularly along rivers. Bangladesh experienced the worst ever flood of the century in the recent past that inflicted colossal damage to the economy. Detailed studies with general circulation models conducted in the US that suggest that some regions of the United States may have more rainfall during wet season which would increase river and lake levels. .

Measuring the warming that has already taken place is relatively simple. The trick is unraveling the causes and projecting what will happen over the next century. To do that, IPCC scientists used different climate models feeding their computers with a wide range of scenarios , involving varying estimates of population and economic growth, changes in technology and other factors. They used seven different versions, which yielded 235 independent predictions of global temperature increase. That's where the range of 1.4 degree centigrade to5.8 degree centigrade comes from.

It won't take the extremes of warming to make life uncomfortable for large numbers of people. Even slightly higher temperatures in regions that are already drought- or flood-prone would exacerbate those conditions. In temperate zones, warmth and increased carbon dioxide would make crops flourish at first. But beyond 1.5 degree of warming, says Bill Easterling, a professor of geography and agronomy at Penn State and a lead author of IPCC report, "there would be a dramatic turning point. US crop yields would start to decline." In the tropics, where crops are already at the limit of their temperature range, the decrease would start right away.

Even if temperatures rise only moderately , some scientists fear, the climate would reach a "tripping point"-- a point at which even a tiny additional increase would throw the system into violent change. If peat bogs and arctic permafrost warm enough to start releasing the methane stored within them, for example, that potent greenhouse gas would suddenly accelerate heat trapping process. More catastrophic scenarios may be in store for us, the denizens of the planet earth.

By contrast , if melting ice caps dilute the salt content of the sea, major ocean currents like the Gulf Stream could slow or even stop, and so would their warming effects on northern regions. More snowfall reflecting more sunlight back into space could actually cause net cooling. Global warming could paradoxically, throw the planet into another ice age.

Md Asadullah Khan is a former teacher of Physics and Controller of examinations, BUET.