TODAY, carbon dioxide causes about half the greenhouse effect. Each year, our skies receive 7 (seven) billion tonnes of carbon dioxide from the burning of fossil fuels. This could double over the next century. Global warming was largely triggered by El Nino, caused by minute changes in ocean temperatures. Movement of warmer water into currents that encircle the Pacific disrupted trade winds and weather patterns in half the world.
What El Nino has wrought, some scientists believe, is just a curtain raiser to what can happen on globe-girdling scale as a result of gradual warming of the earth’s atmosphere. And that might come because of the steady buildup of some gases, particularly carbon dioxide. The gases trapped in the atmosphere tend to reflect light, gradually warming the earth. Scientists believe that a doubling of emissions would, over the next 50 years, raise the average temperature of the planet’s surface by 1.5 to 4.5 degrees Celsius. And that’s a frightening prospect for humans and other living creatures.
The oceans have an enormous capacity to absorb carbon dioxide, but even more amazing is the life that came from the oceans and its role in regulating carbon dioxide levels in the atmosphere. The white cliffs of Dover demonstrate the way nature healed the fevers of past greenhouse warming. About 160 million years ago, ocean plankton took carbon dioxide from the atmosphere and used the carbon to make their protective shells of limestone. When plankton died, their shells sank to the ocean bottom, locking the carbon away in mineral deposits that one day would rise from the sea as white cliffs of chalk.
As a warming climate increases plankton breeding, growing communities of small organisms emit more dimethyl-sulphide (DMS) gas into the air. DMS triggers the formation of usually small water droplets that can reflect more sunlight than ordinary clouds do, thus helping to cool the earth’s climate.
These droplets are called aerosols and work as another key component of the earth’s atmosphere. They are suspended liquid and solid particles including soot from fires and volcanic eruptions, sea salt, bacteria and viruses. Aerosols affect the earth’s energy budget by scattering and absorbing radiation. They exert a cooling effect, because many of the particles tend to prevent radiation from reaching the planet’s surface.
Slight changes in temperature may lead to higher ozone levels near the earth’s surface. This could significantly increase smog in large cities, and also change the way clouds form and dissipate. Warmer temperatures near the ground cause lower clouds to evaporate, letting heat rise further into the atmosphere. As this heated air rises and cools, higher clouds form. But lower clouds usually reflect sunlight back into space while higher clouds tend to absorb more heat. More high clouds mean more heat trapped near the earth’s surface — so small changes of temperature could set off a cycle in which the atmosphere holds more and more heat over time.
Oceans play a great role in regulating the earth’s climate. Their fundamental role in climate is based largely on their storage and transport of heat around the globe. Oceans store vast amounts of heat around the globe, much more than the heat stored by the atmosphere. As water is 1,000 times denser than air, it has a heat holding capacity four times that of air. Ocean currents are primary highways for transport of heat around the globe.
The Gulf Stream is a wind-driven surface ocean current originating in the Gulf of Mexico and terminating in Northwest Europe. When water from this warm current evaporates, it warms the air, which is why northwestern European countries enjoy a milder climate than Canada at the same latitude. The driving force between the Gulf Stream and ocean currents is simple physics: in the waters west of Europe, evaporation makes sea water saltier and colder. The dense water sinks and warmer surface water streams in to replace it, providing the current’s sustaining pull. Global climate change can seriously disrupt the interaction of warmer and colder masses.
Another possible effect of the continued warming of the oceans is significant rise in sea levels. Melting of polar ice caps is one of the reasons. The major reason, however, is thermal expansion of water. Sea levels have risen over 10 to 15 centimetres in the last hundred years. Continued rise would submerge a few metres or so above the sea level. The Maldives is only a metre above the sea level, for example. Bangladesh would also lose vast areas in the coastal zone. The study by a body of scientists at IPCC predicts a sea level rise by 3 feet by 2100. It highlights the devastating impact of a world hotter by 4 degrees Celsius (7.2 degrees Fahrenheit) by the end of this century. The carbon dioxide that we have already put in the atmosphere makes it a near certainty that oceans will become steadily more acidic, eventually destroying coral reefs and sea life. Glaciers will continue to melt year by year, eventually threatening water supply in different regions for as much as 25% of the human population.
The balance between energy absorbed by the earth and energy reflected back into space is fundamental in determining how warm or cool the planet becomes. The proportion of radiation reflected away by a surface is called “albedo.” Albedo can range between 0 (no reflection) and 1 (complete reflection like a mirror).
The Earth’s average albedo is 0.31which means that, overall, the planet reflects 31% of incoming solar radiation back into space. However, forests, deserts, oceans, clouds, snow and ice have different albedos. For example the albedo of forests lies in the range between 0.07 and 0.15, while deserts have an albedo of around 0.3. The albedo of the Earth’s surface varies from about 0.1 for the oceans to 0.6 to 0.9 for ice and clouds, which means that clouds, snow and ice are good radiation reflectors while liquid water is not. This is because clouds, snow and ice have multiple layers that reflect radiation, while a body of water reflects only from its surface.
A calm ocean is a poor reflector but when it foams up in the surfline, producing many reflecting surfaces, it becomes white, reflecting most of the light hitting it. In fact, ice and snow have the highest albedos. Some parts of the Antarctic reflect up to 90% of the incoming solar radiation.
Climate change resulting from change of weather pattern is already having a disastrous effect. Arctic sea ice reached a record minimum in September 2012 and extreme heat waves and drought in the last decade have hit almost all places around the globe more often than ever.
The impact of climate change resulting from weird weather pattern has hit Asian region including Bangladesh most severely. Syed Iqbal Hasnain, professor of Glaciology in Delhi’s Jawaharlal Nehru University, saw in early 2002 as he trekked to Gangotri glacier high in the Himalayas that the snout that feeds the Ganga had developed giant fractures and crevices along a 10-km stretch, indicating massive ice melts. The temperature at the time was five degrees Celsius higher than earlier years.
Increases in sea levels and temperatures are not the only possible outcomes. When ice and snow melt, they generally expose a much darker underlying surface. Dark surfaces absorb more heat (have a lower albedo) than shiny surfaces. This suggests the possibility that a small amount of melting could lead to warmer surface, which could melt more ice, warming the surface still further, initiating the positive feedback loop for a “runaway” warming trend.
Living things do not respond to climate, they affect it too. Plants consume carbon dioxide and produce oxygen through photosynthesis. Undeniably true, photosynthesis is a “sink” for around 5 billion tons of carbon every year, by far the strongest mechanism of carbon removal from the atmosphere. But that mechanism is being disrupted because we are clearing forest areas in all parts of the country.
Atmospheric scientist V. Ramanathan at the University of Chicago calculates that the earth’s average temperature has risen by 1.8 degree Fahrenheit during this century because of the increase in greenhouse gases. Even without further atmospheric pollution, he estimates that trapped heat from the gases we have already put in our skies will boost global temperatures by another five degrees over 1880 levels in the next century. The IPCC report for the time ahead in this century is quite alarming. It says: “Sea levels could rise by a metre mostly because of melting glaciers and the expansion of water as it warms up. The rise could submerge vast areas of low-lying coastal land including river deltas.”
In many parts of the world, the climate emergency has already arrived. An estimated 26 million people have already been displaced by increases in floods, desertification, and drought brought about by climate change upheavals. Noticeably, the sea level rise in the Bay of Bengal and heavy inundation of its coastal areas give some alarming signals for this disaster-ridden country.
The writer is a columnist of The Daily Star.
E-mail: [email protected]
Source: The Daily Star
