Green house effect

Before green house effect is studied one needs to know what kind of radiation / sunshine we receive on earth.

'Sunshine' is a common man's term indicating all that we receive from sun. In a shallow perspective of common man, it may simply, mean light. But in a deep scientific meaning of 'sunshine' - it includes a spectrum of electromagnetic waves packed with large amount of energy. This is called solar spectrum (slides). A part of solar spectrum is visible (visible spectrum) and we can see the world through reflectable visible spectrum. Human eyes are evolved and designed to identify visible spectrum of electromagnetic waves, while solar spectrum contains large amount of other type of waves like infra-red waves and ultra-violet rays.

 concept of green house effect

The Greenhouse Effect

The concept of green house effect simply codifies all the effects due to trapping of radiation within the earth's surface layer of atmosphere. The world 'Green house' is commonly used because such effect was found first time in a green house (Green house is a controlled atmosphere farm house made out of glass all round the surface). When plants were made to grow under confined atmosphere, it was observed that earth's radiation (short wave) was received in side the glass green house but was not able to be reflected back to atmosphere outside glasshouse. This trapping of radiation (short waves) is called green house effect. Green house effect can happen even without any plants being grown in green house.

This phenomenon was extrapolated to earth's atmosphere. The radiation received by earth but not able to be reflected can become 'Green house' effect. That is short waves of infrared and rear infrared rays are trapped in the layer immediately above the earth's surface - just like a green house.

Mechanism of green house effect

The solar spectrum of electromagnetic waves include energy packed heat generating waves called radiation waves. As they lie beyond red colour waves of visible spectrum, they are also called 'infra-red rays'. These infrared rays lie in the wave length range of 700-900 mm. Two types of infrared rays can be identified - a shorter wavelength radiation rays (700 - 800 mm) and larger wave length radiation rays (800 - 900 mm).

While both these types of radiation are received by earth from sun, longer wave length rays are absorbed by carbon dioxide, carbon monoxide in the air. But shorter waves reach the surface. On receiving these strong waves, the earth absorbs part of them and reflects back major part of it depending on its own capacity (black body effect). Such reflected radiation is usually in the form of long wave radiation. Major part of it may be trapped in the air, if its CO2 concentration is high or obstruction in the form of dust, cloud is strong.

An example of green house effect commonly experienced by all can be cited for easy understanding. Just prior to rainfall especially in early kharif days, the climate is warm and dusty because of thick cloudy cover in air not allowing the earth's reflected radiation to pass through.

The air gets heated up due to absorption of long range waves from both direct and reflected source. This phenomenon is called green house effect. In a global perspective it is relevant and a dynamic process as the composition of air is changing. The concept of reflection of waves from earth's surface is an old natural phenomenon. But the trapping of radiation due to changed composition of air (which is dynamic in character) is an added man-made feature. Hence, the green house effect is due to man-made actions of changing the composition of air.

Reasons for green house effect

a) Out of total air pollutants to be 1 x 1012 tons, man made pollutants are 5 x 108 tons.

b) 25% of world's green house gases are released by USA which supports 4% of global population.

c) Carbon monoxide accounts for 50% air pollution

d) Suspended particulate matter (dusts of various types) form 15% of total pollution.

e) Over 15 million tons of carbon monoxide is added to the atmosphere every year.

f) The level of emission is increasing at a very fast rate. As compared to 1987, CO emission during 1997 was 132 times more.

g) CO2 concetrations in the air has increased by more than 15% in the last 100 years, mainly due to deforestations, burning of fuels, industrialisation and urbanisation. From 275 ppm Co2 during 1890 AD, its level has touched 316 mm in 1980. But in 10 years after 1980, it has increased to 350 ppm. This means CO2 concentration in air has increased from 0.0275% to 0.0350%. Increase of this order is, in fact, a jeopardy in carbon - cycle of environment. Production of CO2 is more than what natural processes in carbon cycle can absorb. The source of absorption (plants) is destroyed at a large scale (deforestation).

Impacts of green house effect

Change in the composition of air due to several human activities has brought about green house effect. Green house effect has brought about following phenomenal changes. The severity of these changes will aggravate further in the next 10-20 years. By 2040, CO2 level in air is expected to reach a level of 450 ppm (0.04%).

  • Increase in mean air temperature during various parts of year is to the tune of 0.8oC to 1.02oC as compared to their normal temperature.
  • Increase in sea level is observed in some parts of world due to excess heating of air - which has caused large scale melting of ice covers.
  • Change in the rainfall patterns due to variations in the convectional current (both in time and direction) caused by difference in heating pattern of earth's air.
  • Change/shifts in seasons and seasonal characters through out the globe. These may even include shifts in classification of earth into temperate, subtropical and tropical climates.
  • Major changes in water resources of world due to disturbances in hydrological cycles. Heavy rainfall tracts are gradually converted into low rainfall tracts, many humid areas being transformed into arid areas, ground water depletion is high and recharging is very low etc.
  • Shifts in disease/pest cycles of plants and animals. Many insignificant pests / diseases are attaining major proportions because composition of microbial population is affected by shift in temperature and hydrological cycles.
  • Reduced agricultural productivity and food shortage.
  • Increased water shortage throughout the globe.

Increase in Air temperature

Increase in air temperature - popularly called as global warming - refers to mean temperature of entire globe averaged over all months and all places. Hence, the change in temperature at a place, in a particular month, may or may not match closely with change in temperature of globe.

Although many scientists have predicted a rise in temperature ranging from 1.5o C to 4.5oC due to doubling of CO2 concentration compared to pre-industrial period, the current change in temperature is to the tune of 0.88oC for a period from 1890 - 1989, while it was 1.02oC for a period from 1950 - 1989 in India. Similar increase in temperature is recorded in entire northern hemisphere. Warming is predicted to be greater in higher latitudes than in tropics and will be more pronounced during winter than in summer.

An increase in air temperature of this magnitude is not substantial enough to result in any changes as per the microclimatic experience. Because, over months, variations of temperature occur from 10o to 35oC. Between places the variation of mean annual temperature or seasonal temperature vary from 11-20oC. In this background most people may conclude that a change in air temperature to the tune of 0.88 to 1.02oC is not significant.

Unfortunately, this is not true. Because at global level, increase in mean temperature over places and over months indicate that in many places the rise in temperature is much higher (even upto 6-8oC) than normal. Variation of this order over normal is of great consequences. Deviation from normal is more significant than variation between months / places, as, such variations are part of normal temperatures.

The biggest increase in air temperatures are recorded from 1980 onwards - which coincides with highest increase in CO2 levels. Increased air temperatures is the cause for several changes in hydrological cycles, vegetation and agriculture, sea level rise and many other modification.

In India temperature anomalies were recorded up to +0.41oC during 1980 - 89. The rise in winter temperature is mostly observed in India than rise in pre-monsoon temperatures.

Increase in Sea Level

Over the past 100 years, sea level has increases by 18 cm. Inter government a panel in climate change has suggested that sea level would rise upto 90cm by 2100. Large scale flooding of California in 1999 and parts of western coast in India in the last 5-8 years, are testimonies to effects of sea level rise. If the sea level rises by 80-90 cm, perhaps many of coastal cities of world will be washed away besides great changes in harbours and their facilities, in sea routes and in fishery industry. With increased sea levels these are the first causalities. If the increase is to the tune of 90 cm, the world is going to face a catastrophe.

Till today, no attempt is successful for containing the effects of sea level rise. There is no method to save the inundating cities. Attempts to construct walls across the coast have met with little success.

There are two schools of thoughts about the actual scientific reason behind rise in sea level. According to traditional school of thought, global warming has resulted in melting of polar ice sheet extending to 12 million square kilometres as well as melting of ice on many mountainous regions of world. Extra water generated by these sources was thought to be the reason for rise in sea level.

Recently, another groups of scientists headed by Dr. Duncan Wingham have enunciated a theory that rise in sea level is due to expansion of sea water. Although it may look impracticable in the light of about 1o c mean rise in atmospheric temperature, the team of scientists believe that thermal expansion of sea water must have taken place in many places where the actual rise in air temperature are more than 7 - 8oC. Higher coefficient of expansion of saline water than normal water offers credence to this theory.

Change in the Rainfall Patterns

The rainfall patterns in the world are governed by differential heating of air layers over different latitudes. Differential heating of air is the main reason for different convectional air current movement from one part of globe to other. The rainfall is associated with these convectional current, because movement of air from high pressure region (due to higher air temperature) to low pressure region (due to low air temperature) will carry large quantity of water vapour, if it is from air above sea.

Due to rise in air temperature, the pattern of convectional current and differential heating pattern have substantially changed. This has resulted in change in amount of rainfall and its distribution. The study of global rainfall pattern over last 100 years, as related to positive and negative anomalies in temperatures has indicated that in many places rainfall increased and in many other places it has decreased substantially. In many other places the rainfall pattern is not following any constant relationship with other climatic parameters.

For e.g.,

Out of 10 highest positive winter anomalies of air temperature in northern hemisphere, two heavy and one poor monsoon years are identified. Similarly out of 10 highest negative winter anomalies of air temperature, two heavy and three poor monsoon years are identified. The difference in mean rainfall for ten highest positive and negative anomalies is 7 cm.

In general, the distribution of rainfall has substantially shifted from normal. In many places the rainfall is delayed, whereas in many other places it is preponed or untimely. In some parts of globe, severe aberrations of distribution pattern of rainfall has caused substantial changes in biosphere. In some parts of world, frequency of bizarre and heavy rain falls have increased by 20% changing their hydrological cycles.

Changes in Seasons and Seasonal Characters

Based on temperature and rainfall, each part of world has definite seasons in a year with many characteristic features about air temperature, hydrological changes, biological growth etc. In most parts of world, summer and winter seasons are clearly distinguished. The onset and closure of these seasons is almost predefined and recurs every year with high degree of dependability. Such repeatable characters of seasons with good degree of dependability has resulted in many biological characteristic features of a region / country.

Due to change in air temperature and rainfall patterns, seasons have preponed on post-poned. Winters have extended in many places, while summer is more severe in other places. The degree of dependability has reduced and an element of uncertainity has increased. For e.g., in India pre-monsoon showers (summer rainfall) have reduced substantially, while cyclonic rainfall during November-December have become a regular feature. Similarly, the length of winter with low temperature and summer with high temperature have increased in many places. Unduly long spell of cold temperatures and unduly long hot dry temperatures have become more frequent in the last 15 - 20 years.

This has also necessitated a rethinking of classification of global climates from present system of equatorial, tropical, sub tropical, temperature and arctic / polar climates. A shift from tropical to temperate climates or tropical to subtropical climates has been recorded at many places.

Shifts in disease / pest cycles of animals and plants

Many diseases and pests of plants / animals follow a particular rhythm because of specific response of causal organism to a set of environmental conditions. Hence, disease / pest is more likely to attack a plant/animal if the environmental situations are favourable. These are called 'pre-disposing' factors. For e.g., sap sucking insects called 'aphids' are favourably multiplied in large number if day temperature is between 15 - 20oC and humidity is 60 - 80%.

Due to change in environment, the disease/ pests may occur with greater virulence or may subside. Hence, the attack of pests and diseases in recent years has been undergoing unpredictable changes. Many new diseases such as neckblast in paddy, wiltin cotton, as well as new pests like Armigera in cotton, nematode attack in paddy have been reported recently. Similarly, some diseases like black arm in cotton, blight in paddy, canker in guava have gradually reduced.

Even in human beings, the attack of some diseases is pre-disposed by many environmental factors.

For e.g., low temperature and high humidity favour asthma, coastal climate favouring elephantiasis, heavy rainfall favouring malaria. Recent spread of mosquito borne diseases like malaria in US and dengue in India are examples of spread of tropical habitat northward favouring growth and development of mosquito population in unconventional areas.

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