CO2 and other gases which make the earth warmer than required. Atmospheric layer now traps more energy which comes from the sun rays. This results in warming of the earth’s surface and atmosphere. So, the earth is getting warmer and climate is changing. This is called green house effect. The average temperature of the Earth's surface increased by an estimated 0.6°C in the 20th century and, according to the most recent projections of the Intergovernmental Panel on Climate Change, could rise 1.4 to 5.8°C above the 1990 average by 2100. Much of this predicted increase is attributed by scientists to increasing concentrations of greenhouse gases such as carbon dioxide (CO2) in the atmosphere.

• Without Greenhouse gases, the world would be seventy-two degrees Fahrenheit cooler
• Very high & Very low concentration of GHG affect our whole earth

The atmosphere carries out the critical function of maintaining life-sustaining conditions on Earth, in the following way: each day, energy from the sun (largely in the visible part of the spectrum, but also some in the ultraviolet and infra red portions) is absorbed by the land, seas, mountains, etc. If all this energy were to be absorbed completely, the earth would gradually become hotter and hotter. But actually, the earth both absorbs and, simultaneously releases it in the form of infrared waves (which cannot be seen by our eyes but can be felt as heat, for example the heat that you can feel with your hands over a heated car engine). All this rising heat is not lost to space, but is partly absorbed by some gases present in very small (or trace) quantities in the atmosphere, called GHGs (greenhouse gases).

Greenhouse gases (for example, carbon dioxide, methane, nitrous oxide, water vapour, ozone), re-emit some of this heat to the earth's surface. If they did not perform this useful function, most of the heat energy would escape, leaving the earth cold (about -18 ºC) and unfit to support life.

Greenhouse Gases Emissions :
CO2 - About half the CO2 released by burning fossil fuels
CFCs - Found in Fridges, Air Conditioners, Aerosols etc. are extremely effective greenhouse gases. Although there are lower concentrations of CFCs in the atmosphere than CO2 they trap more heat. A CFC molecule is 10,000 times more effective in trapping heat than a CO2 molecule. CFCs molecules survive for 110 years in the atmosphere.

Methane - is released during coal-mining activities, oil exploration and when vegetation is burnt during land clearance. The main source of methane though is agricultural activity. It is released from wetlands such as rice paddies and from animals, particularly cud-chewing species like cows Methane molecules survive for 10 years in the atmosphere. Since the 1960s the amount of methane in the air has increased by 1% per year. Nitrous oxide - comes from both natural and man-made processes. mainly: fossil fuel combustion, as in power stations, use of nitrogenous fertilizers, burning rain forests and human and animal waste.

Importance of atmosphere in Global Warming :
Mercury and Venus are the two planets between Earth and the Sun. Earth can sustain life only because it has the right amount of atmospheric cover and the right temperature for life to exist.

Changes in CO2 and Temperature in last 400 000 years :
Graph shows increased concentration of CO2 in present times. There is a sharp increase in the concentration of CO2 because of which the temperature of our planet is increasing.

Sources of Greenhouse Gases :
Human Activity -       

1. Burning of fossil fuels
2. Deforestation contributes to 20% of CO2 emissions worldwide.
3. Land fill site
4. Transportation-27% of all manmade CO2 emissions are transportation related.750 million cars worldwide emit a total of approximately 2.25 billion tons of CO2 each year
5. Industrialization - responsible for 57% of all CO2 emissions.
6. Building Structure

Natural -

• Explosions on the sun ("sun spots")
• Volcanic eruptions on a massive scale
• Changes in earth orbit
• Changes in earth's orientation toward the sun
• Explosions caused by large meteors hitting the earth 

Factors & Sources of Greenhouse Gases :
Molecules of NO2 can stay approx for 250 years in the atmosphere as compare to Carbon dioxide which stays for 5 years. Man made emission of CH4, NO2, and CFC’S. The main reasons for the emission of gases that are a cause of green house effect are burning of fossil fuel like coal in the power plants for the purpose of generation of electricity. Fossil fuel burning leads to high emissions of carbon dioxide gas. Another green house gas is methane. Methane is more than 20 times as effectual as CO2 at entrapping heat in the atmosphere. Landfill sites emits large amount of methane gas.

1. Transportation - We are producing twice as much carbon dioxide in the world today. Automobiles, the second largest source. 750 million cars worldwide emit a total of approximately 2.25 billion tons of CO2 each year .About 20 % of carbon dioxide emitted in the atmosphere comes from burning of gasoline in the engines of the vehicles. All new-born babies, and an estimated one in four people worldwide are exposed to unhealthy concentrations of air pollutants. . Cars produce 0.25 tons of CO2/km whereas two wheeler produces 0.028 tons of CO2. It is surprising that the average number of vehicle per family in Pune possess about 3.3 vehicles probably one of the highest in the country. Every individual in Pune if considered traveling an average of approximately 40 Kms/day, it would lead to 3650 tons /year from 4W and 365 tons/year from 2W of GHG.
   
   
2. Deforestation - When trees are cleared, stored carbon is released into the atmosphere. When the wood dies the CO2 is returned to the air. Forest clearance and wood burning (such as happens in tropical rain forests) is increasing the latter half of the process, adding to the CO2 in the atmosphere. After carbon emissions caused by humans, deforestation is the second principle cause of atmospheric carbon dioxide Deforestation is responsible for 20-25% of all carbon emissions entering the atmosphere, by the burning and cutting of about 34 million acres of trees each year Every year, 20 million hectares of rainforest an area the size of England, Wales and Scotland combined are cut down, releasing millions of tons of carbon emissions into the atmosphere.

   ALERT’s 5 Point Programme :

   1. Reduce fossil fuel
   2. Reduce consumption of electricity
   3. Segregate garbage - vermicompost wet waste
   4. Plant trees
   5. Spread the message