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Greenhouse effect

The earth receives energy from the sun, which warms the earth’s surface. As this energy passes through the atmosphere, a certain percentage (about 30) gets scattered. Some part of this energy is reflected back into the atmosphere from the land and ocean surface. The rest (70%) actually remains behind to heat the earth. In order to establish a balance, therefore, the earth must radiate some energy back into the atmosphere. As the earth is much cooler than the sun, it does not emit energy as visible light. It emits through infrared or thermal radiation. However, certain gases in the atmosphere form a sort of blanket around the earth and absorb some of this energy emitted back into the atmosphere. Without this blanket effect, the earth would be around 30 C colder than it normally is. These gases like carbon dioxide, methane, and nitrous oxide, along with water vapour, comprise less than one per cent of the atmosphere. They are called 'greenhouse gases' as the working principle is same as that which occurs in a greenhouse. Just as the glass of the greenhouse prevents the radiation of excess energy, this ‘gas blanket’ absorbs some of the energy emitted by the earth and keeps temperature levels intact. This effect was first recognized by a French scientist, Jean-Baptiste Fourier, who pointed out the similarity in what happens in the atmosphere and in a greenhouse. Hence the term the ‘greenhouse effect’.

This gas blanket has been in place ever since the creation of the earth. Since the industrial revolution human activities have been releasing more and more of these greenhouse gases into the atmosphere. This leads to the blanket becoming thicker and upsets the ‘natural greenhouse effect’. Activities that generate greenhouse gases are called ‘sources’ and those that remove them are known as ‘sinks’. A balance between ‘sources’ and ‘sinks’ maintains the levels of these greenhouse gases.

Humankind upsets this balance when new sources that interfere with the natural sinks are introduced. Carbon dioxide is released when we burn such fuels as coal, oil, and natural gas. And, when we destroy forests, the carbon stored in the trees escapes as carbon dioxide into the atmosphere. Increasing agricultural activities, changes in land-use patterns, and other sources lead to rising levels of methane and nitrous oxide. Industrial processes also release artificial and new greenhouse gases like CFCs (chlorofluorocarbons), while automobile exhaust fumes lead to ozone generation. The resulting enhanced greenhouse effect is more commonly referred to as global warming or climate change.

Greenhouse gas emission trends

Global greenhouse gas (GHG) emissions have grown since pre-industrial times, with an increase of 70% between 1970 and 2004.

The concentrations of several greenhouse gases have increased over time. These are:
carbon dioxide, methane, nitrous oxide, hydrofluorocarbon, perflurocarbons and sulphur hexafluoride Human activity increases the greenhouse effect primarily through release of carbon dioxide, but human influences on other greenhouse gases can also be important.

Of all the GHGs, the concentrations of carbon dioxide in the atmosphere are at the highest levels. The emissions of these gases have increased at different rates. CO2 emissions have grown between 1970 and 2004 by about 80% (28% between 1990 and 2004) and represented 77% of total anthropogenic GHG emissions in 2004.

The largest growth in global GHG emissions between 1970 and 2004 has come from the energy supply sector.

The emissions of ozone depleting substances (ODS) controlled under the Montreal Protocol, which are also GHGs, have declined significantly since the 1990s.

By 2004 the emissions of these gases were about 20% of their 1990 level.

Examples of greenhouse gases affected by human activities

(Carbon Dioxide)
(Nitrous Oxide)
(Chlorofluoro- carbon-11)
Pre-industrial concentration About 280 ppm About 700 ppb About 270 ppb Zero Zero 40 ppt
Concentration in 1998 365 ppm 1745 ppb 314 ppb 268 ppt 14 ppt 80 ppt
Rate of concentration changeb 1.5 ppm/yra 7.0 ppb/yra 0.8 ppb/yr -1.4 ppt/yr 0.55 ppt/yr 1 ppt/yr
Atmospheric lifetime 5 to 200 yrc 12 yrd 114 yrd 45 yr 260 yr >50.000 yr

Source: www.unfcc.int


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