|When visible light from the Sun hits the earth, some of the radiations are absorbed and used to heat the earth. The heated earth, in turn, gives off infrared radiations,
which is reradiated back into space. However, certain gases in the atmosphere
absorb the infrared heat that would normally be radiated back into space. This
progressive heating of the earth's surface due to the gradual building up of infrared
radiations within the atmosphere, caused largely by the accumulation of Greenhouse
gases is known as Greenhouse Effect. The Greenhouse Effect is so called, as it is similar to the warming of air inside a greenhouse compared to the air outside. The greenhouse effect is critical for the survival of life on earth, as it keeps the surface temperature about 33°C warmer than it would be without the greenhouse effect. This is essential for the evolution and proliferation of life. The gases such as carbon dioxide (CO ), 2 methane (CH ), nitrous oxide (N O), Halofluorocarbon (HFC) & Sulphur hexafluoride (SF ) which absorb the solar infrared radiation, are known as the Greenhouse Gases.|
The greenhouse gases act like a blanket, preventing much of the heat reflected by the earth's surface from escaping directly into space. By slowing the release of cooling radiation, these gases warm the Earth's surface. While this is a natural process that is essential to life on Earth, the trouble starts when the concentration of these Greenhouse gases in the Earth's atmosphere increases. The result is an increase in the Earth's temperature, also known as - Global Warming.
Global Warming is the gradual increase of the average temperature of Earth's atmosphere and oceans,
which can contribute to changes in global climate patterns. Global warming in turn interferes with the
Earth's climatic systems, resulting in climate change. Global warming could have disastrous effects on the environment (polar ice melts, changes in amount and pattern of rainfall, sea level rise, frequent floods and droughts, hurricanes and typhoons). It would cause large-scale species extinction and have serious impacts on human lives (freshwater availability, agricultural yields, increases in the spatial and quantitative ranges of disease vectors), as well as on economic infrastructure (such as energy, transport and industry).
Major Greenhouse Gases
Our planet is surrounded by a thin layer of atmosphere, which comprises of a layer of constantly moving
gases that provides us with the air that we breathe. Almost the Earth's entire atmosphere is made up of
Nitrogen (about 78%) and Oxygen (about 21%). The rest of the 1 % is made up of trace gases (including the greenhouse gases).
Of all greenhouse gases, carbon dioxide is singly responsible for over half the effect of global
warming (about 77%). Although it is naturally present in the Earth's atmosphere and in oceanic and
terrestrial 'sinks' (such as forests), the trouble starts when carbon dioxide concentrations increase
beyond limits that can be absorbed by the Earth's natural cycle. Carbon dioxide concentrations have
been increasing rapidly in the atmosphere since the start of the industrial revolution, when the
world became heavily dependent on carbon-based fossil fuels for economic growth. Ever since then,
human beings have been emitting carbon dioxide into the atmosphere in their pursuit for
industrialization, economic growth, and better lifestyles. The amount of carbon dioxide in the
atmosphere is about 30% higher now than 200 years ago at about 379 ppmv.
The level of Methane (which is the second biggest contributor to Global Warming (about 20%) in the
atmosphere) is about 145% higher now than 200 years ago at about 1774 ppbv. The main causes of
this increase are the digestive processes of cattle and sheep (enteric fermentation in their rumens
produces methane, collectively adding about 100 million tons a year), cultivation of rice,
decomposition of waste in garbage dumps and landfills, and the escape of natural gas into the
The Nitrous Oxide level in the atmosphere is 15% higher now than 200 years ago at about 319 ppbv.
The main causes for this increase are indiscriminate use of nitrogenous fertilizers in agriculture,
burning of vegetation and emissions from industries.
Chlorofluorocarbons are one of the greenhouse gases that have caused a rise of 0.3 ºC in the global
temperatures in the past century. Their concentration ion the atmosphere has been reduced since they
were phased out to protect the ozone layer. However, fluorocarbons like Perfluorocarbons (emitted during aluminium production) and Halofluorocarbons continue to be emitted into the atmosphere.
Although Water Vapour is the most important greenhouse gas with the highest concentration in the
atmosphere, human activity has little or no direct impact on its concentration in the atmosphere.
However, increasing global temperatures inevitably result in greater water vapour concentrations in the
atmosphere through a positive feedback, wherein higher temperatures increase evaporation rate and
release larger volumes of water vapour into the atmosphere.
Please copy Figure: Graph showing contribution of various greenhouse gases to global warming (PG 20 of PDF) and source also