Climate change refers to a statistically significant variation in either the mean state of the climate or in its variability, which is attributed directly or indirectly to anthropogenic activities that alter the composition of global atmosphere and which are in addition to natural climatic variability observed over comparable time periods.

While the world's climate has always varied naturally, the vast majority of scientists now believe that rising concentrations of "greenhouse gases" in the earth's atmosphere, resulting from economic and demographic growth over the last two centuries since the industrial revolution, are overriding this natural variability and leading to irreversible climate change. The implications of "global warming" are far reaching, and include rises in sea levels, changes in rainfall patterns (increasing the threat of drought or floods in many regions) and a greater threat of extreme weather events, such as intense storms and heat waves. Climate change could, therefore, have potentially dramatic negative socio-economic and environmental impacts.

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 infra red waves (which are invisible but can be felt as heat). 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 greenhouse gases (GHGs).

The main greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide, and the chlorofluorocarbons (CFCs). Levels of all key greenhouse gases (with the possible exception of water vapour) are rising as a direct result of anthropogenic activities. Concentration of carbon dioxide (mainly from burning coal, oil, and natural gas), methane and nitrous oxide (due to agriculture and changes in land use), and CFCs (produced by industries) are changing at an unprecedented speed. The result is known as the "enhanced greenhouse effect". This could lead to greater warming, which in turn, could have an impact on world's climate leading to the phenomenon known as "climate change".

The climate system must adjust to rising greenhouse gas levels to keep the global "energy budget" in balance. In long term, the earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate must change somehow to restore the balance between incoming and outgoing energy. This adjustment will include a "global warming" of the earth's surface and lower atmosphere. But this is only part of the story. Warming up is the simplest way for the climate to get rid of the extra energy. But even small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming, others to counteract it.

India scenario @ 1990 shows, total CO₂-equivalent emissions were 1001352 Gg and this figure was approximately 3% of global emissions. Energy sector contributes the main CO₂ source with 55% of national emissions, which includes the emissions from transport sector, coal mining, and fugitive emissions from oil and natural gas. Second largest source for GHGs in India was from the agriculture sector with 34% of national GHGs; leads from methane emissions from enteric fermentation in domestic animals, manure management, rice cultivation, and burning of agricultural residues. Emissions from land use change and forestry (LULUCF) sector, results in negligible emissions (ADB-GEF-UNDP 1998).

Global climate is changing drastically w.r.t the anthropogenic activities. Extensive research and computer models have been developed in recent years to get a coherent relationship between the climatic change & various other factors. Industrialized countries contributes to the major part of the responsibility for growing concentration of green house gases and the major burden is on the developing countries that will receive the imminent negative impacts due to their vulnerable socioeconomic, political, and environmental conditions.

The effects of global warming are difficult to quantify because of the complicated relationships between air temperature, precipitation quantity and pattern, vegetative cover and soil moisture. However, it is likely to harm humanity in following ways:

Though, some scientists realise following potential benefits of global warming:

• Some regions at mid-latitudes will have increased crop yields for increases in temperature of a few degrees Celsius.
• An increase in global timber supply from appropriately managed forests.
• More water in some water-scarce regions-for example, in parts of Southeast Asia.
• Reduced winter mortality in mid-and high latitudes.
• Higher winter temperatures will reduce energy demand for space heating.
  

To slow or reverse greenhouse gas emissions all governments need to develop and implement policies to combat climate change. In the early years these policies tended to be rather fragmented; now they are becoming more coherent and linked to sustainable development. The overall aims are to achieve production that is less carbon-intensive and to increase the use of alternative sources of energy, while boosting energy efficiency - in industries, in buildings, in transportation and appliances. Fiscal measures were among the most widely used in developed countries, often combined with financial incentives such as grants and preferential loan rates and tariffs.

India plays a major role in combating the global climatic change:

• India signed UNFCCC on 10 June 1992 and ratified it on 1 November 1993. 
• India acceded to the Kyoto Protocol on 26 August 2002.
• Ministry of Environment and Forests (Govt. of India), the nodal ministry to deal the climate change issues has a separate cell for climate change and another group known as 'Working group on the UNFCCC and Kyoto Protocol' to deliberate the actions that has to be made with regard to the climatic change issues. 
•  In accordance with the Annex decision 17/CP.7; the Govt. of India has constituted the National CDM Authority (NCDMA) on 16th April, 2004. The Indian NCDMA is operational since December 2003.
• Ministry of Non-conventional Energy Sources has separate Climatic Change Advisory Committee working to create appropriate methodologies for implementing RETs in the country and thus increasing the CDM potential.
• In addition to the above groups established by MoEF & MNES a separate group is working under the Planning Commission on the PMO advice.