The Earth is the only planet in our solar system that supports life. The complex process of evolution occurred on Earth only because of some unique environmental conditions that were present: water, an oxygen-rich atmosphere, and a suitable surface temperature. About 30% of incoming energy from the sun is reflected back to space while the rest reaches the earth, warming the air, oceans, and land, and maintaining an average surface temperature of about 15 oC.
The atmostphere carries out the critical function of maintaining life-sustaining conditions on Earth, in the following way: each day, energy from the sun is assorbed by the land, seas, mountains,etc. If all this enerygy were to be assorbed completely, the earth would gradually become hotter and hotter. But actually, the earth both absorbs and, simutanously releases it in the form of infra red waves. 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), reemit some of this heat to the earth's surface. It they did not perform this useful function, most of the heat energy would escape, leaving the earth cold (about -18oC) and until to support life.
However, ever since the Industrial Revolution began about 150 years ago, man-made activities have added significant quantities of GHGs to the atmosphere. The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have grown by about 31%, 151% and 17% , respectively, between 1750 and 2000.
An increase in the levels of GHGs could lead to greater warming, which in turn could have an impact on the world's Climate, leading to the phenomenon known as climate change. Climate change refers to a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Indeed, scientists have observed that over the 20th century, the mean global surface temperature increased by 0.6 C. They also observed that since 1860 ( the year temperature began to be recorded systematically using a thermometer), the 1990's have been the warmest decade. Basic information about the climate change are given below.
Human activities are releasing greenhouse gases into the atmosphere. Carbon dioxide is producted when fossil fuels are used to generate energy and when forests are cut down and burned. Methane and nitrous oxide ar emitted from agricultural activities, changes in land use, and other sources. Artificial chemicals called halocarbons (CFCs, HFC, PFCs) and other long-lived gases such as suphur hexafluoride (SF) are released by industrial processes..
Rising levels of greenhouse gases are already changing the climate. In response to humanity's emissions, the climate has started to adjust to a "thicker blanket" of greenhouse gases in order to maintain the balance between energy arriving from the sun and energy escaping back into space. Observations show that global temperatures have risen by about 0.6 C over the 20th century. There is new and stronger evidence that most of the observed warming over the last 50 years is attributable to human activities.
Climate models predict that the global temperature will rise by about 1.4 - 5.8 C by the year 2100. This change would be much larger than any climate change experienced over at least the last 10,000 years. The projection is based on a wide range of assumptions about the main forces driving future emissions (such as population growth and technological change) but does not reflect any efforts to control emissions due to concerns about climate change.
Climate change is likely to have a significant impact on the global environment. In general, the faster the climate changes, the greater will be the risk of damage. The mean sea level is expected to rise 9-88 cm by the year 2100, causing flooding of low-lying areas and other damage. Other effects could include an increase in global precipitation and changes in the serverity or frequency of extreme events.
Human society will face new risks and pressures. Food security is unlikely to be threatened at the global level, but some regions are likely to experience food shortages and hunger. Water resources will be affected as precipitation and evaporation patterns change around the world. Physical infrasture will be damaged, particularly by sea-level rise and by extreme weather events.
People and ecosystems will need to adapt to future climatic regimes. Past and current emissions have aleady committed the earth to some degree of climate change in the 21st century. Adapting to these effects will require a good understanding of socio-economic and natural systems, their sensitivity to climate change, and their inherent ability to adapt. Fortunately, many strategies are available for adapting to the expected effect of climate change.
Stablilizing atmospheric concentrations of greenhouse gases will demand a major effort. Without emissions-control policies motivated by concerns about climate change, atmospheric concentrations of carbon dioxide are expected to rise from today's 367 parts per million to 490 -1,260 ppm by the year 2100. This would representa 75 -350% increase since the year 1750. Stabilizing concerntrations at, for example, 450 ppm would require worldwide emissions to fall below 1990 levels within the next few decades.
The Kyoto Protocol would require government to take even stronger action. In 1997, the Parties to the Convention agreed by consensus that developed countries should accept a legally binding commitment to reduce their collective emissions of six greenhouse gases by at least 5% compared to 1990 levels by the period 2008-2012. The Protocol also establishes an emission trading regime and a "Clean development mechanism". Howeever, the Protocol has not yet received enough ratifications to enter into force.