According to the Intergovernmental Panel on Climate Change (IPCC) ,Climate change refers to “Any change in climate over time whether it is due to natural variability or as a result of human activity”. Since the third Assessment Report (TAR), new observations, enhanced understanding of uncertainties, broader geographical coverage of observations, improvements and extensions of many datasets and data analyses have brought progress in the understanding of how climate is changing in space and time.

Ever since 1750, human activities have led to a severe increase in global concentration of carbon dioxide, methane and nitrous oxide in the atmosphere. The global increase of nitrous oxide and methane in the atmosphere is mainly due to agriculture while carbon dioxide concentration is associated primarily to the use of fossil fuel and land use change.

Changes in the atmospheric concentration of greenhouse gases (carbon dioxide, methane and nitrous oxide etc), aerosols, solar radiation and changes in surface albedo have altered the energy balance of the climate system. These changes are expressed in terms of radiative forcing which is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the earth atmosphere system. Positive forcing tends to warm the earth surface while negative forcing has instead a cooling effect..Evidence have shown that ever since 1950, human activities have been one of warming (positive forcing) with a radiative forcing of +1.6Watts per square Meters (Wm^-2). Changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12wm^-2..Erosols on the other hand have a net negative radiative forcing and hence a cooling effect.

According to the IPCC,Increase in the global concentration of anthropogenic greenhouse gases in the atmosphere is very likely to be responsible for most of the observed increase in global average temperatures since the mid-20^th century. Lots of direct observations of recent climate change have been made at the continental, regional and ocean basin scales. Examples

-Ever since 1900 temperatures in the frozen grounds have increased in the Northern hemisphere due to temperature rise.

-Changes in the amount of precipitation have been observed in many large regions of the world. Eastern parts of north and South America and Northern Europe have experienced a severe precipitation increase while drying has been observed in the Sahel and Mediterranean. Also severe droughts have been observed over large

regions especially the tropics and subtropics since the 1970s.This is due to higher temperatures and decrease in precipitation.

-An Increased in ocean salinity has been observed in low latitude waters.

-Ever since 1960s, mid-latitude westerly winds have become stronger in both hemispheres.

-Intense tropical cyclones activities have been observed in the North Atlantic since about 1970.

Projected warming of the 21^st century shows that warming will not be evenly distributed around the globe. It is expected to be greatest overland and least over southern oceans and parts of the North Atlantic Ocean (partly due to waters ability to store heat)

-warming is expected to be greatest at most high northern latitudes (partly due to positive feedback effects from melting ice)

-Extensive increases in thaw depth are projected over most permafrost regions. Reduction of the Greenland ice sheet is projected to continue to contribute to sea level rise after 2100.

-Under all the IPCC Special Report on Emission Scenarios (SRES), sea ice is projected to shrink in both the Arctic and Antarctic.

-More frequent hot extremes, heat waves and heavy precipitation events are projected to occur in the future. Increases in the amount of precipitation are very likely in high latitudes, while decreases are likely, in most subtropical land regions.

-Based on models, more intense future tropical cyclones with larger peak wind speeds are likely to occur.

-Pole ward movement of extra tropical storm tracks with consequent changes in wind, precipitation and temperature patterns are projected to occur in future.

Generally, even if the concentration of greenhouse gas in the atmosphere were to stabilized, anthropogenic warming and sea level rise would continue for centuries due to the time scales required for removal of this gas from the atmosphere. Many of the greenhouse gases that have already been emitted remain in the atmosphere for decades and will continue to contribute to warming for their duration. Based on current understanding of climate carbon cycle feedback, model studies suggest that to stabilize carbon dioxide at 450ppm,a cumulative emissions over the 21^st century needs to be reduced from an average of approximated 670GTC to approximately 490GTC.to stabilize at 1000ppm,the cumulative emission has to be reduced from a model average of approximately 1415 GtC to approximately 1100GtC.

1)Intergovernmental Panel of Climate Change(IPCC)

web site:http://www.ipcc.ch/