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The rise in the average temperature of the earth due to the abnormally high concentrations of greenhouse gases (like CO2, H2O, methane etc) in the atmosphere due to human activity(factories, automobiles etc) or natural(volcanic activities) is called global warming. These greenhouse gases trap heat (in the form of radiation) from the sun by absorbing it and by not letting it escape into the atmosphere, thereby increasing the temperature of the earth surface – this is called the greenhouse effect. Because of greenhouse effect, the average global temperature has increased by almost one degree Farenheit over the past century; an additional 2 to 6oF increase over the next one hundred years is also expected. This may not seem to be much, but it could change the earth 's climate as never before. Here is a comparison : At the peak of last ice age (1800 years ago),the temperature was only 7oF colder than it is today, and glaciers  covered much of North America. When global occur the ice shelves get weaker as temperature rise and creek which will release new ice island from its artic grip, making it an hazardous for ships.  As more and more ice melts, the level of sea is expected to rise by 5-10cm. If this happens, the low lying coastal regions will be completely submerged in water resulting in the loss of life as well as property.