http://www.epa.gov/climatechange/kids/bigdeal.html
Sometimes little things can turn into big things. Think about brushing your teeth. If you don't brush for one day, chances are nothing bad will happen. But if you don't brush your teeth for one month, you may develop a cavity. It's the same thing with global temperatures. If temperatures rise above normal levels for a few days, it's no big deal – the Earth will stay more or less the same. But if temperatures continue to rise over a longer period of time, then the Earth may experience some problems. Average global temperature has increased by almost 1ºF over the past century; scientists expect the average global temperature to increase an additional 2 to 6ºF over the next one hundred years. This may not sound like much, but it could change the Earth's climate as never before. At the peak of the last ice age (18,000 years ago), the temperature was only 7ºF colder than it is today, and glaciers covered much of North America!
Even a small increase in temperature over a long time can change the climate. When the climate changes, there may be big changes in the things that people depend on. These things include the level of the oceans and the places where we plant crops. They also include the air we breathe and the water we drink.
What Might Happen? It is important to understand that scientists don't know for sure what climate change will bring. Some changes brought about by climate change will be good. If you live in a very cool climate, warmer temperatures might be welcome. Days and nights could be more comfortable and people in the area may be able to grow different and better crops than they could before. But it is also true that changes in some places will not be very good at all.