Global warming is the temperature of Earth's surface, oceans and atmosphere going up over ten to thousands of years. Average temperatures today are about higher than before people started burning a lot of coal around 1750. But in some parts of the world it is less than this and some more. Most climate scientists say that by the year 2100 temperatures will be to higher than they were before 1750. The changes people can see most easily is because of this increase in temperature is the melting of ice caps all around the world. Sea level is rising for two reasons. One reason is because of ice on the land, like Greenland, melting into the sea. The other reason is because of water getting bigger when it gets warmer. Many cities will be partly flooded by the ocean in the 21st century. 