Global warming is the temperature of Earth's surface, oceans and atmosphere going up over tens to thousands of years. Average temperatures today are about higher than before the Industrial Revolution, which started around 1750, during the Little Ice Age, an abnormally cold period. But in some parts of the world it is less than this and some more. Some scientists say that by the year 2100 temperatures will be to higher than they were before 1750. The most noticeable changes because of this increase in temperature is the melting of ice caps all around the world. Sea level is rising steadily because of continental ice melting into the sea. Many cities will be partly flooded by the ocean in the 21st century. Humans are primarily the cause of global warming.