Climate change has been going on for more than a century. The global climate gets warmer and warmer and the average annual temperature keeps rising. This makes sea water heat up and expand, thus taking up more space and causing a significant rise in the sea level. Another cause of sea level rise has to do with icebergs. As icebergs melt due to rising temperatures, huge blocks of ice move into the sea at the poles. This phenomenon also significantly elevates the sea level. Currently, the sea level is eight inches higher than it was a century ago. Scientists predict that by the end of this century, the sea level is very likely to be three feet higher than it is today. Sea level rise is threatening to submerge some countries. Among them, island countries are especially vulnerable. For example, Tuvalu, an island in the South Pacific Ocean, will likely become the first country to disappear underwater over the coming decades. Other victims suffering the same fate include Kiribati, the Maldives, and the Marshall Islands, to name just a few. Human activities are impacting the climate system. People have been burning coal and petroleum for a long period of time. Such fossil fuels release greenhouse gases to the atmosphere when they are burned. Greenhouse gases act like a sweater around the Earth for they are good at trapping heat and making temperatures rise. As it stands, sea level rise does not pose threats to island countries alone. Scientists are already warning that some of the world's major cities could be underwater by the end of the century. Needless to say, action to protect them should be taken as soon as possible.