One of the most serious problem in front of the world is the global warming.There are some reasons that can cause such a problem.First,people cut down trees and forests freely.Besides,people burn things everywhere - even it is not allowed.People waste things out of control is another reason.So,we should try our best to save things as much as we can.We should encourage others to do so.If we do it,the stage will be better.If we don't take any action,as the warld is becoming warmer and warmer,the water level is rising.It can cause subsidence and threaten people's activities.