This trend probably began thousands of year ago
many areas used to be covered with ice
in recent years the warming trend has speeded up
scientist think that part of the reason for this maybe carbon dioxide pollution from industry and automobile
this pollution covers the earth like a blanket and keeps it warmer
Another reason for the warming may be the fact that the earth is losing its forests.
forests are important part of the earth's system for using up carbon dioxide and keeping the air temperature cool