Hi there!
With humanity's constant burning of fossil fuels, large amounts of carbon dioxide get pumped into the atmosphere.
Under normal circumstances (without humans), greenhouse gases are good, as they help keep heat energy in, much like a blanket.
Now, with such large amounts of carbon dioxide, too much heat is being kept inside of this blanket. Our ice caps are melting, threatening the survival of many species of animals. Along with their melting, sea levels are rising (and will keep rising).
With our increasing population, the last thing we can afford is to lose land as well.
Hope this helps.
:)