Because it is not a local problem, it affects entire planet. First, thermal radiation from the Earth's surface is absorbed by a greenhouse gas (which is in the atmosphere). Then it is spread in all directions. 
THE GREEN HOUSE are the trees in the planet that absorbs the carbon dioxide and all other gases to facilitate it or to convert it into oxygen again but trees are cut down easily fast more than the period of growth that it needs to have a new one or to replace it,
by that way the absorption of the unwanted gas around could not be process by the trees because they are now less in quantity so the continuing heat which the sun gives each day cannot be process and the daily heat keeps on trapping on the ground so even the ice on the north and south pole keeps on melting,that gives the climate change to be not in order