Rainforests are placed in belts around the equator in Brazil, Africa and Asia and play an important role in keeping the atmosphere moist by creating a circulation of moisture as they pull up water from the ground, which then again evaporates into the atmosphere.
By removing rainforests, the circulation of moisture will disappear, as there are much fewer trees to emit and control moisture.
Atmospheric moisture, fresh water supplies and circulations interact with global weather patterns. As rainforests dry out and disappear, resulting in large desert areas or savannas being created, global climatic patterns will change in ways which could be very serious for mankind at large.
Rainforests store billions of tons of carbon (CO2) in their biomass. In fact they store almost twice as much compared to a normal European forest per unit of forest.
About 20% of the worlds humanly created CO2 emissions come from the removal of rainforests, which greatly exceeds emissions coming from all global transportation, which roughly represents 14%.