TOO MUCH carbon dioxide is changing the climate.  Those large quantities of carbon dioxide are being produced by human activity, namely the heavy burning of fossil fuels (coal, gasoline, etc.). When it was only a matter of plants consuming CO2 and animals producing CO2 by their respiration, things were in balance.  Once humans had figured out how to control and manage fire, we started producing extra CO2.  For most of human history, the amounts of CO2 produced by human activity were still pretty small.  However, in modern times, we have figured out how to utilize VERY concentrated sources of carbon, namely coal and petroleum, to produce energy.  This results in the release of larger quantities of CO2 into the atmosphere which tends to hold in the heat of the sun and prevent it from radiating back out into space.  This extra heat, in turn, is  changing the climate of the earth, making some areas dryer and less suited to agriculture.