Weathering is the breakdown of rock by physical, chemical or biological processes. Limestone areas are predominantly affected by chemical weathering when rainwater, which contains a weak carbonic acid, reacts with limestone. This causes the limestone to dissolve. Carbon dioxide from the respiration of animals (and ourselves) is one cause of increased carbon dioxide in the atmosphere. Burning fossil fuels also contributes to this. Carbon dioxide in the atmosphere forms very dilute carbonic acid when it dissolves in rain.
Evidence of chemical weathering can be seen in areas where limestone is exposed to the elements. Rainwater erodes the vertical joints and horizontal bedding planes in limestone (see image below). In doing this karst scenery is created.