There may be several reasons why this may be true.
The food production done by rain forest vegetation feeds air-breathing animals.
These animals produce carbon dioxide as a result of their oxygen consumption. Oxygen is consumed when the body metabolizes food to convert it to sugars. As a result the consumption of the food manufactured by the rain forest contributes directly to the increase of carbon dioxide.
However, carbon dioxide is not the only greenhouse gas. Methane and other hydrocarbon gases contribute greatly to the ...