C4 plants utilise much less Carbon dioxide than other, older plants. C4 plants came into existence about the same time as Antarctica began to ice over. C4 plants came into existence because free Carbon dioxide in our atmosphere had reached the lowest ever level, since Earth first gained an atmosphere. Many of the plant based foods that humans have come to rely on, are C4 plants.
Clearing the forests.
Forests hold huge amounts of water. Forests transpire causing it to rain. Forests hold the ecosystem together. Forests retain the soil. In the first instance, often the most prolific crops can be grown by removing the forest. The soil is deep and fertile, it is alive with microorganisms, it is essentially those microorganisms that aid the flourishing of the forests.
One the forest is removed, everything goes quickly downhill.
It rains less. When it does rain, it is often more intense and washes the soil away. The microorganisms die out, the fertility of the soil dies out. Winds are more problematic, blowing away the dry soil. Fertilizers have to be used and irrigation employed, rather than the Earth managing itself, man is managing the Earth.
The more men there are, the more managing that is required.
All this plays out, even without using fossil fuels.