6.geochemical process that release carbon dioxide
7.store carbon underground
8.human activities that release carbon dioxide

Geochemical:  I guess when volcanoes erupt? 
Store Carbon: when lots and lots of plants die and then get buried or covered up by more organic material and soil, carbon builds up underground.
Human Activities: burning fossil fuels (like burning coal for power plants, driving cars, lighting fires for stoves or farming), breathing 

It's either -limiting factors or -scarce resources but i'd go with my first answer since the food and water are abundant they really aren't scarce