Carbon dioxide is a greenhouse gas that helps to trap heat in Earth's atmosphere. We need some CO2 in the atmosphere to maintain a warm planet, but excess
carbon can cause considerable warming of the planet.
B. What percentage decrease in fossil fuel usage is required to achieve this goal?

Atp is is our little "pocket" of energy. of course, if you have too little, then it means you have too little energy, and you'll pass out. the way to not have enough, is to have no energy sources (glucose, fats, proteins). you can't have too many atp molecules. cycles like the krebs cycle produce it, but if you have too much of any substrate (or too little for that matter), the cycle stops and atp is no longer made (it stops until substrate levels are such that the process can go on). in any case, this is unlikely since atp is constantly hydrolyzed and remade, even while you're asleep. you won't have too much. if you have zero, you are dead or dying.

He used pea plants to study the patterns of heredity.