he theory is due to the hormones in the brain responsible for "wakefulness", or the amount of time we spend awake. CO2 levels can affect these hormones, leading us to go to bed later, which affects our metabolism making it easier for us to gain weight.
Carbon doesn't just affect the atmosphere, it also makes us eat. Breathing in increased CO2 makes our blood more acidic, affecting our brain patterns, making us want to eat more.
If that still sounds perculiar, consider that a study conducted in 2010 has also seen the same effects in different species of animal — so it's not just the proliferation of fast food restaurants leading to humans piling on the pounds.