The NYT had a great article about how Scientists have figured out a way to turn CO2 into gasoline. They would suck CO2 out of the atmosphere, put it through a chemical process, and create gasoline. This sounds like the cure for our greenhouse gas and energy problem, right? Wrong, it takes a lot of energy to do this and in my opinion the payoff doesn't make sense.
There is, however, a major caveat that explains why no one has built a carbon-dioxide-to-gasoline factory: it requires a great deal of energy. [via NYT]
Now the article didn't detail what the net energy loss was in this equation (meaning you put in more energy than you get out) but I assume this won't make sense to do just like biofuels when combating greenhouse gases.
Remember, you should think of the producing energy just like trading. Your reward to risk ratio should always be greater than 1. That means if I'm using 1 Energy Unit to make more energy, the output should always be greater than 1 Energy Unit. Disclaimer: Laws of Physics apply!