You can't just compare like that. You've got to look at volume.
For instance, you say cow farts are "natural" sources. Natural how? As in, because they're produced by animals? How would you explain the unnatural population of animals that we've bred into being, solely for consumption? All of those extra animals contribute, too, but can't be considered "natural", at least in the way you were meaning it.
The truth is that there is not, never was, and can't be, a single canonical "right" amount of CO2 in the atmosphere. We're living in a world that changes drastically over the course of 10,000 years. Millions of years ago, insects were the size of us, just because of a _slight_ change in the O2 percentage in the atmosphere. Was that wrong? Or was that right? How about a few millions of years before that, when CO2 was king, and plants evolved because it was the most plentiful, and they exuded a caustic gas, O2?
This biosphere adapts. The animals (including us) come and go, and change and adapt to the circumstances of thousands and millions of years, but there's no "wrong" or "right", there's only "right now".
Now, you could argue from the point of view that since we're the dominant form of life, most intelligent, and technologically advanced, we have a sort of noblesse oblige to "fix" things. Especially since there's evidence that we "broke" them.
There's a sort of universal guilt among the ecologically-friendly people that attempts to repent for their lifestyle. "Carbon credits", for one. Buying organic food, for another. People feel guilty for their "footprint" and try to buy the new age equivalent of indulgences. "I fly a lot, so I buy carbon credits". Great. I mean, not as good as not flying, but at least you feel better about yourself. "I buy organic because pesticides hurt the environment". Awesome. Unfortunately, you had to work nearly twice as much to pay for those organic foods, not to mention that it's unbelievably inefficient, and much more susceptible to disease than the cheaper, prettier, just-as-healthy food 20 feet down the row at the grocery store.
We need to get past the guilt for breaking our planet, because we haven't. It isn't broken. We might have changed our planet, but it's not broken. As soon as we change the terminology, we can stop focusing on the guilt, and start focusing on what's really happening. We want to change the planet again, but in the other direction. We want to change it, because it's going to be more comfortable for us like that. It's what we're used to. It's how we like it, and we (might) have the technology to do it. So stop concentrating on guilt, and start concentrating on the real goal. We're being selfish, by trying to adjust the planet for our own gain, and there's nothing wrong with that. We've been doing it ever since we killed the first snakes that lived under the rocks we were moved when we built the first house. It's only a matter of scale.
