Animals, plants, fungi, algae, even many bacteria produce CO₂ . Every time an animal breathes out, there goes CO₂ that we make when we break down food to release energy. Fungi do the same thing, even they don’t quite breathe. When they have light, plants and algae do photosynthesis to make sugar, but they also break down sugar when they don’t have light, just like us, making CO₂. Think about it, a tree is made of cells, but only the cells in the leaves are doing photosynthesis, the rest live off the sugar the leaves make. When plants lose their leaves, they survive on sugar or starch (long chains of sugars) until they get leaves again.

So why do people blame cows for global warming? That’s more about what comes out the other end. Cows produce a lot of methane gas as the bacteria in their stomachs break down food. Methane is a powerful greenhouse gas. Pound for pound it causes over 20 times more warming than carbon dioxide. I found a source that said, “average cow produces enough methane per year to do the same greenhouse damage as four tons of carbon dioxide.” That’s a lot of gas. Their manure is also a source of methane as bacteria break it down. One source said that a cow produces about 100 kg of methane a year. This has about the same impact as driving a car about 8,000 miles. As you can see, these estimates are very different, but the idea is the same: cows produce a lot of methane.

Global warming is not all due to cows. Any time we burn gas, oil, coal, wood, or similar things, more carbon dioxide is released. You might think that you don’t burn any of those things, but most of our electricity in the US comes from burning fuels. When we cut down trees and other plants, we get rid of things that remove carbon dioxide from the environment.

What are some things that people could do to produce less carbon dioxide (“reduce their carbon footprint”)?

Calculate the number of calories that a cow intakes in a day, multiply that by the number of days in a cow's life, and you get your answer. I don't know what either of those numbers are, but CO₂ output is just the chemical byproduct of burning sugars, which is used to burn energy, so if you know the energy requirement of an animal, you can calculate its CO₂ output. It isn't related to waste.

You may be thinking of methane, i.e. CH₄, not CO₂, which I know cow stomachs also produce but I know less about the mechanism (I know that the methane actually comes from archaea and that the chemical reaction is the oxygen-less fermentation of the sugars in the grasses, but not the amount of methane produced per kilogram of grass).

A cow can produce anywhere from 70-120kg of methane in it’s lifetime. Methane is a greenhouse gas, like CO₂, but it’s much worse for our climate. To compare, a cow that releases 100kg of methane per year is the equivalent of it producing 2,300kg of CO₂ that year. That’s the same amount of carbon dioxide generated by a car after 7,800 miles of driving! So yes, cow waste, whether it be burping, farting, or manure, significant contributes to our world’s greenhouse gas emissions. The good news, though, is there’s a lot of research and experimentation going on to try and turn cow manure into a renewable energy source!