Slicing the greenhouse gas pie: What gases?

We keep talking about the need to reduce the greenhouse gas we produce. But how do these gases get in the atmosphere? To answer that question we need to look at what I like to call the “greenhouse gas pie,” and it turns out there are lots of ways to slice it up.

One way is to slice the pie up by the type of gases we’re talking about. There are dozens of human-produced greenhouse gases, three of them get special attention: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Here’s why.

CO₂ is the most important because it causes about 67 percent more warming than all the other greenhouse gases combined. Methane causes less than a third as much warming as CO₂, but it is responsible for about three times more warming than nitrous oxide.

Greenhouse gas pie: Sliced by the amount of warming contributed by each human-produced greenhouse gas.

That’s somewhat interesting, but doesn’t do much to show us opportunities to reduce emissions of these gases. What are the principal man-made sources of each gas? That requires us to look at another set of pies.

Let’s first look at CO₂ in more detail, since it’s the biggest slice of the pie above. Globally, about 80 percent of CO₂ comes from burning fossil fuels like coal, oil and gas. (If you’ve ever wondered why they’re called “fossil” fuels, it’s because they are made from the remains of plants and animals that died hundreds of thousands or millions of years ago – in other words, they are fossils.) The biggest sources of fossil fuel CO₂ are coal and petroleum (about 40 to 45 percent each), with natural gas contributing the rest. The other 20 percent of global CO₂ is released through deforestation.

Carbon dioxide pie: Sliced by human-produced sources, which are burning fossil fuels and deforestation.

Turning next to methane — worldwide, the major source is agriculture, primarily rice farming and livestock. Methane is a normal digestive byproduct in domesticated animals like cows, sheep and goats. As the EPA diplomatically describes it, the animals “exhale” the gas after digesting their food (it comes out the other end, too). Animals also produce manure, which releases methane as it decays.

Other important methane sources are deforestation, coal mining and the production and distribution of natural gas and oil. In the United States, landfills are the largest source of methane emissions. The good news is that it is relatively easy to capture methane from manure and landfills. Not only does this prevent emission of methane into the atmosphere, the captured methane can also be burned to generate electricity.

Finally, the other major greenhouse gas, nitrous oxide, comes primarily from agricultural soils. The opportunity to reduce emissions here is in more efficient use of fertilizers. Applying fertilizer in a different form, at a different time, or in a different amount can mean more is used by the crops instead of being transformed into nitrous oxide.

Coming up shortly, another pie showing another view of where the greenhouse gases come from.