Does the band saturation effect prevent global warming from CO2? If not, what effect does it have?

Methane, carbon dioxide, nitrous oxide, and fluorinated gases are collectively referred to as greenhouse gases. Although some of these gases are naturally occurring, their volume has increased rapidly since the Industrial Revolution. Greenhouse gases are byproducts of industrial processes, like fossil fuel combustion and modern agricultural practices. The absorption and emission rates of heat between Earth and space depend upon the makeup of the atmosphere. Greenhouse gases alter the makeup of the atmosphere and contribute to global warming.

People who deny the role of greenhouse gases in global warming sometimes cite carbon dioxide’s band saturation as the reason. Their theory looks like this: an increase in carbon dioxide concentration doesn’t matter when you’re dealing with high levels, and the levels are already high. This means that when you add CO2 to an atmosphere with no CO2, it’s a big deal. When you add more CO2 to an atmosphere that’s full of it, the effect is smaller—the level is already saturated. So far, this is mostly true. The doubling of CO2 from 10ppm to 20ppm has about the same global warming impact as the doubling of 1000ppm to 2000ppm. The amount of CO2 currently in the atmosphere is already absorbing as much heat as it can, so even if the CO2 levels doubled, it wouldn’t lead to much more heat absorption. This is true but misleading. CO2 probably can’t absorb much more heat, but that doesn’t mean it is saturated. The effect of CO2 will continue despite diminishing returns.

Most importantly, though, is that the anti-climate change theory of CO2 saturation only looks at what is happening between Earth’s surface and its atmosphere; it ignores what is happening between the atmosphere and space. Even if CO2 has already reached its peak absorption capacity from Earth, adding more CO2 would still lead to global warming. This is because increasing the concentration of CO2 is like adding another heat-insulating layer between Earth’s atmosphere and space.

Consider what you would wear on a cool autumn day. A sweatshirt or fleece, probably—something warm but breathable. A permeable layer keeps most of the heat from the body in, but not all of it. Some heat escapes into the atmosphere, which keeps you comfortable. Now think about what would happen if you added an impermeable layer, like a water-resistant shell. Body heat would no longer be able to escape through the knit fibers like it could with the fleece. Pretty soon, you’d overheat. An increase in CO2 concentration has a similar effect: it adds another layer to the atmosphere, which makes it harder for heat trapped by CO2 to radiate off of the planet.