This question came from a young relative of mine doing a school project on climate change and it’s a really interesting one, bringing in a lot of important concepts about the climate that aren’t explained as much as they should be. Here we go!
Ultimately, it’s the sun that warms the planet. Well, okay, there’s some heat coming from the core, but the crust’s rock layer does quite a good job insulating so we can pretty much neglect it. I think we can agree that the sun is the hot thing – much much hotter than the ground.
As the sun heats up Earth, Earth needs to be being cooled down in some other way, otherwise it’d just get indefinitely warmer. There’s enough heat coming in from the sun that if we didn’t lose any, the surface of the planet would get considerably hotter every day!
We lose it through “earth-shine” or outgoing long-wave radiation, in the proper lingo. In the same way that the sun gives off light and heat we can feel when we go outside, the earth is giving off its own electromagnetic waves, just at a wavelength we can’t see. Wavelength is like colour: red, blue and yellow light are all different colours, but so are x-rays and radiowaves and what wifi and cell phone signals are transmitted through.
But since the sun-shine and earth-shine are at different wavelengths, they’re blocked by different things. Glass isn’t see-through to all light and nor is the atmosphere. Let’s take a close look at the diagram below:
The wavelength is measured in micrometers, which are million times smaller than a meter. The middle of the sun’s shine (“black-body”) curve is in visible radiation, very little of which is absorbed by the atmosphere (see the same region on the lower plot). For the earth-shine, it’s not so lucky. About 80% of the outgoing earth-shine doesn’t make it out – it gets trapped and re-emitted by the molecules in the atmosphere.
Those molecules work to block radiation because of their specific shape and how well they resonate (match) with the light at each wavelength. Water is really really good at it absorbing radiation at loads of wavelengths, but CH4 (methane) and CO2 (carbon dioxide) do a reasonable job too. They all block the outgoing radiation and make the surface of the earth heat up, like the glass in a greenhouse lets sunlight in but keeps heat from going out. That’s why we call them greenhouse gases.
Water has a special place because there’s so much of it coming in and out of the atmosphere in clouds and rain. We think of it as a response rather than a cause because you can’t inject it into the atmosphere and have it stay. The same is absolutely not true for carbon dioxide, methane and HFCs (hydro-flouro-carbons) and CFCs (chloro-flouro-carbons). They get added to the atmosphere by some natural processes and, unfortunately, by humans who have use them for fire extinguishers and refrigerants, and once they’re there, they stick around, blocking the earth-shine and changing the natural balance.
The question of how bad a particular greenhouse gas is is a difficult one. The first is whether it’s doing a job nothing else can do. The atmosphere has a lot of carbon dioxide and water in it, so adding a little more doesn’t make as much difference as adding something which absorbs in the gaps. If you look at the diagram below, you can see that CFCs (and the same is true for HFCs) absorb in the ‘atmospheric window’.
This means that every molecule added absorbs some radiation that would otherwise have gone through. Methane is powerful like that too. For CO2, a molecule added doesn’t have so much power – there’s already a lot of them so that each additional molecule doesn’t go so far. This is measured in the radiative efficiency.
The other thing that makes a molecule strong is how long it sticks around for (the lifetime). Methane turns into CO2 after a little while (10 years) in the atmosphere thanks to the active chemistry (driven in part by the sunshine) that goes on up there. Some CFCs and HFCs stick around much longer – check out the IPCC table here.
Now, suppose you put different things in the atmosphere and wanted to know how bad they are. You’ve got to combine two things: one is how much of a difference they make themselves (that is, how good they are at blocking earth-shine and whether anything else would have been blocking it anyway) and the other is how long it sticks about. That’s why we think about global warming potential.
From the EPA glossary
Global warming potential (GWP): A measure of how much heat a substance can trap in the atmosphere. GWP can be used to compare the effects of different greenhouse gases. For example, methane has a GWP of 21, which means over a period of 100 years, 1 pound of methane will trap 21 times more heat than 1 pound of carbon dioxide (which has a GWP of 1).
It adds up how much damage each gas does times how much of it is around over the course of 100 years. Take those together and you get a table like the one here. CFCs and HFCs come out pretty badly, with a GWP in the thousands!
But there is good news: we don’t make as much of them as some of the less nasty things, especially since the Montreal Protocol which came in in 1989. That’s one big success for global political agreements to curb climate change!
A puzzle: 100 year timeline over which to calculate global warming potential doesn’t do such a good job of taking into account your great-grandchildren! Over the molecule’s lifetime, something long-lived (like CO2) might do a lot more harm than something short-lived (like methane). So what should we prioritise? Comparing 100-year global warming potentials or calculating warming per molecule over the molecule’s entire lifetime?