The Nuance of Greenhouse Gas

Next time you and your climate change friends are sitting around on a Saturday night trying to stump each other with a game of  little known climate change facts —yes we know you do it—we have a winner for you. The question you pose is:

I was surprised when I found this out from a colleague here at Sweet Lightning’s global headquarters when we were playing, er...working...on this question. I thought it was CO₂, but it turns out it's not. 

Okay let’s unpack this assertion and define some terms so we can build some understanding. First what do we mean by “potent”? 

On a molecular level, climate scientists use a measure called Global Warming Potential (GWP) to compare how effective each type of molecule is at trapping infrared energy and causing warming. These measurements are made over time: usually 20 to 100 years. As the Earth’s most pervasive greenhouse gas, CO₂ was chosen as the basis for comparison, and the GWPs of other gases are usually expressed in terms of CO₂ equivalent (CO₂e). For example, methane has a GWP of roughly 100, which means it’s 100 times more effective at absorbing energy than a molecule of CO₂.

Time is important here. Methane’s GWP is 100 if we consider its effect over 20 years, but is roughly 20 if we consider its effect over 100 years. This is because methane breaks down in the Earth’s atmosphere into CO₂ and water vapour. Kind of the GHG that keeps on giving. So on a molecular level methane is more potent than CO₂, as are many other gases.

In this case, water vapour is the most potent. It contributes about 50% of the overall earth warming, while CO2 contributes about 25%. The remaining 25% of warming comes from clouds (20%) and other green house gases (5%). By way of reminder, clouds are not made up of water vapour (a gas): they are made up of water in as either a liquid (water droplets) or solid (ice crystals). This is a nuance or as some of your colleagues around the climate-change trivia table might suggest, cheating. 

Okay, let’s get into the nuances, because there are a few of them. Firstly, how does the water vapour compare to CO₂ as a green house gas, or what is it’s GWP? It can’t really be compared in the same way as other GHGs because it only exists as water vapour in the atmosphere for about 10 days before it condenses and rains back to Earth or becomes a liquid droplet or ice crystal in the form of clouds. If you had to come up with a GWP number is would be about zero, since it is so short-lived. By way of contrast, CO2 persists in the atmosphere for centuries to millennia.  So nuance.

The fact that water vapour only lives in the atmosphere for 10 days doesn’t tell the story. Water vapour is part of a massive hydrological cycle wherein: water evaporates or is transpired into the atmosphere from oceans, rivers, land and plants, it condenses to form clouds and returns to earth as precipitation. While it doesn’t stay in the atmosphere for long it is continually replaced by more water vapour, in a constant cycle, so it is always there. What is different is that water vapour is not a gas that is “forced” into the atmosphere on its own. Water vapor in the atmosphere is a part of several feedback loops driven by other factors.

Water vapour is not the originator of warming. That work is done by the other green house gases (ofter anthropogenic) like CO₂, methane, etc.; but water vapour amplifies the amount of warming that occurs. Nuance….

Here is a rough analogy of how this works. Think of the system that heats your house. The anthropogenic GHGs like CO₂, methane, and nitrous oxide (NO₂) act as the thermostat in the house in that they set the temperature. Water vapour is the furnace. At a certain temperature, set by the thermostat, the amount of water vapour in the atmosphere is determined and therefore the green house heating. As anthropogenic gases in the atmosphere increase, atmospheric temperature rises and the amount of water vapour the the atmosphere can hold increases, which further increases the global heating caused by the greenhouse effect. That is, the added water vapour amplifies the global heating. Since water vapour responds to atmospheric temperature, it works just the same in reverse. If we reduce anthropogenic gases (turn down the thermostat), the global temperature comes down, the amount of water vapour the atmosphere will support is reduced and the amplification causes further reductions.  

Okay, enough nuances. As you might imagine there is a lot more to all of this than we have covered here. For those of you who want to better understand how this all works, we have prepared an excellent, more-accurate, and more in-depth description of how this all works. You will find it here, in an article called "The Greenhouse Gas No One Talks About."

In the meantime, we suggest you print this little article out to deal with any doubters amongst your climate change friends next time you play your game of little-known climate change facts. Oh, and remember to say nuance…a lot.