Greenhouse gases make up less than half a percent of the atmosphere - why are they so potent?

Yesterday we covered off the basics about our atmosphere and what’s in it. We discovered that the atmosphere is about 78% nitrogen, 21% oxygen and almost 1% argon - meaning that over 99% is made up of those three gases alone. A mere 0.4% of the atmosphere is made up of greenhouse gases.

So what are these greenhouse gases and why are they so potent given they make up such a small proportion of the atmosphere? According to the Environmental Protection Agency in the US, the make up of greenhouse gases in the atmosphere is as follows:

Almost 80% of the greenhouse gases in the atmosphere is carbon dioxide, another 11.5% is methane, just over 6% is nitrous oxide, and the remaining 3% or so are fluorinated gases.

It’s important to note that water vapour can be considered a greenhouse gas too, however:

“Water vapour is a very effective absorber of heat energy in the air, but it does not accumulate in the atmosphere in the same way as the other greenhouse gases. This is down to it having a very short atmospheric lifetime, of the order of hours to days, because it is rapidly removed as rain and snow. The amount of water vapour that the atmosphere can hold increases as the atmosphere gets warmer, so the greenhouse properties of water vapour are usually considered to act as part of a feedback loop, rather than a direct cause of climate change.”

In other words, increased water vapour doesn’t cause global warming, it is a consequence of it.

Each greenhouse gas has a different influence on global warming:

“How much any one greenhouse gas influences global warming depends on three key factors. The first is how much of it exists in the atmosphere. Concentrations are measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt). For example, 1 ppm for a given gas means that there is one molecule of that gas in every 1 million molecules of air. The second is the gas’s lifetime—how long it remains in the atmosphere. The third is how effective the gas is at trapping heat. This is referred to as its global warming potential, or GWP, and is a measure of the total energy that a gas absorbs over a given period of time (usually 100 years) relative to the emissions of 1 ton of carbon dioxide.”

Here’s the Natural Resources Defense Coucil’s take on the potency of the various greenhouse gases along with some notes from Carbon Brief via The Guardian on how each is eventually removed from the atmosphere:

• Carbon dioxide (CO2) - once emitted into the atmosphere, 40 percent still remains after 100 years, 20 percent after 1,000 years, and 10 percent as long as 10,000 years later. Much of the CO2 is absorbed by so-called carbon sinks such as forests, soil, and the ocean, but some is removed by slower processes that take up to several hundreds of thousands of years, including chemical weathering and rock formation.

• Methane - persists in the atmosphere for far less time than carbon dioxide (about a decade), but much more potent in terms of the greenhouse effect. In fact, pound for pound, its global warming impact is 25 times greater than that of carbon dioxide over a 100-year period. Mostly removed from the atmosphere by chemical reaction.

• Nitrous oxide - has a GWP 300 times that of carbon dioxide on a 100-year time scale, and it remains in the atmosphere, on average, a little more than a century. Is destroyed in the stratosphere and removed from the atmosphere more slowly than methane.

• Fluorinated gases - emitted in smaller quantities than other greenhouse gases but trap substantially more heat. Indeed, the GWP for these gases can be in the thousands to tens of thousands, and they have long atmospheric lifetimes, in some cases lasting tens of thousands of years. Fluorinated gases are destroyed only by sunlight in the far upper atmosphere.

As to why greenhouse gases drive global warming when they account for such a small proportion of the atmosphere, a lengthy explanation from Jason Smerdon, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory, is available here. I’ve taken out an extract that helped me understand why though:

“Today, CO2 levels are higher than they have been in at least 3 million years. And although they still account for only 0.04% of the atmosphere, that still adds up to billions upon billions of tons of heat-trapping gas. For example, in 2019 alone, humans dumped 36.44 billion tonnes of CO2 into the atmosphere, where it will linger for hundreds of years. So there are plenty of CO2 molecules to provide a heat-trapping blanket across the entire atmosphere.

In addition, “trace amounts of a substance can have a large impact on a system,” explains Smerdon. Borrowing an analogy from Penn State meteorology professor David Titley, Smerdon said that “If someone my size drinks two beers, my blood alcohol content will be about 0.04 percent. That is right when the human body starts to feel the effects of alcohol.” Commercial drivers with a blood alcohol content of 0.04% can be convicted for driving under the influence.

“Similarly, it doesn’t take that much cyanide to poison a person,” adds Smerdon. “It has to do with how that specific substance interacts with the larger system and what it does to influence that system.””