Why are some greenhouse gases more powerful than others?


Why are some greenhouse gases more powerful than others?

In: Chemistry

It has to do to the property of each gas to reflect back heat to the ground in the form of infrared light.

Greenhouse gases operate by absorbing infrared radiation. This is light coming from the sun – on the way to the earth, it has a lot of energy so can pass through most of the atmosphere without too much issue, but once it bounces off the earth it loses a lot of its energy. This makes it much easier for greenhouse gases to absorb the radiation. When a gas absorbs this infrared, it basically traps the heat inside the atmosphere, instead of allowing it to dissipate into space.

Different molecules have different abilities to absorb infrared radiation. This is a very complicated sciency thing that essentially boils down to the types of atoms it’s made from and the way in which those atoms are arranged.

We use a measurement called GWP to describe how impactful a gas is on climate change. It’s a measure of how much energy 1 ton of the gas will trap over a 100 year time period, relative to carbon dioxide. Since its relative to carbon dioxide, CO2 has a GWP of 1.

Methane is much better at absorbing energy than CO2 is, so it has a GWP of 28-36 (estimated), however, methane is also a lot less stable – methane only remains in the atmosphere for about a decade, whereas carbon dioxide can stay in the atmosphere unchanged for thousands of years.

N2O is another big offender, which has a whopping GWP of 265-298. This means that one ton of N2O absorbs 298 times more energy per 100 years than CO2 does.

Greenhouse gases warm the planet by absorbing infrared radiation from the surface of the planet. There are three important properties that define how effective a greenhouse gas is at any given moment. (In other words, not accounting for how long it remains in the atmosphere.)

1. How effectively it absorbs the infrared energy. This should be obvious.
2. At which wavelengths is absorbs the infrared energy. The reason this is critical is because water vapor is a highly effective greenhouse gas. (Providing something like 95% of the net warming of the greenhouse effect.) Some gases may be very effective at absorbing infrared but they can still have a low impact on temperature because so much energy at that wavelength is already being absorbed by water vapor. Since the wavelengths where CO2 absorbs energy only partially overlap with water vapor, it also affects energy absorption.
3. How much of the gas is already present in the atmosphere. The more of a greenhouse gas you have in the atmosphere, the less effect a marginal increase will have on absorption. In other words, going from 300 ppm of CO2 to 400 ppm of CO2 (increase of 100 ppm) has a much bigger impact on energy absorption than going from, say 800 ppm to 900 ppm.