# Which greenhouse gas has the most influence over global warming?

Carbon dioxide is the gas everyone talks about because of how much we emit, but it is the most influential greenhouse gas ? I know there are a lot of other gasses are way more potent to retain heat within the atmosphere (water vapor, methane) but exist in smaller quantities.

Can we quantify the relative influence of each greenhouse gas over global warming ?

• Please specify whether you want to talk about the greenhouse effect in general, or the human contribution. Also, please have a look at this wikipedia-entry: en.wikipedia.org/wiki/Global_warming_potential - it should cover your question as I understand it. – Erik Apr 23 '19 at 15:04
• Erik is right but do note that this is a also matter of ongoing debate, you'll get different answers depending who you reference. – Ash Apr 23 '19 at 15:19
• I believe my question is unambiguous but if it is, then I do not understand the difference between 'the greenhouse effect in general' and 'the human contribution', could you please explain a bit so that I can edit my question ? thanks – Magix Apr 23 '19 at 20:25
• The greenhouse effect in general would occur even if there were no humans. The human contribution is mostly our carbon dioxide and methane contribution, though to a lesser extend water vapor through the combustion of fossil fuels. I say, "lesser extend," because whatever water vapor humans put into the atmosphere just becomes part of the water cycle. – BillDOe Apr 23 '19 at 22:17
• Title asks only for "most influential" and that of course is water. But the body of the question asks to "...quantify the relative influence of each greenhouse gas on global warming". I think that's what you want to ask, so maybe you can adjust the title to match? – uhoh Apr 23 '19 at 23:31

The annual amount of CO2 we emit from using carbon that was in relatively long-term sequestration (coal, natural gas, petroleum) is on the order of 40 GT. To put this in perspective: the amount of food humans consume over the same period of time is on the order of 4 GT. Our CO2 production is an order of magnitude larger than worldwide food consumption. And the problem with it is that it takes weathering to remove it from the atmosphere - meaning it can stay around for millennia, and we're very definitely the ones extracting it from the ground.

H2O only stays in the atmosphere as long as the temperature allows. This means that it won't be the driving factor. There is a slight positive feedback with more H2O leading to a warming so it can hold more H2O, but there's also a negative feedback of about the same size with cloud formation reflecting the sunlight back to space.

Ozone in the upper atmosphere is responsible for the warming at the Stratopause. It's the reason why we have our atmosphere divided into layers with the Troposphere getting cooler with height, the Stratosphere getting warmer with increased height, and the Mesosphere getting cooler again with height. The Ozone absorbs the very energetic solar radiation that would otherwise destroy the DNA in your skin and cause cancers, but as it's a triatomic molecule, it's also very good at absorbing longwave radiation. In this case, a hole in the ozone above the Antarctic may be allowing slightly more longwave radiation to escape - and repairing the hole so we humans can survive might also be trapping additional heat. https://climate.nasa.gov/faq/15/is-the-ozone-hole-causing-climate-change/

CH4 stays in the atmosphere on the order of a decade before chemically reacting with oxygen molecules to become CO2 and H2O - but while it's present, the effect is about two orders of magnitude greater than that of CO2. But since the methane hydrates have been stable, they're not culpable in the warming. Although there are large quantities in tundra and ocean shelves, there's little point in making it the talking factor for anything except scare tactics.

This page https://www.epa.gov/ghgemissions/overview-greenhouse-gases discusses the relative sizes emitted, but temper this with the measurements of these gases. CH4 us currently less than 2 ppmv and CO2 is over 400 ppmv, making it easily the biggest factor in our warming, and in fact, the warming from CO2 is what's causing the tundra to melt and the East Siberian Arctic Shelf (the largest on the planet) to warm and release its methane.

This is why we concentrate on CO2 (it being the largest factor and also the only one we seem to have much control over).

Hopefully this puts it into perspective, and you can do your yandex.com searches for more details.

• "But since the methane hydrates have been stable, they're not culpable in the warming." Methane hydrates? Or Carbo hydrates? Also I'm not sure how to understand that phrase, as $\rm CH_4$ is a GHG. – AtmosphericPrisonEscape May 5 '19 at 15:49
• @AtmosphericPrisonEscape Methane hydrate is a special form of methane trapped in a lattice of water molecules. It's typically found at the bottom of the ocean. – Spencer May 5 '19 at 18:42
• @AtmosphericPrisonEscape: Also look for the term "methane clathrates". They refer to the same thing, but I think clathrates is the more common. – jamesqf May 8 '19 at 17:36