Is CO2 “well mixed”?

Just to pick up an ongoing conversation where we left off over at the recently closed How to Talk to a Climate Sceptic thread, I wanted to provide a more serious answer to a likely unserious visitor because I think myhrr’s issue deserves an answer, even if myhrr obviously doesn’t. Kind of like the “we can’t predict the temp 2 weeks from now” argument, this one has an intuitive appeal to perfectly fine people who are just not well informed for whatever reason.

myhrr is wondering why CO2, being heavier than air, does not just stay close to the ground. Okay, he actually claims entire scientific disciplines are frauds or idiots because they are unaware of this obvious truth, but let’s not talk to him, let’s talk to passers by who might breifly say to themselves, “hey, wait a minute, that’s a good point”.

First things first, we can just sample the air, as has been done at all kinds of altitudes all around the globe and the simple fact is that CO2 is what is called a “well mixed gas”.

You can go here to see dozens of CO2 sampling station records from sea level to mountain top, from pole to equator, that show unequivically that CO2 spreads very evenly throughout the global atmosphere, all theory or prediction aside. We can even see that CO2 rise in the southern hemisphere lags behind CO2 rise in the northern hemisphere by a few years as most anthropogenic CO2 is produced in the north where the majority of fossil fuel combustion takes place and it takes some time to spread around. The annual rise and fall is the natural result of plant growth and die-off as seasons cycle through the northern hemisphere, again where most plant growth takes place.

I note that in the other thread, in his denial of CO2’s mixing properties, myhrr confidently proclaims “all measurements show [CO2] isn’t [well-mixed]“. It is a pretty safe prediction that he will never provide any source of measurements to back this up.

So CO2 mixes, why is this? Pure CO2 is indeed heavier than air and there have even been suffocation deaths caused by volcanic emissions of CO2 many times in human history. The one word answer is wind. The atmosphere is very turbulent (windy) and this turbulence easily dilutes many kinds of gases in the atmoshpere and overpowers any small differences in bouyancy.

A simple sanity check thought experiment shows this must be true for most gases in the atmosphere. The fact is “air” is not a single gas, it is a mixture of many. If the atmosphere layered itself out according to the relative molecular weights of its components we would not have air as we know it anywhere. We would have what adelady amusingly described as

a layer cake atmosphere, ozone as a pastry base, CO2, then big thick slabs of oxygen and nitrogen, icing would be water vapour – all topped off with methane sprinkles.

I think she forgot the delicious, if thin, Argon layer that would be between O2 and N2. Yum!

So, sorry myhrr, but I guess on this point at least, the experts do in fact know just a little more than you do about their topic.