Automatic-Poet-1395 t1_j9zh1e0 wrote on February 25, 2023 at 6:30 PM

Reply to comment by PureImbalance in how accurate is the greenland ice core oxygen isotope study in regards to earth's climate history ? by Additional-Rhubarb-8

Don’t the ice cores show levels of atmospheric CO2 which I’d think would be the same globally?? Or are these different cores?

paulHarkonen t1_ja00h53 wrote on February 25, 2023 at 8:41 PM

Atmospheric CO2 is not homogeneous globally (no atmospheric constituents are). It may not vary a ton (I haven't looked at CO2 specifically so I can't say) but it does vary.

We think of the atmosphere as this uniform mix that's the same everywhere but the reality is that it's lots of distinct chunks with different conditions throughout. They mix some, but those chunks are remarkably distinct as they move through the atmosphere. The easiest example is a fog bank, you can see a distinct difference between the air in the fog and outside of it and see how they mix at the edges but they don't spread out to mix evenly everywhere and remain fairly distinct.

When you take a sample at a location you are only sampling that one spot. To properly sample the entire globe and comment on the Earth as a whole you need lots of samples (which we use for analysis today).

epi10000 t1_ja0crjc wrote on February 25, 2023 at 10:06 PM

Weeelll... Average lifetime of CO2 molecule in the atmosphere is some centuries or thereabouts, and the troposphere is fairly turbulently mixed on timescales of weeks, i.e. for long lived atmospheric compounds the atmosphere is actually very uniform in remote regions without local sources or strong sinks. Fog banks are just due to local T & RH fluctuations which are extremely transient when compared to most atmospheric gases. So you're partially right, but given that we aim for precision here I just had to jump in :)

paulHarkonen t1_ja0f231 wrote on February 25, 2023 at 10:23 PM

That's fair, I'm more familiar with tracking things that have more local sources/sinks (which arguably CO2 has as well at the surface) which is why I noted that while all constituents have local variation it may not be very large in the case of CO2.

I appreciate the clarification though. I should really go look into some of the datasets and see how much surface variation you actually get when not intentionally chasing sources/sinks.

The fog bank example was intended to highlight how distinct different atmospheric "chunks" can be, not necessarily that the CO2 content would change. But again, the clarification is worthwhile here.

CrustalTrudger t1_ja0iu29 wrote on February 25, 2023 at 10:51 PM

The idea that there are remote areas where even the lower troposphere is sufficiently well mixed enough that sampling in one location represents a reasonable approximation of a global average is the whole concept behind the Mauna Loa Observatory and Keeling Curve.

paulHarkonen t1_ja0wg8r wrote on February 26, 2023 at 12:33 AM

Oooooh, I see what you're saying. You can find sample points that are sufficiently far from sources/sinks that your local measurements are (essentially) just measuring the aggregate of the troposphere.

That's different from saying that a local measurement at an arbitrary location is representative. Ok, I'm onboard now that I understand what you're saying. I was just commenting that local measurements are not necessarily reflective of larger scale measurements (which it sounds like is accurate with the exception of a few selective spots where the local readings happen to be better reflections of the average).