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For the entire Earth's climate history, it is not one core from one place that will tell us everything. It's why climate catastrophe denialists keep bringing up the "medieval warming period" which anybody in the field knows was a local anomaly, not a global one (or you'd see it reflected in records of other places). So standalone, the Greenland ice core is quite accurate for the local climate of Greenland. In context with the various measures of our past climate, it contributes another degree of certainty to the consensus global climate, more accurately reflecting the overall global climate.

Slight tangent but this phenomenon of local vs global is quite important when we think about other questions too, e.g. does the COVID vaccine cause heart disease (insert one statistic from one country that seems to correlate the two) - here you can ask if this is reflected in all the other countries as well, or might more related to something local (which could be as benign as how data was collected in one place)

About the medieval warming period, aren't there tree rings studies from around the world that suggest it was not a local anomaly ?

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

There are tree rings that show it, but only from the local region where it happened.
The tree rings outside of that region show no sign of it.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001GL014580

I think this XKCD is related to the local/global phenomenon you’re talking about. For a statistically chaotic measure, if you take enough samples, one of them is bound to randomly exhibit the behind you’re looking for (a warming period, say, or heart disease) simply by chance.

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).

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 :)

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.

Many of these events (e.g., the Medieval Climate Anomaly or the Roman Warm Period, etc.) are better described as "patchy" as opposed to global. Both the MCA and RWP were the most intense in the northern hemisphere, and specifically in portions of western Europe. This does not mean that there were no effects elsewhere, but these were generally less extreme and the global effects were (1) asynchronous, (2) differed in magnitude, and (3) sometime even differed in sign (i.e., in some places the MCA or RWP represented anomalously cool periods). This is discussed in lay terms moderately well in this Skeptical Science post, but it's a bit dated. There are however more recent papers highlighting the same point (e.g., Neuokm et al., 2019). Thus, in the context of comparing events like the MCA or RWP to modern conditions, they break down pretty quickly because in the modern anthropogenically induced warming, we consistently see rapid warming pretty much everywhere globally and synchronously (and specific to the New Zealand example, follow up work - e.g., Lunning et al., 2019 - has specifically highlighted that while the MCA is recognizable in records through much of Oceania, it's not exactly synchronous, appearing to occur up to several hundred years later in different areas). So, in the form of a direct response, finding that the MCA represented a warm period in 2 places (i.e., western Europe and Oceania) within a broad band of time, does not imply that it's a global event in the same sense as what we are seeing today.

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.

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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).

Define accuracy…

[TL/DR] Good estimates is absence of other data but open to nit-picking

One of the primary uses of oxygen isotopes is for a proxy of environmental temperature at the time the ice was deposited, since there is no historical weather station data reaching back hundreds of thousands of years.

Primarily this is a measurement between O-16 and O-18. In a sample. (This ratio can also be measure in seashells of very small marine animals) Neither oxygen isotope is radioactive so that variable is eliminated.

Because O18 is 1.125 x heavier than O16, this makes for slight physical differences between water made of O16 and O18. Think boiling point and vapor pressure.

It turns out that evaporation and sublimation very slightly favors O16 water molecules leaving and O18 water molecules remaining behind.

This is know as fractionation and fractionation is temperature dependent.

The relative abundance of O16/O18 in a sample can be measure with precision in a laboratory and so there is good, reproducible data documenting the ‘curves’ in the lab.

The environment is much more complicated, for example during ice ages more O18 water might be locked up in massive ice sheets leading to some skewing of the temperature estimates. The magnitude is the skew is a function of your assumptions about ice volume and such. Regardless, the estimated local environmental conditions based on Oxygen isotope ratios give a valuable albeit imperfect proxy of the temperature when there is no other data.

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