[svara]

You're probably confused by the fact that (1) they are plotting ice core based CO2 concentrations as years before 1950 and (2) the time resolution of the ancient ice core data is quite low, while all the anthropogenic action happened in the last 150 y or so (and about 50% of the CO2 addition happened in the last 50).

Look at [0] for a more authoritative source and specifically compare the 800 and 400 ky data to the 2000-year (Law Dome, Antarctica) data. This will show you that we are indeed at the top of a very slow CO2 cycle, but that we added about 100 ppm on top of that in the past ~100 y! This is more than the amplitude of the underlying cycles.

[0] https://cdiac.ess-dive.lbl.gov/trends/co2/ice_core_co2.html

[allovernow]

What I'm saying is that all of our historic CO2 data comes from cores, which are a proxy for paleo-CO2 levels. The theory is that bubbles of atmospheric gas are trapped in ice as it forms over thousands of years. What I'm saying is that there's the possibility that this trapping is imperfect, and ratios of atmospheric gassed may change after being sequestered, such that past ppm values are underestimated, and the currently measured values are not unusual in history, and are rather consistent with the peak that historic core data shows us to be at.

In fact, here [1] is a well sourced article which highlights at least one indication that this is the case - historic CO2 estimates derived from plant stomata are much higher and closer to contemporary measured levels than ice core data. Then there is the idea of time averaging smoothing out peaks, and we can only speculate/model the loss of resolution.

It's very easy for a well intentioned group of researchers to reason themselves into a corner, particularly when things like institutional momentum, political biases, and understandable doomsday concerns influence the kinds of questions scientists ask and the paths they are willing to take to find answers. Because this research has broad effects on economic, social, and corporate policy, it is dangerous to close off one side of questioning behind an apparent consensus, not to mention it's simply bad science.

>the time resolution of the ancient ice core data is quite low, while all the anthropogenic action happened in the last 150 y or so

All the more reason to be open to questioning.

1. https://wattsupwiththat.com/2010/12/26/co2-ice-cores-vs-plan...

[svara]

The blog post you cited seemed pretty dodgy to me, so I went ahead and looked closely at the work they base many of their conclusions on, a PhD thesis from 2004 by Kouwenberg [0]. Specifically the data in Fig. 5.4 (p. 57) is what they base much of their argument on.

Kouwenberg herself however doesn't believe the crazy 300-700 AD CO2 excursion to around 400 ppm and spends a lot of time looking at alternative explanations, finally concluding on p. 65:

"""

The extremely low number of stomata per mm needle length in the Tsuga heterophylla record at Jay Bath between 300 and 700 AD does not appear to result from extremely high atmospheric CO2 levels at the time, but coincides with the establishment of the species during a period of major disturbance at the site. The open, exposed setting after this disturbance probably provided highly stressed growth conditions for pioneering, early-successional T. heterophylla trees.

"""

Of that, there is of course no mention in the blog post you cited.

Importantly, of course, none of this is really that relevant to the discussion of anthropogenic global warming, since we know that we are putting CO2 into the atmosphere, and that CO2 in the atmosphere traps heat.

I'm really starting to see a pattern here, where facts that contain some grain of truth are used out of context to sow doubt, and everything just evaporates as soon as you look more closely (and then the next tangentially relevant fact is brought up)... It's probably quite effective - I mean, who is going to read that blog post you linked and then do what I just did and actually dig into the sources? I did that because I took the "advice" I got in this comment section to heart and wanted to give my "opponents" the benefit of the doubt.

[0] PDF available at https://dspace.library.uu.nl/bitstream/handle/1874/243/full....

[kaybe]

> What I'm saying is that there's the possibility that this trapping is imperfect, and ratios of atmospheric gassed may change after being sequestered..

Do you seriously think that we don't consider these things? Seriously?