It's around 370 ppmv, which is 22 percent higher than the 304 ppmv reading 340,000 years ago, assuming that all the numbers are very accurate.
However, are all these data accurate to +/- 22 percent. ? What is the percent accuracy ?
The actual ppmv 340,000 years ago could have been 400 ppmv, for all we know.
Also,there appears to be a long lag between a temperature increase and a CO2 increase---- quote ---"Using semiempirical models of densification applied to past Vostok climate conditions, Barnola et al. (1991) reported that the age difference between air and ice may be ~6000 years during the coldest periods instead of ~4000 years, as previously assumed."
--- link
But did human activity cause the CO2 and temperature increases 340,000 years ago ?
Of what? Temperature or CO2 concentration? If you have a reference, tell me: I wouldn't be able to find this easily with a Web search.
Your link notes this:
"The extension of the Vostok CO2 record shows the present-day levels of CO2 are unprecedented during the past 420 kyr."
which is rather obvious. They probably make the assumption that the numbers are reasonably accurate and that the uncertainty bounds are low probability, which is statistically reasonable.
But did human activity cause the CO2 and temperature increases 340,000 years ago ?
Of course not. A warming global climate caused the CO2 increases 340,000, 240,000, and 130,000 years ago, and also about 10,000 years ago at the end of the last glacial period. After which CO2 concentrations remained relatively stable until a few years into the 1800s. Also note that the Vostok ice core data define a CO2 concentration zone that has only been exceeded (due to anthropogenic emissions) in the past two centuries.
That's the problem that's keeping climate scientists hot and bothered -- what happens to the climate when one of its controlling factors jumps out of its normal range by so much, so fast? And that's hard to predict.
Now I have to get back to AG.