how can scientists accurately compare atmospheric composition today with atmospheric composition thousands of years ago?

They accurately compare the current atmosphere with the previous one, by assessing the amount of atmospheric gases present today, with the amount of atmospheric gases trapped in ice cores that they believe are millennial, that is, they have existed for thousands of years ago.

Explanation:

As we know, it is difficult to compare current characteristics of the planet earth with characteristics that the planet had in the past, thousands of years ago. This is because the ancient characteristics on earth are not fully available to be analyzed, since most of them have been modified over time.

To do this, scientists need to find factors that may have existed thousands of years ago and assess whether these factors have any account of the characteristics of the earth. In the case of atmospheric characteristics, it is common for scientists to compare the current atmosphere with the atmosphere of thousands of years ago, with the assessment of the amount of atmospheric gases trapped in ancient ice cores, with the amount of atmospheric gases present in the atmosphere today.