How are ancient CO2 levels measured?

Ancient CO₂ levels can be measured from

1) Air bubbles trapped in deep ancient ice cores (up to 800,000 years ago) or

2) Tree rings (up to 10,000 years ago)

Explanation:

The percentages of CO₂ concentration in the atmosphere from previous or ancient times are found by analyzing the CO₂ content in trapped air located in the deep ancient ice cores present in Antarctica and Greenland such as the EPICA Dome C cores

As the ice is drilled though, the air released from trapped bubbles are captured for analysis and the time frame for the captured bubbles can date back up to 800,000 years

Ancient CO₂ levels can also be measured from tree rings which provide ancient CO₂ levels up to 10,000 years

the correct answer to this question is 2