Answer: Â A. Air bubbles trapped in a layer indicate how much CO2 was in the atmosphere the year the layer formed.
Explanation:
When ice cores were formed, they included air bubbles that got stuck during the formation. Scientists can study these air bubbles to find out how much carbon dioxide is in them as this will show the proportion of carbon dioxide in air at the time the core was formed.
They can then compare those figures to current carbon dioxide levels to see if there has been a change. This change will give proof of climate change.
