re: When
You obviously have a comprehension problem.
It doesn't matter WHEN the ice core measurements were made, but WHICH TIME they relate to.
Suffice it to say that very accurate records exist for CO2 levels in the atmosphere for the past several hundred thousand years.
Are you asking when scientists first became aware of the problem, and what they then did about it? That's a different question.
Please ask your FULL question, not the loaded half-question...