The scientist know much carbon dioxide was present in the atmosphere as far back as 800,000 years ago.
The Scientist use the ice cores to study the concentration of carbon dioxide present at that time. The snow keeps falling and the air in the atmosphere is trapped in the ice.
The air contains carbon dioxide which gets trapped in the snow and it becomes a time capsule storing the sample of air which lies deep inside the old snowflakes.
The concentration of carbon dioxide at that time is compared with the carbon dioxide concentration found today.
