By 1960 , he had established that there are strong seasonal variations in carbon dioxide levels with peak levels reached in the late northern hemisphere winter .
Keeling started collecting carbon dioxide samples at the base in 1958 .
By 1960 , Keeling had established that there are strong seasonal variations in carbon dioxide levels with peak levels reached in the late northern hemisphere winter .
Keeling started collecting carbon dioxide samples at the base in 1958 .