When talking about the correlation between CO2, often the Vostok ice core is given as an illustration (see right). It leaves no doubt there is a correlation, but unfortunately it only goes back a couple of 100.000's of years.
The latest edition of Science magazine presents an article by Aradhna Tripati (abstract) who has developped a technique by studying the ratio of the chemical element boron to calcium in the shells of ancient single-celled marine algae, allowing her to go back some 20 million year.
Tripati says :
The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet [= 25 - 40 m - J] higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland.
Now thàt is alarming news.
a Dutch summary can be found here.