I dont have the exact graph, but one of the points I saw earlier today suggested exactly this. Warming trends preceded CO2 increases in some of the "deniers" theories.
https://www.newscientist.com/article/dn11659-climate-myths-ice-cores-show-co2-increases-lag-behind-temperature-rises-disproving-the-link-to-global-warming/
Biological activity
The source of this extra carbon was the oceans, but why did they release CO2 as the planet began to warm? Many factors played a role and the details are still far from clear.
https://www.nature.com/articles/35038000