That's not true. Natural warming warms the oceans and a warmed ocean holds less CO2, just like a warmer soda is fizzier than a cold one because it holds less CO2.
Natural warming -> warmer oceans -> oceans release CO2 -> trend in increased CO2.
That's a credible explanation that does not involve human activity.
The warming of the oceans during the 5 deg. C global temperature change between glacial and interglacial conditions is only responsible for about 10 ppm of the 80 ppm change in atmospheric CO2 concentrations (see my profile, point #5). For the currently much smaller temperature increase, the increase in CO2 from warmer oceans is essentially negligible.