Yes, I'm not sure what to make of that wording either. Based on my quick read of the article, it should probably just be "rise". Presumably it is knowable from isotope analysis if carbon added to the atmosphere is recent or fossil. We certainly know that we are adding fossil carbon to the atmosphere at a prodigious rate, and if "natural" carbon content is rising as well, then that is quite bad news because it would likely mean we had crossed some tipping point or other where the Earth system itself has begun contributing to carbon emissions.