> “Our research identified the fastest rates of past natural CO2 rise ever observed, and the rate occurring today, largely driven by human emissions, is 10 times higher.”

They are investigating historical trends. (They are then comparing their findings to modern trends.)

If I understand the paper correctly,

Southern Hemisphere westerly winds are intensifying due to a natural cycle on the scale of decades. This makes the southern ocean warmer (and the arctic colder, but the ocean is mostly in the south). Warm water doesn't take up so much carbon dioxide. (Water takes up carbon to produce carbonic acid, so if the water is warmer it incidentally reduces ocean acidification, a possible benefit?)

Or it might be about convection.

> The drivers and source of these centennial-scale CO2 jumps are unknown. The proposed mechanisms include enhanced ocean–atmosphere CO2 exchange in the Southern Ocean (via wind-driven upwelling of carbon-rich deep waters or invigorated convection), surface ocean temperature changes (impacting CO2 solubility), biomass burning (via NH subtropical wildfires), or a combination of mechanisms.

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.