New observations from the North Sea, a NW European shelf sea, show that between 2001 and 2005 the CO2 partial pressure (pCO(2)) in surface waters rose by 22 mu atm, thus faster than atmospheric pCO(2), which in the same period rose approximately 11 matm. The surprisingly rapid decline in air-sea partial pressure difference (Delta pCO(2)) is primarily a response to an elevated water column inventory of dissolved inorganic carbon (DIC), which, in turn, reflects mostly anthropogenic CO2 input rather than natural interannual variability. The resulting decline in the buffering capacity of the inorganic carbonate system (increasing Revelle factor) sets up a theoretically predicted feedback loop whereby the invasion of anthropogenic CO2 reduces the ocean's ability to uptake additional CO2. Model simulations for the North Atlantic Ocean and thermodynamic principles reveal that this feedback should be stronger, at present, in colder midlatitude and subpolar waters because of the lower present-day buffer capacity and elevated DIC levels driven either by northward advected surface water and/or excess local air-sea CO2 uptake. This buffer capacity feedback mechanism helps to explain at least part of the observed trend of decreasing air-sea Delta pCO(2) over time as reported in several other recent North Atlantic studies.