It has been speculated that the partial pressure of carbon dioxide (pCO ) in shelf waters may lag the rise in atmospheric CO . Here, we show that this is the case across many shelf regions, implying a tendency for enhanced shelf uptake of atmospheric CO . This result is based on analysis of long-term trends in the air-sea pCO gradient (ΔpCO ) using a global surface ocean pCO database spanning a period of up to 35 years. Using wintertime data only, we find that ΔpCO increased in 653 of the 825 0.5° cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of +0.5 μatm yr (p < 0.05). Although noisier, the deseasonalized annual data suggest similar results. If this were a global trend, it would support the idea that shelves might have switched from a source to a sink of CO during the last century.