It has been speculated that the partial pressure of carbon dioxide (pCO(2)) in shelf waters may lag the rise in atmospheric CO2. Here, we show that this is the case across many shelf regions, implying a tendency for enhanced shelf uptake of atmospheric CO2. This result is based on analysis of long-term trends in the air-sea pCO(2) gradient (Delta pCO(2)) using a global surface ocean pCO(2) database spanning a period of up to 35 years. Using wintertime data only, we find that Delta pCO(2) increased in 653 of the 825 0.5 degrees cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of + 0.5 mu atm yr(-1) (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 CO2 during the last century.