Stable oxygen isotope ratios d18O of foraminifera arewidely used to reconstruct the climatic history ofEarth. It is well known that temperature reconstructions basedon d18O are complicated by factors such as the unknown isotopic compositionof the ocean. In addition, recent experimental and theoretical workhas shown that the seawater pH has a marked effect on the d18O offoraminifera. Here I employ this effect to demonstrate thatreconstructions based on d18O of foraminifera may underestimate seasurface temperatures in the geological past. Ocean surface temperatures forthe mid-Cretaceous are estimated to have been ~2-3.5 deg Chigher than previously thought.