Fossil fuel use, land use change and cement production have perturbed the global carbon cycle and have led to the accumulation of carbon dioxide in the atmosphere. This has two major consequences, namely global warming and ocean acidification (?the other CO2 problem?). Sea surface water pH has decreased already by 0.1 unit since pre-industrial time, and based on atmospheric CO2 scenarios, it is projected to further decline by 0.0015-0.002 unit per year over the coming century. However, observations on the Washington coast and in the North Sea (Rijkswaterstaat monitoring) show stronger decreases of 0.045 and 0.02 unit per year, respectively. The North Sea is apparently acidifying 10 times faster than global ocean model predictions. Here we propose a detailed investigation of the spatial and temporal patterns of pH in the North Sea at a basin-wide scale using the high quality methodology in use by the international CO2 research community. This will generate the needed data to see whether the acidification of the North Sea is indeed occurring at such high pace. In addition, we will also elucidate the biogeochemical mechanisms governing the pH in North Sea waters, in particular the balance between production and respiration and the generation of alkalinity. As part of this investigation, we will apply a recently developed modelling technique to attribute pH changes to changing environmental parameters.