Observation data from DYFAMED site, in northwestern Mediterranean Sea between 1995 and 2011, are used to study mathematical forecasts of sea water surface pH evolution over the next century. In a preliminary study, daily and monthly data have been used to compute total inorganic carbon ( ) and total alkalinity ( ) concentrations. Due to the arbitrary number of missing monthly observations from 1995 to 2011, mean pH values have been calculated from the available data in order to obtain a convenient monthly time series. Based on these results, we used in this paper a cubic spline method for interpolation within the range of known time series and then tested two extrapolation methods: linear and exponential smoothing. A 100-year simulated period is performed in order to have information beyond seasonal variations and observations. The mean seasonal variation allows us to draw forecast evolutions from 0.3 to 0.4？pH units decrease in the water surface at the end of the century. Although these simple forecasts do not pretend to present realistic predictions, these obtained theoretical results provide limits on pH variations in the northwestern Mediterranean Sea similar to those in the open ocean. 1. Introduction Since the beginning of the 19th century, the industrial era has produced an increasing amount of CO2. The evolution of CO2 concentration in the atmosphere during the last decades has been extremely important and several studies show and underline its effects on climate change (see [1–5]). The increasing trend of global atmospheric CO2 concentrations roughly follows that of the global anthropogenic injection into the atmosphere. Nowadays, we know that the world ocean (covering 71% of the Earth’s surface) acts as the biggest buffer for the atmospheric CO2 concentration by absorbing an important part of it. Studies show that the ocean absorbs about 2？ · (see [6, 7]). Ocean pH is decreasing, as shown in several models and studies (see [1, 4, 7, 8]), due to CO2 absorption across air-sea interface. Predictions for the end of the century suggest a mean decrease of about 0.3？pH units to 0.5？pH units (see [3, 7]). The absorbed CO2 affects the ocean through several factors. Partial pressure of CO2 in the atmosphere rises with anthropogenic evolution, while in water partial pressure is affected by surface absorption and biogeochemical processes. Those modifications of properties directly affect sea surface CO2 absorption. The variations in total inorganic carbon ( ) due to physical and biological processes will modify the carbonate system. Therefore
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