The Consejo Regulador (regulatory authority) for the vineyards and wines of Rioja was established in 1926. They carefully maintain and publish statistics of the Rioja Wine Industry, which allows a number of important parameters to be tracked through time.
In the 2013 vintage, the wineries within the Rioja DOCa produced 2.64 million hl of wine. As part of the Rioja wine approval process, there is a regulatory requirement for a sample from each tank of wine to be subjected to analytical (and sensory) investigation. Last year 3,973 wine samples were analysed by the Rioja Control Board, for parameters such as pH, volatile acidity and Alcohol content (abv). Although the raw data has not been published, the summary data provides a useful snapshot of the vintage, and provides the opportunity to compare with earlier vintages as far back as 2001.
In the case of alcohol content for red wine there is, as would be expected with any agricultural product, a good deal of variation between vintages. Underlying this scatter is an upward trend which amounts to an increase of abv of ca. 0.5% from 2001-2013. (The p-value associated with the trend line is 0.03; values of less that 0.05 are generally considered significant). The causes of this increase (e.g. climate? picking dates?) are presently unclear, and there is currently no reason to believe that it will necessarily continue in future.