How Many Muslims Are There in Europe?

I recently received an inquiry from a researcher in the Netherlands, Morly Frishman, who was trying to track down the origin of a purported projection that by 2020 one in four Europeans would be Muslim. I quote part of the note with his permission:

This is what my question concerns. As you may know, one of the issues taken by the Dutch politician Geert Wilders is the present and future of Muslims in Europe (or, as he puts it, the "Islamization of Europe"). In this context, Wilders quotes a 'study by the University of San Diego', according to which "a staggering 25 percent of the population in Europe will be Muslim just 12 years from now." Wilders does not provide more information regarding this study, which makes it difficult to trace. Other sources seem to refer to the same study, attributing it to a 'Jan Wax' (see e.g. http://www.mediareviewnet.com/index2.php?option=com_content&do_pdf=1&id=302; or otherwise, see Google search results for the query "San Diego University researcher Jan Wax"). I suspect "Jan Wax" might actually be a failed attempt to refer to you; your name, lost in translation, so to say. Furthermore, I suspect that not only your name has been twisted, but also your data is presented by others in a deceiving way. But I do not know that for sure, as all my attempts to find such a study have failed. 

Since the San Diego State University International Population Center, which I direct,  has produced various estimates of the Muslim population in the world that have been published by US News and World Report as well as CNN, it is certainly possible that someone associated my name with Muslim population figures. However, I have never suggested, and I have never seen anyone else in academics suggest, that Muslims could account for 25 percent of Europe's population by 2020. Our estimates suggest that as of 2009 (the most recent date for which we have data), the 18 million Muslims living in the European Union account for about 5 percent of the total population. Even if we include Russia and Turkey in our definition of Europe, Muslims account for only 18 percent of the population and it is impossible to imagine that the figure could rise to 25 percent by 2020.

My own Google searches suggest that the origin of the misinformation might have been an article published in 2006 by Peter Jenkins at Penn State University ("Demographics, Religion, and the Future of Europe," Orbis, 50(3):519-539, Summer 2006), in which he disputes Bernard Lewis's suggestion that Europe could have a Muslim majority by 2100 by arguing instead that 

A Muslim population of around 25 percent by 2100 is more probable—a historically striking statistic, with enormous political implications, but nothing like a majority.(p. 533)

Even this comment, though, does not make a projection for the year 2020, but rather a date 80 years beyond that--2100. I do mention in my text that an estimated one in four humans globally is Muslim, but that is a far cry from suggesting that by 2020 one in four Europeans will be Muslim. 

This whole case of misinformation is a reminder that there is an enormous amount of information on the internet, but we must all be very careful about "reliable sources," as the founder of Wikipedia keeps saying.