The share of Muslims living in Europe may double to more than 10 percent of the population by mid-century, according to new research on the continent's growing Muslim population.

The projections of the Pew Research Center are likely to fuel further debate over immigration after a record influx of migrants and refugees into Europe in recent years.

The US-based public opinion and demographic research center modelled Muslim population growth in Europe, defined as the 28 European Union countries plus Norway and Switzerland, on three scenarios taking into account natural population growth, future regular migration — such as for work or school — and refugee migration. 

Even under the unrealistic assumption that all migration to Europe stopped today, known as "zero" migration, the percentage of Muslims in Europe would rise to 7.4 percent in 2050 from 4.9 percent in 2016. In Germany, the Muslim population in 2050 would rise to nearly 9 percent from 6 percent today.

Researchers cautioned that it is very difficult to anticipate the future and underscored that the projections are hypothetical. Push factors that impact migrant and refugee flows, such as instability in Africa and the Middle East, may wane or increase. Much also depends on economics and European governments, which have tightened migration policies domestically and on the EU's borders.

The researchers based their projections on those people who identify themselves as Muslims using data from 2,500 data points, including official statistics and polls carried out in countries that do not collect information on religious identity.

The percentage rise can be accounted for by differences in age structure and fertility rate between Muslims and non-Muslims. Muslims in Europe are on average younger (30.4 years) than non-Muslims (43.8), meaning more women are of child-bearing age.

The researchers predict that a Muslim woman will have 2.6 children, one more than the 1.6 children that a non-Muslim woman living in Europe will bear. The reseachers pointed out that while not all children born to Muslim parents will identify as Muslim, kids tend to take on the religious identity of their parents. 

'Medium' and 'high' migration scenarios

Under two other projected scenarios, Muslims would account for between 11 and 14 percent of Europe's population by mid-century. For Germany, which has taken in many Muslim migrants and refugees in recent years, the percentage of Muslims is projected to lie between 11 and 20 percent.

The so-called "high" migration scenario assumes record refugee flows witnessed between 2014 and 2016 will continue, as well as regular migration. It also assumes that future refugees will be mainly Muslim, as was the case between 2010 and 2016.

The "medium" migration scenario assumes refugee flows will stop but regular migration will continue at previous levels. 

"The zero migration and the high-migration scenario are really thought experiments, kind of what it could be like at either end of the spectrum," said Conrad Hackett, one of the lead researchers of the study. "It seems like there is a good chance that the medium scenario, or something a little higher than that, is a more reasonable guess."

Regular migration

Between 2010 and 2016, 7 million people from all religious backgrounds arrived in Europe as regular migrants or refugees. More than half (3.7 million) were Muslims.

Only 1.6 million of the total 7 million people were refugees. But the vast majority of refugees were Muslim (1.3 million), reflecting war-torn Syria, Iraq and Afghanistan as top origin countries. 

"Although there is understandably a lot of discussion about the impact of asylum-seekers on the population broadly and particularly the Muslim population in Europe, in fact the flow of migrants who are coming to go to school, to seek jobs and other kinds of regular non-asylum-seeker migration have been, and will likely continue to be, a significant part of the increase in the Muslim population," Hackett said, referring to regular migration. 

However, in Germany's case, the country's increase in Muslims mostly come from refugees as opposed to regular migration. Germany took in 670,000 refugees between 2010 and 2016, 86 percent of whom were Muslims, according to the report. During the same period Germany also took in 680,000 regular migrants, of whom 40 percent were Muslim.

Read more: Muslims 'integrate' well into Germany - but aren't accepted

Many variables

Importantly, the figure of 7 million regular migrants and refugees does not include some 1.7 million asylum-seekers who have had their asylum applications rejected or are not expected to receive protection. Around 1 million of these 1.7 million individuals are Muslim.

Read more: When an asylum request is rejected

Another factor that may affect the number of Muslims in Europe is whether rejected asylum-seekers return to their country of origin voluntarily or through forced deportation.

An additional factor that may impact future Muslim population numbers is family reunification. Accepted asylum-seekers are generally allowed to bring immediate family, although in countries such as Germany the government has moved to impose some restrictions.

Read more: Family reunification in Germany - what you need to know

"If family reunification policies allow existing refugees [to bring their families], that would push us towards higher numbers, not necessarily towards levels that we've seen in last couple years, but higher than the medium scenario which is only regular migration," Hackett said.