Migration from African nations has increased dramatically in the last three decades, going from just 1% in the 1990s to 31% by the 2000s, a new study shows.

As of 2017, some 25 million sub-Saharan migrants lived outside their nation of birth, according to the Pew Research Center. With the exception of Syria, where a violent conflict has created a humanitarian disaster since 2011, the region accounted for nine of the 10 fastest growing international migrant populations since 2010. With a global average growth of just 17%, emigration from sub-Saharan nations grew by 50% or more between 2010 and 2017.

A majority of those leaving is forced out by conflicts, leaving their homes in countries such as South Sudan, the Central African Republic, Burundi, the Democratic Republic of Congo, and Sudan. Many of them are also children and women, who are fleeing inter-communal violence, economic decline, disease, and hunger. But some people are also moving from peaceful and economically stable countries too, like Namibia (190,000), Botswana (80,000) and Sao Tome & Principe (80,000).

The sub-Saharan region also continues to host a large and growing number of refugees, which exerts enormous pressure on public services and local infrastructure in neighboring nations.

Over the last three decades, the number of African migrants in Europe and the United States has also increased—albeit at a slower pace. Many of those leaving the continent have also used dangerous routes across the Sahara, with some being auctioned off as slaves in car parks, garages, as well as public squares in Libya. Many Eritreans and Sudanese migrating to Israel have also survived torture and extortion while fleeing persecution and conscription at home.

Pew said the exodus of African migrants will only continue to increase in the coming decades, given the population spurt in the continent. By 2050, around 1.3 billion people will be added to Africa’s population, and more than one in three people will live in the continent by 2100.