Climate was a major reason why people bearing some of Americaʼs most famous family names, including Trump, Pfizer, and Heinz, emigrated from southwest Germany in the 19th century, a new study reveals.

Over 5 million Germans moved to North America from 1816 to 1886, mainly to the US, which is why German-Americans are America's largest single ethnic group today. The 19th century Germans, including Trump’s grandfather Frederick, left their homes after a number of chilly winters and cool summers, as well as other extreme weather events such as droughts and floods, said Rüdiger Glaser, a professor at the University of Freiburg and lead-author of the study published today in the journal Climate of the Past.

Although southwest Germany experienced a great deal of poverty, war, and revolutions in the 19th century, climatic factors explain almost one-third of the migration to North America based on a quantitative analysis, Glaser told Motherboard in an interview.

“Unfavourable weather conditions led to low crop yields, which led to higher food prices and resulted in emigration,” said Glaser.

Temperatures in Europe during the 19th century were nearly 1℃ cooler in comparison to the 1961–1990 climate normals and had more seasonal extremes and a number of floods, frost periods, and droughts. Glaser and colleagues studied official migration statistics and population data from the 19th century, as well as weather data, harvest figures, and cereal-price records. The first wave followed the eruption of the Tambora volcano in Indonesia in 1815. The volcanic ash and gases that spewed into the atmosphere caused temperatures to drop around the world for a few years after the eruption. The ‘year without summer,ʼ 1816, was wet and cold causing widespread crop failures, famine, and emigration.

Frederick Trump's family. On the left is Fred, Donald Trump's father. Image: Wikimedia Commons

“Another peak-migration year, 1846, had an extremely hot and dry summer leading to bad harvests and high food prices,” said Annette Bösmeier, a researcher at the University of Freiburg who also involved in the study. “These two years of high migration numbers appear to be quite strongly influenced by climate changes, while for other migration waves other circumstances appeared to be more important,” Bösmeier said in a statement.

Poor climatic conditions are more likely to be a strong migration trigger in agrarian-based societies, said Glaser. Hotter and drier conditions in the Middle East led to food shortages and were a major factor in sparking the Arab Spring in 2010-2011, he noted. Other research has suggested that climate change was a factor in creating those conditions. Climate change is expected to lead to mass migration (‘climate refugeesʼ), as sea levels rise and extreme weather events, such as floods, droughts and hurricanes, become more frequent, he said.

Roughly 25 million people have been displaced annually due climate change-related events in the last few years, mainly on island states such as Cuba, Fiji, and the Philippines. During the 2017 hurricane season, 1.7 million people in Cuba were displaced—thatʼs 15 per cent of its population, said Camila Minerva, Humanitarian Programme Manager with Oxfam, at a press conference during last week’s UN Climate Conference (COP23).

"Climate refugees—people forced to leave their country due to impacts of climate change—are not officially recognized as genuine refugees under current international laws," said Marine Franck, an official in the Office of the UN High Commissioner for Refugees, at the press conference. Franck suggested that countries could provide assistance using humanitarian protection visas, temporary protection and stay arrangements, as well as migration laws.

Read More: Climate Change Is Creating an Entirely New Kind of Refugee

New Zealand’s recently elected government may be the first to offer a special ‘climate refugee’ visa to Pacific Islanders who are forced to migrate because of rising sea levels. “We want to get ahead of this before it turns into a real problem,” climate minister James Shaw recently told Reuters.

The 19th century mass migration of Germans to North America may have been seen as problem at the time, but in the end it was a highly beneficial for the US and Canada, said Glaser.

“Today, it seems America has forgotten it is a nation of immigrants,” he said.