Fifty million "environmental refugees" will flood into the global north by 2020, fleeing food shortages sparked by climate change, experts warned at a major science conference that ended here Monday.

"In 2020, the UN has projected that we will have 50 million environmental refugees," Cristina Tirado, a professor at the University of California, Los Angeles, said at the annual meeting of the American Association for the Advancement of Science (AAAS).

"When people are not living in sustainable conditions, they migrate," she said, outlining with the other speakers how climate change is already impacting the amount of food we have -- food security -- and food safety, or the healthfulness of that food.

Southern Europe is already seeing a sharp increase in what has long been a slow but steady flow of migrants from Africa, many of whom risk their lives to cross the Straits of Gibraltar into Spain from Morocco or sail in makeshift vessels to Italy from Libya and Tunisia.

The flow recently grew to a flood after a month of protests in Tunisia, triggered by food shortages and widespread unemployment and poverty, brought down the government of long-time ruler Zine El Abidine Ben Ali, said Ewen Todd, a professor at Michigan State University, predicting there would be more of the same.

"What we saw in Tunisia -- a change in government and suddenly there are a whole lot of people going to Italy -- this is going to be the pattern," Todd said.

"Already, Africans are going in small droves up to Spain, Germany and wherever from different countries in the Mediterranean region, but we're going to see many, many more trying to go north when food stress comes in. And it was food shortages that put the people of Tunisia and Egypt over the top.

"In many Middle Eastern and North African countries, you have a cocktail of politics, religion and other things, but often it's just poor people saying 'I've got to survive, I've got to eat, I've got to feed my family' that ignites things," he said.

Among ways that climate change has impacted food security is by making winters warmer, which allows pests that carry plant diseases to survive over the cold months and attack crops in the spring, soil physicist Ray Knighton of the US Department of Agriculture said.

An increase in fungal pathogens caused by more rainfall during harvest time -- another result of climate change -- can also "dramatically impact yield and quality," said Knighton, while Tirado noted that floods caused by heavy precipitation can spread diseases carried in animal waste.