"We are facing a tragedy," United Nations Secretary-General Antonio Guterres warned on Wednesday, according to Global News.

Why? Because unless the U.N. receives $4.4 billion dollars by the end of March, over 20 million people — including 1.4 million children — could be at risk of death to severe famine in four countries. "We must avoid it becoming a catastrophe," Guterres said.

The last time a famine was formally declared was in 2011 in Somalia. Reuters reports that 260,000 people died.

Famine isn't a word that's thrown around lightly. TIME explains that a famine, "in technical terms, doesn’t just mean people are going hungry. It means they are already starving to death — two adults or four children a day per every 10,000 people." That's why it's crucial that action is taken quickly.

South Sudan, Nigeria, Somalia, and Yemen are all in danger of famine.

Acording to Global News, the U.N. says only $90 million of the $4.4 billion it needs has been collected so far." To put that in perspective, President Donald Trump's wall on the U.S.-Mexico border alone would cost an estimated $12-15 billion.

Of the four countries at risk of famine, UNICEF estimates 1.4 million children are "at imminent risk of death from severe acute malnutrition this year."

The majority of the children at risk are in Yemen, where there are an estimated 462,000 starving kids, "a nearly 200 per cent increase since 2014," according to the UNICEF press release. 

Yemen is especially vulnerable to famine due to the armed conflict in the country that has been raging March of 2015 after Houthi rebels, aligned with the country's former dictator, pushed out the internationally recognized government. The BBC reports Yemen has since been "devastated by war" as "more than 6,800 people have been killed and 35,000 injured ... the majority in air strikes by a Saudi-led multinational coalition that backs the president [Abdrabbuh Mansour Hadi]."

The effects of a famine are felt for years after its over. Those who survive are often so severely malnourished that it can lead to stunted brain development, as Adeiza Ben Adinoyi of the International Federation of Red Cross and Red Crescent Societies told TIME.

"They are not equipped with the skills to grow and be active members of society," Adinoyi said. "Others will likely constitute a burden on the wider society, and they will not be able to contribute to the betterment of their country. It leads to a vicious cycle of poverty."