How millennials are creating new ‘mega cities’ by Leena Rao @FortuneMagazine October 13, 2015, 1:21 PM EDT E-mail Tweet Facebook Linkedin Share icons There will be 40 million mega cities (cities with 10 million or more people) in the world by 2030, up from 28 million cities in 2015, according to engineering consulting company CH2M. Why? Rural economies are declining, and millennials are looking to urban economies to find work. Millennials will eventually comprise 70% of the U.S. workforce, said Jacqueline Hinman, CEO of CH2M, speaking on a panel about the future of cities Tuesday at Fortune‘s Most Powerful Women summit in Washington D.C.. Part of the reason why cities are becoming more popular is that millennials are staying single and childless longer and living in cities that accommodate a non-family lifestyle. The sharing economy, bolstered by companies like ride sharing giant Uber and lodging site Airbnb, allows for easier transportation and housing, Hinman added. Belinda Johnson, chief of business affairs and the chief legal officer at Airbnb agreed, explaining that as cities have become more dense, home sharing allows people to make extra money by renting their apartments or homes. The average Airbnb host, she said, makes an addition $7,500 per year on top of any existing salaries. And as rents go up in more popular cities, Airbnb is seeing many hosts rent their homes or rooms out so they can afford to continue to afford the rising cost of living. “It’s a game changer for these people,” Johnson said. Johnson added that Airbnb is also helping grow cities, by spreading urban tourists to new neighborhoods. According to Airbnb, 76% of the company’s hosts are located outside of traditional hotel districts. Airbnb currently has 1.5 million listings for rooms and homes on its site, in 34,000 cities and 190 countries. But despite the growth in number of cities and city populations, Hinman warns that many of these urban centers may not be ready to accommodate a burgeoning population. She maintained that 70% of infrastructure that will be needed in cities by 2016 still has not been built, and that without these improvements, cities will face major challenges in accommodating fast growth. “Without infrastructure the rest doesn’t happen,” she added. Subscribe to The Broadsheet, Fortune’s daily newsletter on the world’s most powerful women.