Discuss as:

World's cities to expand by more than twice the size of Texas by 2030

AP file

Building denser cities like Manhattan -- as shown in this aerial view Wednesday, Aug. 29, 2007, in New York - could be part of the answer.

Cities worldwide are on track to expand by nearly 580,000 square miles – more than twice the size of Texas – in less than 20 years, according to experts at a major international science conference.

Yale University professor Karen Seto said the North American suburb had “gone global, and car-dependent urban developments are more and more the norm.”

The world’s population is expected to grow from the current 7 billion to about 9 billion by 2050, according to the United Nations.

Experts meeting at the Planet Under Pressure 2012 conference in London said in a statement released by the organizers Tuesday that unless changes were made, “humanity’s urban footprint” would increase in size by 1.5 million square kilometers (nearly 580,000 square miles) by 2030.

This is significantly more than twice the size of Texas or, according to a "back-of-the-envelope calculation" by Seto, more than 43,000 football fields every day for the next 18 years.

”The way cities have grown since World War II is neither socially or environmentally sustainable and the environmental cost of ongoing urban sprawl is too great to continue,” Seto said in the statement.

“People everywhere, however, have increasingly embraced Western styles of architecture and urbanization, which are resource-intense and often not adapted to local climates,” she added. “The North American suburb has gone global, and car-dependent urban developments are more and more the norm.”

Eco-friendly skyscrapers?
Seto was one of the authors of a report in the journal PLoS One about global urban sprawl, along with Michail Fragkias of Arizona State University, who is one of some 2,800 participants at the London conference.

The Planet Under Pressure conference is designed to give an idea of the health of the globe ahead of the Rio+20 United Nations Conference on Sustainable Development in June.

Fragkias told msnbc.com that "the answer [to urban sprawl] is denser cities."

"The main message is we are not going to get away with cities like Phoenix or cities like Los Angeles," he said. "These are the typical cities of the single-family house, with a huge lot and huge highways that connect various areas of the cities because there is no way you can have an efficient or cheap enough mass transit system to support them."

Instead, densely populated areas such as Singapore or Manhattan -- but not New York City's surrounding urban sprawl -- provided possible models for the future. "If cities can develop in height rather than in width that would be much more preferable and environmentally not as harmful," Fragkias said.

Census: 8 out of 10 Americans are now urbanites

But Seto told msnbc.com that density was only part of the answer, saying someone who lives close to where they work in Phoenix could have a low environmental impact, compared to someone living in a densely packed city who commutes through congested streets in a car.

Cities are not 'bad'
And she said that increasing the urban population was "absolutely" the way to deal with the rising number of people in the world.

"For a long time, environmental activists said 'cities are bad.' The rationale was cities take up space and that could be better used for gorillas and butterflies," and other wildlife, Seto told msnbc.com.

But she said the global population was increasing and "we certainly don't want them strolling about the entire countryside. We want them to save land for nature by living closely [together]."

The conference statement said that creating more environmentally friendly cities included better infrastructure planning; “reversing the trend to ever larger homes;" and ending subsidies that favor cars over public transport.

Beijing's 'hubs' haven't curbed population pressures

Inner city schools should also be improved and other urban issues such as income inequality and crime rates addressed, the statement said.

"Cities are being built so quickly, we have to rethink how we do things ... Rome wasn't built in a day, but Chinese cities are," Seto quipped, saying most urban growth was taking place in Asia.

UNICEF: Millions of kids live in urban squalor

On Tuesday, organizers of the conference also highlighted a website called Welcome to the Anthropocene, referring to a term adopted by scientists and environmentalists to describe what they say is a new epoch, one uniquely influenced by human actions.

The idea the world had entered the Anthropocene was first put forward in 2000 by Dutch Nobel laureate Professor Paul Crutzen and U.S. academic Professor Eugene F. Stoermer.

"This century is special in the Earth's history. It is the first when one species -- ours -- has the planet's future in its hands," Martin Rees of the Royal Society, Britain's academy of sciences, said at the conference Monday according to the AFP news agency. "We've invented a new geological era: the Anthropocene.”

More from msnbc.com and NBC News:

Follow us on Twitter: @msnbc_world