While the future (as a whole) is very much unwritten, the future of what our urban centers will look like is less uncertain.

We can assume, to a degree, that they'll become more crowded and, as a result, more prone to delays as time goes on; 75% of the world's population, in fact, is expected to reside in cities by 2050, putting more stress on already overburdened urban infrastructure and public transportation systems.

To mitigate overcrowding and increased consumption, cities in the future will likely to continue to adopt smart technologies — like pneumatic waste disposal and grid sensors — and encourage the development of sustainable architecture and low-carbon energy production.

In 2016, ABI research projects, cities will spend $39.5 billion on smart city technologies.

Here's a glimpse of what green design, smart technologies and sustainable architecture will look like in four cities by 2030.

1. Songdo, South Korea

Built with technology in mind (and not retrofitted with it), Songdo, South Korea has been billed as the world's first "Smart City."

Songdo, a 15-minute drive from Incheon International Airport (South Korea's busiest airport), is among a growing number of aerotropolises worldwide — urban communities that are built around an airport, not the other way around.

While work in Songdo isn't expected to be completed until the end of this decade, the city is fully livable today. It houses nearly 40,000 people and another 55,000 commute to Songdo every day for work.

Though developers thought about proximity to an airport when they broke ground on Songdo in 2003, they didn't just stick the city in a crowded area. Rather, the 1,500-acre site was placed in a marshy part of the Yellow Sea over which workers dumped 500 tons of sand from the bottom of the ocean. Because salty soil kills trees, engineers designed a special buffer soil to go between the sand and the top soil to keep the plants hydrated.

While Songdo will certainly have its fair share of big buildings (its 68-story Northeast Asia Trade Tower is the tallest building in South Korea), 40% of the city has been allocated for open green space. Songdo's Central Park, which opened in 2009, sits on 4.2 million square feet and contains a museum, ecotarium and water taxi service.

Developers also believe Songdo's infrastructure and technology will be as cutting-edge and sustainable as its architecture. The city houses dozens of LEED-certified buildings and has designed a pneumatic waste disposal system that uses pipes to suck trash from individual homes into processing centers that automatically sort the material and recycle it. In the future, the city plans to turn that waste into renewable energy.

Underneath Songdo's streets, sensors detect traffic conditions and alter signals based on congestion. At street level, bikes pass by on Songdo's 15 miles of bike lanes, and cars are fitted with radio identification tags to report gridlock. Elsewhere, sensors monitor salt water flow in Central Park.

"In Songdo International Business District, nearly all aspects of life are digitally networked, from sensors that help control traffic and public transportation schedules, to Cisco TelePresence-based personal video services linking residents to businesses and service providers, to the centralized control systems that manage city services like waste disposal and energy generation," says Tom Murcott, the executive vice president-international at Gale International, the group developing Songdo.

2. Malmo, Sweden

New cities aren't the only smart cities. All around the world, cities are coming up with novel methods to become more energy efficient and create eco-friendly infrastructure.

Malmo, Sweden, for example, used to be a polluted industrial center that was home to nuclear power plants. Since 2000, however, city officials have closed both plants and pledged to make Malmo both carbon neutral by 2020 and able to run on entirely renewable energy by 2030. By the end of this year, they also expect Malmo's fleet of municipal vehicles to run on either hydrogen, electricity or biogas, the latter of which is created from mandatorily collected food waste.

Within Malmo, the Western Harbour district has become ground zero for sustainable building. At the Bo01 development in the harbor, nicknamed the "City of Tomorrow", a wind turbine occupies space that used to be part of an abandoned shipyard and today provides electricity for parts of the city.

The district also uses an aquifer storage system to collect rainwater that's pumped using energy from the turbine to eventually heat homes in the winter and cool homes during the summer. Another development near Bo01, Flagghusen, has made vegetated roofs and walls part of mandatory building codes.

Malmo officials are optimistic that the Western Harbour will continue to be a green population center. They say the area can house 10,000 people and expect an additional 20,000 to work or study there.

3. New York

With its abundant skyscrapers and near-constant congestion, New York is more often thought of as an ugly capital of consumption and waste than it as a budding haven of green design.

That's a reputation designers and developers are hoping to change in the coming decades, and they've already devised a few promising projects that'll help New York transform some of its unused areas into sustainable public spaces.

One more well-known urban design project in New York is the Lowline, a planned subterranean park in an abandoned trolley station that uses the sun, a glass shield and a parabolic collector to illuminate the space and provide energy for plants and trees growing there.

Because the planned Lowline is underground, it'll hover at a lower stable temperature, meaning it won't need artificial air pumped in. Architect James Ramsey, the co-founder of the Lowline project, tells Mashable that he hopes the project will be open to the public in 2018 or 2019.

"We get to take advantage of the natural properties of the Lowline being underground, so it'll have a very low footprint," he says. "We conceive of the park as an amenity for the neighborhood that it's in. My hope is that this is a means to spur excitement about science, technology and design locally and in the city as a whole."

Above ground, local developers and designers are working on a pair of exciting projects that both use unoccupied space in the Hudson and East rivers. In the Hudson, on Pier 57, which was formerly a bus depot and a temporary jail, plans are underway to create a "Super Pier" with 270,000 square feet of leasable retail and office space available on the pier and underneath it in underwater caissons. On the pier's roof, developers hope to build a 3.5-acre public park.

On the other side of New York, a team of designers hope to break ground (or water) on a self-filtering, floating pool in the East River, called the Plus Pool, that filters out sewage from the river water and makes it safe for the public to swim in. The project, in its testing phase, is entirely crowd-funded so far. The team of designers hopes the Plus Pool will open in 2016.

4. Masdar City, Abu Dhabi, United Arab Emirates

Though not yet inhabited like Songdo, Masdar City, near Abu Dhabi, aims to be a global paragon for low-carbon smart cities — a city that runs mostly on renewable energy and emits zero waste.

Masdar is so dedicated to being carbon free that it has banned non-electric vehicles from the nearly two-mile city. Instead of generating power by burning oil, coal or gas, Masdar receives energy from solar farms. The result is a city that consumes 20% the energy that a similarly sized conventional city does.

Though officials have pushed back a completion date for Masdar to 2025, they still believe it'll be a hospitable hub for eco-entrepreneurs and residents alike. Planners say Masdar could house 40,000 people and see 50,000 commuters daily.

Despite development setbacks, numerous investors and companies remains committed to Masdar. Siemens opened its Middle Eastern headquarters there last year and the Massachusetts Institute of Technology (MIT) helped found the Masdar Insitute of Science and Technology in 2009. Since opening, the Emerati university has attracted a faculty of 91 PhDs and filed more than 60 invention disclosures.