200,000 people migrate to cities daily around the world. Nearly 70 percent of the world will live in cities by 2050, closer to 90 percent in North America and Europe.

The problem?

Our cities weren't designed to accommodate the rapid influx of people now settling within their area codes. Key infrastructure that much of our cities' depend on to support their growing population was largely build 50 to 100 years ago. Moreover, our cities were built for an entirely different climate and not well adjusted for the extreme climate events happening today, and that will continue to happen over the coming decades as climate change intensifies.

Overall, for our cities—our homes—to survive and grow into the next quarter century, they need to be rebuilt stronger and more sustainably. Over the course of this concluding chapter of our Future of Cities series, we'll explore the methods and trends driving the rebirth of our cities.

Infrastructure crumbling all around us

In New York City (2015 figures), there are more than 200 schools built before the 1920s and over 1,000 miles of water mains and 160 bridges that are more than 100 years old. Of those bridges, a 2012 study found that 47 were both structurally deficient and fracture critical. NY's subway mainline signaling system is exceeding its 50-year useful lifespan. If all of this rot exists within one of the world's wealthiest cities, what can you assume about the state of repair within your city?

Generally speaking, the infrastructure found in most cities today was built for the 20th century; now the challenge lies in how we go about refurbishing or replacing this infrastructure for the 21st century. This will be no easy feat. The list of repairs needed to achieve this goal is long. For perspective, 75 percent of the infrastructure that will be in place by 2050 doesn't exist today.

And it's not just in the developed world where infrastructure is lacking; one can argue that the need is even more pressing the developing world. Roads, highways, high-speed rail, telecommunications, plumbing and sewage systems, some regions in Africa and Asia need the works.

According to a report by Navigant Research, in 2013, the worldwide building stock totaled 138.2 billion m2, of which 73% was in residential buildings. This number will grow to 171.3 billion m2 over the next 10 years, expanding at a compound annual growth rate of just over two percent—much of this growth will happen in China where 2 billion m2 of residential and commercial building stock is being added annually.

Overall, 65 percent of global construction growth for the next decade will happen in emerging markets, with at least $1 trillion in annual investments needed to bridge the gap with the developed world.

New tools to rebuild and replace infrastructure

Just like buildings, our future infrastructure will benefit greatly from the construction innovations first described in chapter three of this series. These innovations include the use of:

Advanced prefabricated building components that allow construction workers to build structures much like using Lego pieces.

Robotic construction workers that augment (and in some cases replaces) the work of human construction workers, improving workplace safety, construction speed, accuracy, and overall quality.

Construction-scale 3D printers that will apply the additive manufacturing process to build life-sized homes and buildings by pouring out cement layer-by-layer in a finely controlled fashion.

Aleatory architecture—a far future building technique—that allows architects to focus on the design and shape of the final building product and then have robots pour the structure into existence using custom designed building substances.

On the materials side, innovations will include advances in construction-grade concrete and plastics that have unique properties. Such innovations include a new concrete for roads that is amazingly permeable, allowing water to pass right through it so as to avoid extreme flooding or slippery road conditions. Another example is concrete that can heal itself from cracks caused by the environment or by earthquakes.

How are we going to fund all this new infrastructure?

It's clear that we need to fix and replace our infrastructure. We're fortunate the next two decades will see the introduction of a variety of new construction tools and materials. But how are governments going to pay for all this new infrastructure? And given the current, polarized political climate, how are governments going to pass the gargantuan budgets needed to make a dent in our infrastructure backlog?

Generally speaking, finding the money isn't the issue. Governments can print money at will if they feel it will benefit enough voting constituents. It's for this reason one-off infrastructure projects have become the carrot politicians dangle in front of voters prior to most election campaigns. Incumbents and challengers often compete over who will fund the newest bridges, highways, schools, and subway systems, oftentimes ignoring the mention of simple repairs to existing infrastructure. (As a rule, creating new infrastructure attracts more votes than fixing existing infrastructure or invisible infrastructure, like sewer and water mains.)

This status quo is why the only way to comprehensively improve our national infrastructure deficit is to increase the level of public awareness about the issue and the public’s drive (anger and pitchforks) to do something about it. But until that happens, this renewal process will remain piecemeal at best until the late 2020s—this is when a number of external trends will emerge, driving the demand for infrastructure construction in a big way.

First, governments throughout the developed world will begin to experience record rates of unemployment, largely due to the growth of automation. As explained in our Future of Work series, advanced artificial intelligence and robotics are going to increasingly replace human labor in a wide range of disciplines and industries.

Second, increasingly severe climate patterns and events will occur due to climate change, as outlined in our Future of Climate Change series. And as we’ll discuss further below, extreme weather will cause our existing infrastructure to fail at a far faster pace than most municipalities are prepared for.

To address these dual challenges, desperate governments will finally turn to the tried-and-true make-work strategy—infrastructure development—with enormous bags of cash. Depending on the country, this money may come simply through new taxation, new government bonds, new financing arrangements (described later) and increasingly from public-private partnerships. Regardless of the cost, governments will pay it—both to simmer down public unrest from widespread unemployment and to build climate-proof infrastructure for the next generation.

In fact, by the 2030s, as the age of work automation accelerates, grand infrastructure projects may represent one of the last great government funded initiatives that can create hundreds of thousands of non-exportable jobs in a short period of time.

Climate-proofing our cities

By the 2040s, extreme climate patterns and events will stress our city infrastructure to its limits. Regions that suffer from extreme heat could see severe rutting of their roadways, increased traffic congestion due to widespread tire failure, dangerous warping of railroad tracks, and power systems overloaded from air conditioners put on blast.

Regions that experience moderate precipitation could experience an increase in storm and tornado activity. Heavy rains will cause overloaded sewer mains leading to billions in flooding damage. During winter, these areas could see sudden and sizable snowfalls measured in feet to meters.

And for those populated centers that sit along the coast or low-lying areas, like the Chesapeake Bay area in the US or most of southern Bangladesh or cities like Shanghai and Bangkok, these places could experience extreme storm surges. And should sea levels rise faster than expected, it could also cause massive migrations of climate refugees from these affected areas inland.

All these doomsday scenarios aside, it’s fair to note that our cities and infrastructure are partly to blame for all of this.

The future is green infrastructure

47 percent of global greenhouse gas emissions come from our buildings and infrastructure; they also consume 49 percent of the world's energy. Much of these emissions and energy consumption is entirely avoidable waste that exists due to a lack of funding for wide-scale building and infrastructure maintenance. They also exist due to structural inefficiencies from the outdated construction standards prevalent in the 1920-50s, when most of our existing buildings and infrastructure were built.

However, this present state presents an opportunity. A report by the US government's National Renewable Energy Laboratory calculated that if the nation's stock of buildings were retrofitted using the latest energy efficient technologies and building codes, it could reduce building energy use by 60 percent. Moreover, if solar panels and solar windows were added to these buildings so that they could produce their much or all of their own power, that energy reduction could increase to 88 percent. Meanwhile, a study by the United Nations Environment Programme found that similar initiatives, if implemented worldwide, could cut emission rates and achieve energy savings of over 30 percent.

Of course, none of this would be cheap. Implementing the infrastructure improvements needed to reach these energy reduction targets would cost roughly $4 trillion over 40 years in the US alone ($100 billion per year). But on the flip side, the long-term energy savings from these investments would equal $6.5 trillion ($165 billion per year). Assuming the investments are financed through the future energy savings generated, this infrastructure renewal represents an impressive return on investment.

In fact, this kind of financing, called Shared Savings Agreements, where equipment is installed and then paid for by the end-user through the energy savings generated by said equipment, is what's driving the residential solar boom in much of North America and Europe. Companies like Ameresco, SunPower Corp., and Elon Musk affiliated SolarCity have used these financing agreements to help thousands of private homeowners get off the grid and lower their electricity bills. Likewise, Green Mortgages is a similar financing tool that allows banks and other lending companies to offer lower interest rates for businesses and homeowners that install solar panels.

Trillions to make more trillions

Worldwide, our global infrastructure shortfall is expected to reach $15-20 trillion by 2030. But as mentioned earlier, this shortfall represents a huge opportunity as closing this gap could create up to 100 million new jobs and generate $6 trillion a year in new economic activity.

This is why proactive governments who retrofit existing buildings and replace aging infrastructure will not only position their labor market and cities to thrive in the 21st century but do so using far less energy and contributing far fewer carbon emissions into our environment. Overall, investing in infrastructure is a win on all points, but it will take significant public engagement and political will to make it happen.

Future of cities series

Our future is urban: Future of Cities P1

Planning the megacities of tomorrow: Future of Cities P2

Housing prices crash as 3D printing and maglevs revolutionize construction: Future of Cities P3

How driverless cars will reshape tomorrow’s megacities: Future of Cities P4

Density tax to replace the property tax and end congestion: Future of Cities P5