One defining characteristic of a city is that it is full of hard surfaces. Streets, sidewalks, buildings and bridges shed water when it rains. All that water has to go somewhere, and it usually gets there fast. So for a city to function more like a forest or a field, step one is to slow down the water.

The issue is becoming more urgent. According to the Natural Resources Defense Council (NRDC), a leading New York–based policy, science and advocacy organization, over the past 50 years the number of days with heavy precipitation events has increased more than 50 percent. If this trend continues, as climate models suggest it will, flash floods will pose an ever greater risk both to people and to the infrastructure built over decades — even centuries, in some places — to handle stormwater. On the flip side, droughts elsewhere will put drinking water supplies under greater stress and increase conflicts among agricultural, industrial and residential water users.

The systems that deliver water literally are crumbling beneath our feet. In 2009, the American Society of Civil Engineers — the same group that gave the nation’s levees a D — gave the same lousy grade to U.S. wastewater facilities. For example, as of 2010, the budget for Washington, D.C.’s water department, funded entirely from residents’ water bills, was so low and the water mains and other infrastructure in such disrepair that replacing the entire aging system would take 300 years.

One estimate says that at least an annual investment of $180 billion is necessary for urban water infrastructure to deal with rapid population growth. If even 10 percent of this investment could be made in green infrastructure, that would represent a figure greater than the combined annual budgets of the major conservation NGOs, greatly enhancing the goals of conservation while providing necessary water to people.

The role of water is so important to green infrastructure that some experts speak of blue-green or turquoise infrastructure. The reason is clear: in natural conditions, rocks, soil, plants and trees keep water where it falls, or slow water down on its way into wetlands, streams and rivers. As a result, only 10 percent of rain becomes runoff, half gets absorbed and the rest goes back into the air as water vapor. The páramo in Ecuador is a great example; a giant sponge that soaks up water where it falls. When the ground cannot absorb rain — whether because cattle have pounded it solid or because people have built office towers, apartment complexes, roads and parking lots on it — nearly all the rain washes away, carrying pollution with it, and with increasing frequency overrunning stormwater systems.

City/tree image by Parinya via Shutterstock

NRDC says that even though just 3 percent of the United States is classified as urban, urban stormwater runoff pollutes water far beyond city limits. In the restrained bureaucratese of the EPA, an “impaired” river, lake or estuary is so polluted that it cannot support current or desired uses. In these terms, stormwater runoff from cities is responsible, at a minimum, for 3 percent of all impaired river miles, 18 percent of impaired lake acres and 32 percent of impaired square miles of estuaries.

These polluted waters harm fish and wildlife populations, kill native vegetation, contribute to stream bank erosion, foul drinking water supplies and make recreational areas unsafe and unpleasant. Pollution makes people sick and those illnesses in turn become a further drag on the economy.

Landscape architects and engineers have devised a variety of techniques for making cities more porous or capturing rainwater where it falls. For example, rain barrels at the end of a gutter downspout catch water from rooftops and rain gardens along sidewalks keep water out of sewer drains. Gravel driveways and parking lots, paving stones and high-tech materials such as pervious concrete allow water to soak into the ground, significantly reducing runoff. Planting sod and other vegetation on rooftops not only slows the flow of water into storm sewers but also provides habitat for birds, cools and cleans the air and, by adding a layer of insulation, saves on heating and cooling.

Making cities more like natural landscapes with green roofs and rain gardens may seem tiny solutions to massive problems. Indeed, one big hurdle in dealing with anything related to climate change is getting past the overwhelming feeling that anything less ambitious than continental-scale projects will fail to make any difference. Enough green infrastructure in enough places, however, will matter.

In 2009, researchers from NRDC and the University of California Santa Barbara found that building green infrastructure at new and redeveloped residential and commercial properties in Southern California and the San Francisco Bay Area could increase local water supplies annually by up to 405,000 acre-feet — more than 130 billion gallons — by 2030. This represents roughly two-thirds of the volume of water used by the entire city of Los Angeles each year. Because more plentiful local water reduces the need for energy-intensive imported water, these savings translate into electricity savings of up to 1,225,500 megawatt-hours — decreasing the release of carbon dioxide into the atmosphere by as much as 535,500 metric tons per year. Perhaps most important, researchers say, these benefits would increase every year.

Conservation of upstream forests that act as green infrastructure saves money for New York, Boston and other cities. These efforts are largely invisible to most city residents, but green infrastructure right downtown, even more front and center than Staten Island’s Bluebelt, will save money as well. In the United States, Philadelphia is a leader in this area.

Like many older cities, Philadelphia has one system of pipes and drains to handle both sewage and stormwater. The combined system serves three-quarters of the city’s residents and underlies the oldest and most densely populated part of the city. On most days, the system works fine and transports water to a number of wastewater treatment plants. In heavy rain or snow, however, the system cannot handle the volume, resulting in what engineers call a combined sewer overflow (CSO). Rather than suffer a catastrophic failure and widespread flooding, the city dumps millions of gallons of raw sewage into the Schuylkill and Delaware Rivers, which bracket the city to the west and east, as well as local creeks.

CSOs occur in dozens of other cities as well. Not only does this kind of dumping pour all manner of pollutants into the rivers and streams, the sheer volume of water scours riverbeds and strips vegetation off banks, leading to erosion and making it easier for nonnative species to become established. If this continues, urban waterways become smelly, chewed-up eyesores — just as Olmsted predicted for Boston’s Muddy River.

In June 2011, the Pennsylvania Department of Environmental Protection approved Philadelphia’s ambitious “Green City, Clean Waters” plan to reduce runoff over the next 25 years. The city plans to transform at least one-third of currently impervious surfaces into “greened acres,” installing green roofs and rain gardens to capture the first inch of rainfall in any given storm. This could reduce runoff by 80 to 90 percent.

At the other end of the spectrum, cities that choose not to pursue environmentally friendly policies face considerable risks. The fact is, Philadelphia and other cities would have far less incentive to act if not for the hefty fines they face if they do not. The EPA has the authority, under the Clean Water Act, to levy fines or even bring the city to court on criminal charges, as it did in Pineville, La., in 2011. City officials in Pineville knew that a pump was leaking hydraulic fluid but did nothing to repair it. When Pineville flooded in a hurricane in 2011, the fluid got into the nearby river and bayou. The city itself — not the mayor, not the manager of the pumping station — pled guilty, received one year of probation and had to, among other things, issue a formal apology.

Beyond the dark comedy of a city appearing in handcuffs before a judge and making a shamefaced apology, taking a city to court for environmental crimes is serious stuff. Philadelphia and others cities know the possible penalties and the Pineville story highlights the crucial role government must play in maintaining and protecting the benefits of nature. Even when economic benefits of nature are evident, regulation and enforcement are often required to bring various interests to the negotiating table.

Governments generally have been the builders of and investors in the infrastructure that society depends on. Accordingly, they should be the logical champions of natural infrastructure. Governments also provide regulation, the vital underpinning of all aspects of business and markets, and need to do the same with environmental assets as well. If companies can pollute the environment without charge or penalty, less enlightened companies certainly will pollute. The notion of natural capital and its important values should motivate cross sections of society to come together and advocate for well-designed environmental regulations.

Motivated by the threat of fines from the EPA under the Clean Water Act and by a desire to be as frugal as possible, Philadelphia now seeks to reduce the amount of sewage dumped into its rivers each year by nearly 8 billion gallons, with much of that reduction coming from green infrastructure. As of August 2011, Philadelphia’s water department had completed or was in the process of designing a roster of improvements that reveals the project’s gritty practicality: 91 stormwater tree trenches, 33 downspout planters, 24 rain gardens, 12 porous paving projects, nine stormwater bumpouts, nine swales, seven stormwater planters, six infiltration/storage trenches, three stormwater wetlands and one stormwater basin. The water department also plans stream-corridor restoration projects to complement the green infrastructure efforts. Philadelphia considers its green infrastructure efforts part of a broader strategy to provide “more equitable access to healthy neighborhoods” for its residents and to make Philadelphia the “greenest city in America.”

Philadelphia city officials estimate that an all-gray approach to reducing overflows would have cost billions more than the green infrastructure plan that will achieve comparable results. The green infrastructure plan includes at least $1.67 billion of investments in greened acres and $345 million in expanded sewage treatment plant capacity.

Over time, Philadelphia almost certainly will see a positive return on its green infrastructure investment. This return combines the full range of economic, social and environmental benefits of the plan with all the costs avoided — air pollution emissions from gray-infrastructure manufacturing, installation, and runoff pumping and treatment. A 2009 analysis of Philadelphia’s efforts focused on those “fringe benefits”: additional recreational user days in the city’s waterways, reduction of premature deaths and asthma attacks from air pollution and excessive heat, increased property values in greened neighborhoods, the ecosystem values of restored or created wetlands, poverty reduction from new local green jobs, and energy savings from the shading, cooling and insulating effects of vegetation. The study found that over 40 years the total value of the green infrastructure would be nearly $3 billion.

Not everyone has taken Philadelphia’s approach. Washington, D.C., for example, in 2011 began building a pipeline around the city to deal with its stormwater problems, at a cost of nearly $3 billion. The city also elected to spend a mere pittance, perhaps as little at $10 million, to investigate green infrastructure solutions.

Washington’s decision reveals an important challenge for green infrastructure. The experiences of many other cities show that investing in nature works financially, environmentally and socially, yet each year $250 billion in private capital is invested in gray infrastructure instead. This is partially because few government agencies or financial institutions make the effort to link the benefits of natural infrastructure with financing these projects. Changing this situation requires creative financing expertise — expertise too rarely used on environmental projects.

Next page: Green infrastructure works

The private capital necessary to create green infrastructure that will have a far-reaching impact will not flow until savvy investors are satisfied on their own terms that it will be worthwhile. To that end, NRDC, TNC and the investment firm EKO Asset Management Partners in 2011 created the Natural Infrastructure Innovative Financing Lab, whose goal is to shift a substantial portion of the billions in annual private capital investment in traditional infrastructure from gray to green solutions.

Green infrastructure works. Any city can become cleaner and healthier and can save money at the same time by investing in nature. This is about far more than stormwater. Hurricane Sandy almost immediately prompted city officials in New York City and elsewhere to begin thinking about how wetlands, reefs and other green infrastructure can be part of the effort to prepare for the next big flood. And climate change practically guarantees there will be another.

Nearly 3 billion additional people will live in cities by 2050. More than four-fifths of all Americans currently live in cities of 50,000 people or more, and many of these cities, such as Houston or Jacksonville, Fla., sprawl across the landscape, gobbling up farms and fields with few zoning laws. What happens in cities will determine the pace and extent of environmental change, locally and globally. Such changes do not necessarily spell doom for nature and natural habitats.

Thinking about how cities will look several decades from now is essential to urban conservation. This seems counterintuitive, given the urgency of so many environmental challenges facing cities such as Philadelphia. We should not underestimate the value of efforts that offer real short-term payoffs, such as Philadelphia’s initiative, which will make a real difference almost immediately.

Urban conservation is not without risk. The biggest risk may be transforming conservation into something people support only when they are its direct beneficiaries. If this were to happen, then conservation would become a far narrower and desiccated version of itself.

We have a moral imperative to place the interests of those creatures that are at the mercy of humankind above our own narrow self-interest. To abandon that imperative would be a great loss to conservation. Thus, urban conservation must broaden its support base not by preaching utilitarianism, but instead by demonstrating its widespread benefits. Cities offer the opportunity to demonstrate the value of nature writ large.

Excerpted with permission from Nature's Fortune: How Business and Society Thrive by Investing in Nature, by Mark R. Tercek and Jonathan S. Adams.  Available from Basic Books, a member of The Perseus Books Group. Copyright © 2013.