For years scientists have urged national leaders to tackle climate change, based on the  assumption that prevention efforts would require the coordinated actions of entire nations to be effective. But as anyone who has watched the past 15 years of international climate negotiations can attest, most countries are still reluctant to take meaningful steps to lower their production of greenhouse gases, much less address issues such as how to help developing countries protect themselves from the extreme effects of climate change. Frustrated by the ongoing diplomatic stalemate, a number of urban leaders have decided to take matters into their own hands, adopting solutions that already exist or inventing new ones for limiting greenhouse gas emissions and preparing for the effects of ongoing global warming.

Mayors and urban managers are taking over because they have a keener sense about how changing weather patterns will affect their cities’ political and economic future. As Bärbel Dieckmann put it in 2007, when she was mayor of Bonn, Germany, “cities are already experiencing flooding, water shortages, heat waves, coastal erosion and ozone-related deaths.” Since the mid-1990s, according to a 2009 report, the number of intense hurricanes has been increasing in the Atlantic Ocean, and the size of wildfires has been growing in the western U.S. As temperatures continue to rise, such extreme events may become even more frequent and severe. Most of the world’s major metropolises were originally built on rivers or coastlines and are therefore subject to flooding from rising seas and instances of heavier rainfall.

Many civic leaders point to Hurricane Katrina and the devastation it visited on New Orleans in 2005 as their moment of awakening. They saw how the multiple failures of an aging and inadequate infrastructure, plus indifferent planning, sharply increased the death toll of a catastrophe that had long been predicted. Indeed, two major alliances of city mayors to combat climate change formed within months after Hurricane Katrina’s landfall. The organization now known as the C40 Cities Climate Leadership Group launched in London in October 2005, and the World Mayors Council on Climate Change (WMCCC) got its start in Kyoto that December. As of June 2011, more than 190 mayors and other local authorities, representing some 300 million people from around the world, have also signed a voluntary pact sponsored by the WMCCC to reduce greenhouse gas emissions.

In some ways, cities may be in a better position than nations to do something about climate change. By conservative estimates, the cities of the world emit no less than 40 percent of such greenhouse gases as carbon dioxide, methane, nitrous oxide and fluorinated gases. According to a 2011 study by Daniel Hoornweg and his co-authors in the journal Environment and Urbanization, cities may actually be responsible for roughly 80 percent of emissions if one takes into account their consumption of electricity, food and other commodities that require the burning of fossil fuels. Indeed, the article found that if the C40 cities were a country, its population would be about 290 million people, and it would be the fourth-largest emitter of greenhouse gases, after the U.S., China and Russia.

Cities are already assessing the greatest climate risks they face and are beginning to try out solutions to the most obvious challenges. Scientists and engineers are helping the effort by sizing up existing programs and evaluating proposed initiatives, using the best available evidence. The people leading the charge still have much to learn—particularly with respect to integrating the efforts of multiple players in the public and private sectors. But it is already apparent that cities have the power to reduce the sources of climate change while softening the blow from whatever weather extremes have already become unavoidable.

Stepping Up
Each urban center faces its own unique constellation of climate-related problems. The risk of damage varies depending on its physical features (such as whether it is built on a delta or floodplain), its particular layout (a compact, high-density arrangement or urban sprawl), and its built environment (such as the amount and location of pavement that promotes runoff during storms). Urban planners need to know precisely which neighborhoods and what services are most vulnerable.

Nevertheless, cities are beginning to address four interconnected issues:

Reducing emissions. Commercial and residential buildings account for a significant portion of urban energy use. The combination of rising energy costs and concerns about climate change is pushing many cities to try to tame consumption by improving the energy efficiency of new buildings and by retrofitting old ones. For example, about 75 percent of New York City’s carbon emissions stem from energy used in buildings. Mayor Michael Bloomberg has begun tackling this issue with a program that evaluates the energy use of the city’s largest buildings and mandates improvements in cost-effective energy efficiency. To reduce emissions, water use and heat buildup, cities can adopt more renewable energy sources; Oakland, Calif., now meets 17 percent of its needs with electricity generated from wind, solar and geothermal plants. In cities in developing countries, lack of access to reliable energy is more often the key problem. In many cases, improved energy systems are needed to aid in development rather than to combat climate change. But the two may be linked if renewable sources are encouraged.

Some cities are further along than others in reducing the production of greenhouse gases. The Hoornweg study found that each person living in Denver emits the equivalent of 21.5 metric tons of carbon dioxide every year. Residents of New York City, in contrast, produce about 10.5 metric tons apiece. New York City’s higher population density, milder winters and lesser use of cars for commuting allow its inhabitants to produce less than a third of the per capita average of greenhouse gases for the entire U.S. population. Lest New Yorkers starting patting themselves on the back, however, they should consider the citizens of Amsterdam, who are each responsible for only 6.7 metric tons of carbon dioxide a year—and other European cities are lower. As part of Amsterdam’s plan to reduce its carbon footprint by 40 percent from 1990 levels by 2025, it is generating heating and electricity from waste and sewage and is placing additional wind turbines in its port and upgrading old ones.

Preserving water. Most climate change models predict a long-term decline in the availability of freshwater in southwestern regions of North America, southern Europe, the Middle East and southern Africa. With a water-conservation program that began in 1983, Austin, Tex., has pioneered the large-scale use of low-flush toilets, rebates to residents who replace turf grass with native plants that are better adapted to drought conditions, and progressive water rates for residential customers that become more expensive as more water is used. Cities can also use “gray water”—wastewater that has been cleaned enough by treatment plants to be dumped into rivers but not enough to drink again—to keep city parks green instead of using freshwater. Since 2002 Melbourne, Australia, has responded to a continuing drop in rainfall by enacting increasingly stringent water restrictions. Sanitation officials, however, anticipate that the sharp drop in water flow, combined with increasing temperatures, will make wastewater warmer and more concentrated, increasing the chance of corrosion in sewer pipes; they will have to change their inspection and maintenance programs to keep up.

Keeping transportation systems moving. Key transportation infrastructure is often located near waterways and is thus vulnerable to sea-level rise and inland flooding. When tunnels, ramps and vent shafts flood, pumps are needed to remove the water. Debris must be cleared, and essential elements of the system, such as motors, relays, resistors and transformers, must be repaired or replaced. Entrances to the Taipei subway in Taiwan have been raised to avoid inundation from flash floods and high tides. Sweltering temperatures can also disrupt equipment such as overhead electrical wire and steel rails, eventually causing them to sag or even to buckle. Installing transformers and wiring that are able to function efficiently at higher temperatures and keeping equipment dry are minimum first steps.

Protecting public health. The rise in average global temperatures will likely lead to a worsening of urban health problems such as respiratory ailments related to poor air quality and bring about new difficulties, such as a greater range for certain illnesses caused by rodents and other disease-carrying animals. Perhaps the most immediate effect, however, will be more frequent and severe heat waves, which are already the most deadly weather-related events in the U.S. Chicago and Paris are planning for the changes, but there has been little research to show public health authorities which interventions—such as opening cooling centers or identifying particularly vulnerable individuals ahead of time—actually save lives or reduce hospitalizations. Some adaptation strategies can pay off in multiple ways; for example, improving energy efficiency reduces power generation, which lessens the heat and pollution a city generates, thus lowering cases of heatstroke and asthma.

As soon as civic leaders have a clearer picture of their own city’s individual risks, they need a strategy for prioritizing initiatives. My colleagues and I encourage cities to concentrate on efforts that result in multiple wins. For example, greenery planted on rooftops decreases water runoff from storms and acts as an insulator that reduces a building’s energy consumption, thereby lessening carbon emissions.

Helping One Another Succeed
Many cities do not have the expertise within their own governments to accurately assess their risk from climate change and to develop a comprehensive response plan. Various groups of international researchers, including the Urban Climate Change Research Network, have come together to try to fill that gap by linking scholars with decision makers. The network’s first in-depth assessment covering some 50 cities—including Buenos Aires, Delhi and Lagos—was released this year and found, among other things, that severe flooding is as bad as unrelenting drought when it comes to loss of power or the provision of clean water. The goal of such reports is twofold: to provide scientific analysis of the specific challenges cities face because of climate change and to evaluate potential adaptations that might limit the most deleterious effects. 

Going forward, it makes sense to develop common sets of standards for reporting greenhouse gas emissions and reductions, the impacts of climate change on cities, and the efforts to lessen the toll in human lives and property. Such universal benchmarks would allow cities to measure their own progress, compare their results with those of other municipalities and share their innovations.

Just as important, cities have to engage larger groups of citizens—especially those from the poorest and most vulnerable neighborhoods because they are the people who are likely to suffer most from climate change and may need to make the biggest adjustments. The Ecuadorian city of Quito, for example, provides technical support to nearby impoverished farmers that helps them switch from growing potatoes and corn to native Andean crops such as quinoa, which require less water and better prevent soil erosion. Such changes improve the amount of water available as well as its quality in both rural and urban areas.

In the six years since Katrina, climate change initiatives by some of the world’s largest cities have shown that progress is possible when motivated partners work together. Much must be done, and cities in many nations still need to get onboard. But the momentum is growing. Let us hope it is not too late to save lives and safeguard the future.