NEW YORK CITY—Here is how climate change could shut down a city: On the morning of August 8, 2007, a thunderstorm paralyzed the largest rail transit system in the U.S.—New York City's subway—during morning rush hour. Flash floods deposited more than 7,000 kilograms of dirt and debris on tracks that stretch more than 1,350 kilometers and carry 1.5 billion passengers annually. A December 1992 storm had a similar impact, including flooding portions of Lower Manhattan and the East River Drive.

Such powerful storms are exactly what scientists predict to become more frequent as the climate changes. In fact, the New York City Panel on Climate Change (NPCC), co-chaired by NASA climate modeler Cynthia Rosenzweig, predicts a 5 to 10 percent increase in the mean precipitation around New York City by 2080—the bulk of that in extreme precipitation events that deliver a large amount of rainfall in a short span.

At the same time, the panel predicts a temperature rise of 2 to 4 degrees Celsius from the current average temperature of 13 degrees C by 2100. "You need vents to get hot air out of the [subway] system, especially if we are going to have hotter weather," says Adam Freed, acting director of the city's Office of Long-Term Planning and Sustainability. But, he adds, "open grates increase the risk of flooding."

Issues like that make adapting to climate change a tricky balancing act for this city government and others, and shoring up infrastructure such as New York's subway lines will be the work of decades. In the meantime, the city has already planted more than 322,000 trees as part of its MillionTreesNYC initiative as well as converted 25 percent of the city's taxi fleet to more fuel-efficient vehicles, such as hybrids.

And the city purchased on April 8 more than 11,500 hectares of additional land upstate in its watershed in an effort to ensure its water supply, which could be impacted by the droughts and unusual precipitation events that climate change brings, potentially fouling drinking water. Officials also are fighting to keep that land free from drilling for natural gas, which can contaminate groundwater.

New York City is not alone in attempting to address climate change, of course. Chicago has attempted to reduce energy use in buildings by encouraging green roofs—planted gardens on rooftops. Green roofs, which diminish the heat island effect, could also help forestall an expected increase in deadly heat waves such as the one that killed 700 people there in 1995. King County, which comprises the Seattle metro area, in Washington State aims to reduce its greenhouse gas emissions by 80 percent from 2007 levels by 2050. Abroad, efforts range from London's attempts to stem the rising tide of the Thames River with flood barriers to an effort by the (relatively) small city of Rizhao, China, to become carbon neutral.

Solar solution
Back in the city that never sleeps, the urban landscape offers roughly 150 million square meters of rooftop. Much of that roofing is black tar, baking under the summer sun, cracking in winter, and generally contributing to the so-called urban heat island effect that compounds the warming impact of climate change. To seed an alternative future, Mayor Michael Bloomberg's office announced the creation of three "solar empowerment zones" on June 8.

The three zones—Brooklyn's downtown and Greenpoint neighborhoods as well as Staten Island's eastern shore—were chosen both for their expansive rooftop acreage and for an electricity use profile that peaks during the day, when the maximum amount of power can be produced from rooftop photovoltaic panels. The train yards in Coney Island, Brooklyn, already use the sun's heat to warm water used to wash subway cars.

"That came out of [the question]: 'Where are the load pockets in the city that need additional assistance to reduce demand?' Then we target resources to that area to put off more costly infrastructure upgrades," Freed says. "It helps increase resilience."

The efforts to bring solar power to New York, along with the tree-planting, fuel-efficient taxis and watershed efforts, are all part of so-called PlaNYC—an effort to adapt the city to the realities of global warming and other challenges by 2030.

As part of that PlaNYC effort, kicked off April 22, 2007, the city convened a panel of experts to assess the risks posed by climate change. The number-one risk, according to a report issued this past December: infrastructure failures—from a shutdown of the power grid, as occurred in the 2003 blackout, to a degradation of the city's water quality.

"You're looking at a city that could have its infrastructure compromised for periods of time by these climate change impacts," says Steven Cohen, executive director of The Earth Institute at Columbia University, which advised the Bloomberg administration on PlaNYC and continues to assist the effort. "The question is: 'What can you do to prepare for that?'"

It's electric
Mitigating New York City's contribution to the greenhouse gas emissions causing global climate change is another part of the PlaNYC strategy. But it is a tricky balance to strike. On the one hand, abundant natural gas has been discovered in the Marcellus Shale Formation beneath New York State, and burning it rather than coal to produce electricity results in 40 percent fewer emissions of carbon dioxide—the primary greenhouse gas causing climate change. On the other, such drilling could foul the municipal water supply.

The region's only large-scale, local low-emission source of electricity—the Indian Point nuclear power plant that provides the single largest portion of New York City's electricity—faces intense local opposition and may be shut down by New York State's efforts to control how it uses cooling water. With or without this power plant—and the local natural gas—the city hopes to reduce its emissions of CO2 by 30 percent below 2007 levels by 2030. The city government itself aims to reach that same target by 2017.

Moving to distributed generation, such as the rooftop solar power initiative, may help with that. But, more importantly, the city is attempting to reduce its overall electricity demand. "In Manhattan, there is a 2,500-megawatt-per-square mile demand," electrical engineer Reza Ghafurian of Consolidated Edison, the city's power utility, explained at a smart grid event at New York University in February. The city hopes to reduce that through new energy audits and efficiency standards.

After all, 85 percent of the buildings that will be weathering a changed climate in 2030 already exist, according to Freed. And "buildings represent 75 percent of emissions in the city," he adds. That means that retrofitting older multiunit apartment buildings and office towers with everything from better windows to increased insulation is more important for reducing emissions and improving resilience than building new ultra-green buildings. Says The Earth Institute's Cohen: "If the population is going up but energy use is not, that's a positive sign….New York City is already the most energy-efficient place in America, and I think it can become even more energy efficient."

Freed adds: "We are making sure that changes that are necessary are part of our codes and standards, so building resilience is not optional." Fostering such resilience will be the key to adapting to climate change, he says. Just as the city does not attempt to stop a snowstorm but instead prepares for its aftermath, New York also must plan how it will come through an increase in extreme weather events, whether heat wave or flood.

Regional impact
Whereas other cities plan for climate change, New York may be alone in having at least some estimates for its local impacts, thanks to a relatively new and imprecise modeling effort. That effort is imprecise because the computer models for global climate change typically employ grid cells larger than the city itself. Nevertheless, in addition to the temperature and precipitation changes, the NPCC predicts sea level rise of at least 30 centimeters (and as much as 140 centimeters) by 2100.

"This is a coastal city," Cohen notes, with at least 965 kilometers of coastline and an average height above sea level of roughly five meters. Much of New York's critical infrastructure—from power plants to marine transfer stations for garbage—lies on the waterline. "We need to—and can—figure out ways to operate [coastal infrastructure] with an acceptable level of risk," Freed says.

A mayoral task force of 40 government agencies and other stakeholders is currently evaluating what the estimates for sea-level rise and other climate change risks will ultimately mean for New York City, particularly for critical infrastructure like the water supply, in order to prioritize upgrades. "You have to be constantly investing in infrastructure and thinking about vulnerabilities as well as new technologies," Cohen says. "Concern for climate change helps you do that."

Already, efforts have begun, such as raising the generators as well as installing flood doors and other barriers, at a wastewater treatment plant in the Rockaways, Queens, that is currently prone to flooding. "When the waters recede we can flip the proverbial switch and only temporarily shut down," Freed notes.

The NPCC, for its part, stresses such flexible adaptation in any core infrastructure plans. An effective response to climate change, the panel wrote in its December 2009 assessment, promotes "strategies that can evolve through time as climate risk assessment, evaluation of adaptation strategies, and monitoring continues."

Of course the ultimate flexible adaptation will mean updating the plan to deal with a continuing climate of change, in both the political and physical realms. The first revision of PlaNYC is due April 22, 2011. "You can't actually climate-proof a city," Freed says. But "the benefits of the things that make sense to do today greatly increase as our climate changes."