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The city's greenest asset – its subway system – also became its greatest weakness this week.

Slate posted a smart story last week about the inherent tension cities face in trying to be "resilient" and "sustainable" at the same time. "Resiliency" is often another way of talking about redundancy: Communities are more likely to withstand disasters, terrorist attacks or future consequences of climate change if they have multiple systems backing up access to important social assets (whether that’s the Internet, or a street network, or a power grid). If a tree falls in Brooklyn, the neighborhood will be more resilient if bikers and cars can route around it.

Slate used the example of New York City’s combined sewer system:

When there's simply too much water in the sewers for the city's wastewater treatment plants to cope, the proverbial flood gates are opened and rainfall mixed with sewage flows into area waterways, such as Newtown Creek or the Gowanus Canal.

In that example, New York’s redundant backup system is Mother Nature herself. Writes David Biello:

That isn't exactly sustainable. Especially once you consider that downpours and the like are predicted to be on the increase as a result of climate change, as are little threats like sea level rise that could turn outflows into inflows.

Slate gets serious props for foresight publishing this a full week before Sandy. The Gowanus Canal did, in fact, just overflow to predictably disgusting effect. But Biello’s point got us thinking about another reality in post-Sandy New York. The very asset that makes New York one of the greenest cities in the country also created a huge vulnerability for it during the storm. I'm talking about New York’s exhaustive subway system, the envy of transit advocates everywhere.

Because of that transit system, and the dense development it enables, 55 percent of New York City households don’t even own a car (that’s far and away the highest rate of any large city in America). And that statistic helps contribute to the fact that, per capita, New Yorkers produce only about one-third of the greenhouse gas emissions [PDF] of the average American.

But because the city relies so heavily on the flood-prone underground subway network, that effectively means that New York grinds to a halt when its transit network does. Literally, that's what happened Wednesday morning. The above Google map shows the nightmarish surface gridlock in New York yesterday morning as the city tried to get back to work without the subway. Here’s just one snapshot, posted by Aaron Naparstek on Twitter, of the parking-lot-like standstill on Brooklyn’s Atlantic Avenue:

This morning, Twitter user @virginialaird captured this shocking image out her window of the line of Brooklyn commuters waiting to board the Manhattan-bound bus service the city is running this week to get workers onto the island while the subway tunnels are still flooded:

New Yorkers are being punished this week for the city’s over-reliance on the subway. And yet that transit network is the city’s greatest contribution to combating the climate change that could make Sandy-like storms even more likely in the future.

This is exactly the resiliency-vs.-sustainability tension Biello was talking about. Often, he writes, we use the two words interchangeably. But in fact they mean very different things, and, as goals, they can be in direct conflict with each other. Sustainable cities are the most efficient ones (they're efficient with dense housing, with multi-modal transportation, with mixed-use development). But efficiency sometimes entails forgoing all those backup systems that keep us "resilient" in the face of an event like Sandy. In this case, New York simply wasn't designed with the street infrastructure to handle all these cars carrying commuters who'd otherwise take the train. Via Biello:

Some of the most obvious ways to become more resilient are not sustainable. For example, if you are concerned about reliable electricity, you can increase the resilience of your local grid by buying a diesel generator, or two, or more. In effect, that's what the Googles, Facebooks, and Twitters of the world do. But extra diesel generators are certainly not an efficient, or particularly sustainable, way to create electricity. It's not ideal for the environment to be burning all that extra diesel, with attendant air pollution and the like.

In fact, the NYU Hospital would have needed even more generators – located somewhere other than the basement – to survive Monday night’s double-whammy of power-outage-plus-flood.

All of this doesn’t mean that cities like New York must pick between one ambition over the other. As Biello argues, we need to thoughtfully balance the two going forward (of course, it's an important first step to recognize that these words have different meanings). As for New York’s subway, this doesn't mean we need more highway infrastructure to get these people to work when their transit shuts down. But Sandy teaches us that it may be time to heavily invest in flood-proofing transit so that the city’s greenest asset won’t also be its greatest single point of failure during the next hurricane.

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