The sheer power of Hurricane Sandy and the damage it inflicted upon New York City is both improbable and ominous. Improbable because extreme climatic events are dictated by statistical probabilities of occurrence that seem impossible until they occur. Ominous because it is likely that the familiar statistical record is changing. In the last few days I have heard a lot of discussion of the word resilience. Extreme events provide a ready forum for applying the term resilience, but at the same time, the term resilience has multiple connotations that can imply very different approaches to management and planning.
Over the past year, I have been examining the concept of resilience and its applicability for water infrastructure systems following the lead of a number of earlier researchers. Resilience is a term that spans many fields, including ecology, engineering, network analysis, infrastructure management, natural resources management, and more. Our current conceptions of the term stem from work by the preeminent ecologist Fritz Holling. In 1973, Holling used new knowledge from the study of population biology and ecological systems to contrast resilience and stability. Holling argued that:
Resilience determines the persistence of relationships within a system and is a measure of the ability of these systems to absorb changes of state variables, driving variables, and parameters, and still persist… Stability, on the other hand, is the ability of a system to return to an equilibrium state after a temporary disturbance. The more rapidly it returns, and with the least fluctuation, the more stable it is.
Later, as research from many fields adapted the term and began to muddy its applicability, Holling furthered this distinction between stability and resilience with the terms Engineering Resilience and Ecological Resilience. Engineering resilience describes systems that are designed or managed to return to a design target after being disturbed. Much of our infrastructure in managed in this manner- we want electricity, water, public transit, and the Internet to function regardless of disturbances. On the other hand, ecological resilience borrows from ecology to recognize that systems can be pushed beyond functional limits to a transition point, at which point the system completely reorganizes. Fisheries are a classic unit of study in this regard, as populations persist until stressed by multiple human and ecological factors to a point of collapse. Under such conditions, a system can “reorganize” and be dominated by wholly new species.
What does this all mean for Hurricane Sandy and New York City? After the September 11th attacks and the Homeland Security boom, resilience became an increasingly popular term to describe a desired operational state for infrastructure after a large disturbance such as a climatic or terrorist event. Preparedness actions can limit the impact of such events to a human system even if it is a natural event. Sandy is the latest in a long line of climatic events that test our notions of system stability. Infrastructure is designed to withstand disturbances and continue to function at a particular level of performance. For instance, in flood protection, we can design levees to protect a city from 100-, 500-, or 1000-year floods. But what constitutes the 500-year flood? For that, hindsight becomes 20-20 vision. The notion of “normal” is based on a relatively short (perhaps 100-year) record of hydrologic events. Statistics will not really tell us if that record has changed for another decade. All the while, our cities, including buildings, levees, subways, and roads, were built years ago with infrastructure that is “sticky” and not easily changed.
In the engineering resilience conception, we would simply make sure that our vital energy, transit, and communications systems return to the expected operating state after a disturbance. With enough money, this sort of resilience is almost always possible. In the ecological resilience conception, however, perhaps a city badly damaged by a unforeseen climatic event completely reorganizes. It might eliminate neighborhoods, lay new high-speed telecommunications cable, or change its demographic composition. Or, perhaps, citizens’ notions of transportation drastically alter. These examples are ways to conceptualize system reorganization that allows a system to persist, but in a different form.
In the political realm, change is hard. It becomes harder in the face of established interests. No better opportunity exists to focus time and resources for system reorganization than a crisis. If a city is devastated by some disturbance, it offers the opportunity to consider how it can be remade, which is an opportunity we should readily embrace.
I favor the need for system adaptability and change, especially as cities are the most prolific engines that exist for innovation. As we consider rebuilding East Coast communities after Hurricane Sandy or the next disturbance to come, resilience becomes a complex term. Do we want to return to our typical patterns and systems? Can we use the opportunity to rebuild with updated infrastructure? Achieving engineering resilience with infrastructure is almost always possible with enough money. But, ecological resilience- the persistence of a system through multiple stable states- is a much more delicate social and technical endeavor. Public discussions of resilience would be better served by less liberal use of the term as we enter a new age of greater climatic variability.
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