Urban resilience has conventionally been defined as the "measurable ability of any urban system, with its inhabitants, to maintain continuity through all shocks and stresses, while positively adapting and transforming towards sustainability".
Therefore, a resilient city is one that assesses, plans and acts to prepare for and respond to hazards, regardless whether they are natural or human-made, sudden or slow-onset, expected or unexpected. Resilient Cities are better positioned to protect and enhance people's lives, secure development gains, and drive positive change.
According to the urban historian Roger W. Lotchin the 2nd World War had an profound environmental impact on urban areas in the USA. By 1945 Pittsburgh and other cities along the Mississippi River experienced air pollution comparable to the Dust Bowl. The environmental impact of the 2nd World War turned urban areas around the world into shock cities. Extreme cases of hard hit cities include Hiroshima, Chongqing, Stalingrad, and Dresden. Environmental history first emerged as an academic research topic in the 1970s, focusing initially on rural areas. Pioneers of urban environmental history include Martin Melosi, Christine Rosen, Joel A. Tarr, Peter Brimblecombe, Bill Luckin, and Christopher Hamlin. Recurrent climate changes have prompted renewed interest in this academic field. The concern for urban resilience in the urban planning of cities has become increasingly visible in recent years, partly because urban resilience can be used to describe the change in structure and function of urban areas. Social scientists have taken an increased interest in ecological resilience, because the links between social-ecological systems are being examined. Urban resilience is no longer the preserve of academics, urban policy documents around the globe are putting forward proposals to enhance the urban resilience of cities. The definition of urban resilience may vary, but are no longer limited to the speed at which an urban systems recover after a shock.