The overall vulnerability of a city is a function of three factors: city location, city size, and type of economy. First, climate risks vary with location, with differences existing between coastal and inland risks, riverine and dryland risks, tropical and extratropical risks, and so forth. For example, coastal cities are vulnerable to sea level rise, and many tropical and subtropical coastal locations are additionally vulnerable to tropical cyclones. Cities located on rivers are vulnerable to floods, and cities located in dry vegetated climates are subject to wildfires. Second, large cities have greater total vulnerability to climate change because there are more people and infrastructure to expose to climate impacts and more likelihood that they have a large number of sensitive people. Still, large cities also have more financial, educational, and other resources -- i.e., more adaptive capacity -- for vulnerability reduction. In contrast, smaller cities have a reduced footprint and might be less sensitive overall, but when they do suffer exposure to an event, they have less adaptive capacity. Third, city economies that are resource-dependent are more vulnerable to climate change than cities with diverse, integrated economies. For instance, a city that is largely dependent on forests devastated by climate-induced insect outbreaks is more vulnerable than if the forest industry is one of many facets of the city’s economy. Other characteristics of cities that make them vulnerable to climate change include rapid urbanization and fragile urban infrastructures.
The greatest climate risks to cities are extreme events and sea level rise. Tropical cyclones, floods and landslides resulting from extreme rainfall, wildfire, and heat waves are examples of extreme climate-related events that could devastate a city. As discussed in the coastal impacts lesson, sea level rise is starting to stress some cities and worry many others. Other important climate risks include health impacts, particularly heat stress, air quality, water-borne illnesses, and disease vectors. Cities with strong urban heat island effects are particularly prone to heat stress and air quality issues.