On Exposure and Vulnerability

The IPCC departs from standard methodology when defining risk. Broadly defined, risk comprises some variant of likelihood and consequence, while the IPCC combines hazard, exposure (x) and vulnerability to assemble risk estimates. In either case, risk is a multiple of the components, meaning it follows a hyperbolic form. IPCC further decomposes vulnerability into sensitivity (y) and adaptive capacity, where risk is an increasing function of the former and decreasing function of the latter. Within that tradition, adaptive capacity allows reductions in sensitivity. This latter connection misleads users of the IPCC conceptualization into a singular focus on adaptive capacity to reduce sensitivity to the exclusion of considering adaptive capacity to reduce exposure. Yet examples abound where exposure can be reduced to lower risk. Moving from a floodplain to a hillside reduces risk from flooding but does not reduce the sensitivity to flooding; the effect is from reduced exposure. Considering this risk reduction opportunity, which is especially relevant when making decisions about development in less developed countries, adaptation capacity must be defined more broadly as the potential to reduce sensitivity or to reduce exposure. Recognizing this broader opportunity can deepen our insights into the hyperbolic character of risk and its implications for risk reduction measures. In such a model, and considering only the sensitivity and exposure components, the amount of risk reduction is itself a derivative of the hyperbolic function so that the amount of risk reduction by lessening one source of risk (e.g. sensitivity) depends upon the initial state of the other source (e.g. exposure), a fact easily demonstrated by suitable differentiation of the (hyperbolic) risk function to dy/dx = -y/x .