IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

7.1.3 Human systems in context

Human systems include social, economic and institutional structures and processes. Related to industry, settlement and society, these systems are diverse and dynamic, expressed at the individual level through livelihoods. They tend to revolve around such aims of humanity as survival, security, well-being, equity and progress; and in these regards weather and climate are often of secondary importance as sources of benefits or stresses. More important are such issues as access to financial resources, institutional capacities and potentials for conflict (Ocampo and Martin, 2003; Thomas and Twyman, 2005) and such stresses as rapid urbanisation, disease and terrorism. It is in its complex interactions with these kinds of social contexts that climate change can make a difference, easing or aggravating multiple stresses and in some cases potentially pushing a multi-stressed human system across a threshold of sustainability (Wilbanks, 2003b).

In most cases, climate (and thus climate change) affects human systems in three principal ways. First, it provides a context for climate-sensitive human activities ranging from agriculture to tourism. For instance, rivers fed by rainfall enable irrigation and transportation and can enrich or damage landscapes. Second, climate affects the cost of maintaining climate-controlled internal environments for human life and activity; clearly, higher temperatures increase costs of cooling and reduce costs of heating. Third, climate interacts with other types of stresses on human systems, in some cases reducing stresses but in other cases exacerbating them. For example, drought can contribute to rural-urban migration, which, combined with population growth, increases stress on urban infrastructures and socio-economic conditions. In all of these connections, effects can be positive as well as negative; but extreme climate events and other abrupt changes tend to affect human systems more severely than gradual change, because they offer less time for adaptation, although gradual changes may also reach thresholds at which effects are notable.