Small Climate Events Could Have Large Social Effects

Thresholds or tipping points have received much attention in the literature of physical climate science. In Chapter 3 we discuss evidence on the likelihood, in the next decade, of crossing important physical thresholds that could lead to a sharply altered climate regime. Less commonly examined are the ways in which changes in human systems might sharply alter vulnerabilities and thus contribute to the potential of even small climate events to have major impacts. Such changes could contribute to social and political stresses, even in an unchanged climate regime, and could have greater effects in the presence of climate change. The following examples illustrate some mechanisms by which this could happen.

Loss of “Slack” in Local Life-Supporting Systems

Relatively slow climatic, ecological, or economic changes can shift the balance of supply and use of natural systems at a local or regional level to the point that adequate supply can be achieved only with favorable climate conditions. The effects may not be noticeable until an unusual climate event occurs, but the responsibility for the impact would in fact lie with the combination of the event and the underlying changes in vulnerability. The decline in water availability in Pakistan in 2010–2012, already mentioned, exemplifies this type of situation. For decades water supplies had been increasingly appropriated to irrigate crops and provide electric power, but this situation did not create a crisis for livelihoods until these slow changes combined with the much decreased water flows in the Indus River to create a situation in which the agricultural and energy systems were highly vulnerable to drought. The Indus water commissioner’s claim that the cause of the water shortage was climate change may or may not have been accurate; ordinary climate variation may have been the trigger. Even events within the normal range of climate variability can lead to major disruption if support systems have become vulnerable to them.

Increasing Dependence on Global Markets

Economic development in most countries has generally been marked by a pattern in which livelihoods depend decreasingly on subsistence agriculture and the local manufacture of essential products and increasingly on wage labor and the purchase of necessities in global markets. This transition usually includes a rural–urban shift in national populations as well. Historically, these changes have tended to decrease vulnerability of food supplies to local climate events because when destructive climate events occur locally, necessities can be purchased from places where such events have

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