Weather and climate extremes have always posed serious challenges to society. Changes in extremes are already having impacts on socioeconomic and natural systems, and future changes associated with continued warming will present additional challenges. Increased frequency of heat waves and drought, for example, could seriously affect human health, agricultural production, water availability and quality, and other environmental conditions (and the services they provide).

Extremes are a natural part of even a stable climate system and have associated costs and benefits. For example, extremes are essential in some systems to keep insect pests under control. While hurricanes cause significant disruption, including death, injury, and damage, they also provide needed rainfall to certain areas, and some tropical plant communities depend on hurricane winds toppling tall trees, allowing more sunlight to rejuvenate low-growing trees. But on balance, because systems have adapted to their historical range of extremes, the majority of events outside this range have primarily negative impacts.

The impacts of changes in extremes depend on both changes in climate and ecosystem and societal vulnerability. The degree of impacts is due, in large part, to the capacity of society to respond. Vulnerability is shaped by factors such as population dynamics and economic status as well as adaptation measures such as appropriate building codes, disaster preparedness, and water use efficiency. Some short-term actions taken to lessen the risk from extreme events can lead to increases in vulnerability to even larger extremes. For example, moderate flood control measures on a river can stimulate development in a now “safe” floodplain, only to see those new structures damaged when a very large flood occurs. Human-induced warming is known to affect climate variables such as temperature and precipitation. Small changes in the averages of many variables result in larger changes in their extremes. Thus, within a changing climate system, some of what are now considered to be extreme events will occur more frequently, while others will occur less frequently (e.g., more heat waves and fewer cold snaps). Rates of change matter since these can affect, and in some cases overwhelm, existing societal and environmental capacity. More frequent extreme events occurring over a shorter period reduce the time available for recovery and adaptation. In addition, extreme events often occur in clusters. The cumulative effect of compound or back-to-back extremes can have far larger impacts than the same events spread out over a longer period of time. For example, heat waves, droughts, air stagnation, and resulting wildfires often occur concurrently and have more severe impacts than any of these alone.