Working Group II: Impacts, Adaptation and Vulnerability

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North America has experienced challenges posed by changing climates and changing patterns of regional development and will continue to do so. Varying impacts on ecosystems and human settlements will exacerbate subregional differences in climate-sensitive resource production and vulnerability to extreme events. Opportunities may arise from a warming climate, and some innovative adaptation strategies are being tested as a response to current challenges (e.g., water banks), but there are few studies on how these strategies could be implemented as regional climates continue to change. Recent experience demonstrates high capability in emergency response to extreme events, but long-term problems remain.

Climate Trends and Scenarios

North America has warmed by about 0.7°C during the past century and precipitation has increased, but both trends are heterogeneous (e.g., seasonal reductions in precipitation in some areas).

For a range of emission scenarios produced for this Third Assessment Report, model results suggest that North America could warm by 1-3°C over the next century for a low-emissions case (B1). Warming could be as much as 3.5-7.5°C for the higher emission A2 case. Published regional impact studies have used climate scenarios with global temperature changes that are similar to these new cases, but regional scenarios may not be directly comparable.

Key Regional Concerns

Water Resources

Changes in precipitation are highly uncertain. There is little agreement across climate scenarios regarding changes in total annual runoff across North America.

The modeled impact of increased temperatures on lake evaporation leads to consistent projections of reduced lake levels and outflows for the Great Lakes-St. Lawrence system under most climate change scenarios (medium confidence). The only exception is the HadCM2 transient scenario incorporating IS92a sulfate aerosol emissions, which suggests slight increases in lake levels and outflows.

Where snowmelt currently is an important part of the hydrological regime (e.g., Columbia basin), seasonal shifts in runoff are likely, with a larger proportion of runoff occurring in winter, together with possible reductions in summer flows (high confidence).

Adaptive responses to such seasonal runoff changes could include altered management of artificial storage capacity, increased reliance on conjunctive management of ground and surface water supplies, and voluntary water transfers between various water users. It may not be possible, however, to avoid adverse impacts on many aquatic ecosystems or to fully offset the impacts of reduced summer water availability for irrigation and other out-of-stream and instream water uses.

Where lower summer flows and higher water temperatures occur, there may be reduced water quality and increased stress on aquatic ecosystems (medium confidence).

Possible changes in the frequency/intensity/duration of heavy precipitation events may require changes in land-use planning and infrastructure design to avoid increased damages arising from flooding, landslides, sewerage overflows, and releases of contaminants to natural water bodies.

Responses to recurring and emerging water quality and quantity problems will provide opportunities to develop and test adaptive management options.

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