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EXECUTIVE SUMMARY
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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