Ecosystems: Most ecosystems are moderately to highly sensitive to changes in climate. Effects are likely to include both beneficial and harmful changes. Potential impacts include northward shifts of forest and other vegetation types, which would affect biodiversity by altering habitats and would reduce the market and non-market goods and services they provide; declines in forest density and forested area in some subregions, but gains in others; more frequent and larger forest fires; expansion of arid land species into the great basin region; drying of prairie pothole wetlands that currently support over 50 per cent of all waterfowl in North America; and changes in distribution of habitat for cold-, cool- and warm-water fish. The ability to apply management practices to limit potential damages is likely to be low for ecosystems that are not already intensively managed.

Hydrology and Water Resources: Water quantity and quality are particularly sensitive to climate change. Potential impacts include increased runoff in winter and spring and decreased soil moisture and runoff in summer. The Great Plains and prairie regions are particularly vulnerable. Projected increases in the frequency of heavy rainfall events and severe flooding also could be accompanied by an increase in the length of dry periods between rainfall events and in the frequency and/or severity of droughts in parts of North America. Water quality could suffer and would decline where minimum river flows decline. Opportunities to adapt are extensive, but their costs and possible obstacles may be limiting.

Food and Fiber Production: The productivity of food and fiber resources of North America is moderately to highly sensitive to climate change. Most studies, however, have not fully considered the effects of potential changes in climate variability; water availability; stresses from pests, diseases and fire; or interactions with other, existing stresses. Warmer climate scenarios (4-5°C increases in North America) have yielded estimates of negative impacts in eastern, southeastern and corn belt regions and positive effects in northern plains and western regions. More moderate warming produced estimates of predominately positive effects in some warm-season crops. Vulnerability of commercial forest production is uncertain, but is likely to be lower than less intensively managed systems due to changing technology and management options. The vulnerability of food and fiber production in North America is thought to be low at the continental scale, though subregional variation in losses or gains is likely. The ability to adapt may be limited by information gaps; institutional obstacles; high economic, social and environmental costs; and the rate of climate change.

Coastal Systems: Sea level has been rising relative to the land along most of the coast of North America, and falling in a few areas, for thousands of years. During the next century, a 50-cm rise in sea level from climate change alone could inundate 8,500 to 19,000 km 2 of dry land, expand the 100-year flood-plain by more than 23, 000 km 2 and eliminate as much as 50 per cent of North America's coastal wetlands. The projected changes in sea level due to climate change alone would under-estimate the total change in sea level from all causes along the eastern seabord and Gulf coast of North America. In many areas, wetlands and estuarine beaches may be squeezed between advancing seas and dikes or seawalls built to protect human settlements. Several local governments are implementing land-use regulations to enable coastal ecosystems to migrate landward as sea level rises. Saltwater intrusion may threaten water supplies in several areas.

Human Settlements: Projected changes in climate could have positive and negative impacts on the operation and maintenance costs of North American land and water transportation. Such changes also could increase the risks to property and human health and life as a result of possible increased exposure to natural hazards (e.g., wildfires, landslides and extreme weather events) and result in increased demand for cooling and decreased demand for heating energy,with the overall net effect varying across geographic regions.

Human Health: Climate can have wide-ranging and potentially adverse effects on human health via direct pathways (e.g., thermal stress and extreme weather/climate events) and indirect pathways (e.g., disease vectors and infectious agents, environmental and occupational exposures to toxic substances, food production). In high-latitude regions, some human health impacts are expected due to dietary changes resulting from shifts in migratory patterns and abundance of native food sources.

Conclusions: Taken individually, any one of the impacts of climate change may be within the response capabilities of a subregion or sector. The fact that they are projected to occur simultaneously and in concert with changes in population, technology, economics, and other environmental and social changes, however, adds to the complexity of the impact assessment and the choice of appropriate responses. The characteristics of subregions and sectors of North America suggest that neither the impacts of climate change nor the response options will be uniform.

Many systems of North America are moderately to highly sensitive to climate change, and the range of estimated effects often includes the potential for substantial damages. The technological capability to adapt management of systems to lessen or avoid damaging effects exists in many instances. The ability to adapt may be diminished, however, by the attendant costs, lack of private incentives to protect publicly owned natural systems, imperfect information regarding future changes in climate and the available options for adaptation, and institutional barriers. The most vulnerable sectors and regions include long-lived natural forest ecosystems in the east and interior west; water resources in the southern plains; agriculture in the south-east and southern plains; human health in areas currently experiencing diminished urban air quality; northern ecosystems and habitats; estuarine beaches in developed areas; and low-latitude cool- and cold-water fisheries. Other sectors and sub-regions may benefit from opportunities associated with warmer temperatures or, potentially, from CO 2 fertilization,including west coast coniferous forests; some western rangelands; reduced energy costs for heating in the northern latitudes; reduced salting and snow-clearance costs; longer open-water seasons in northern channels and ports; and agriculture in the northern latitudes, the interior west and the west coast.