The Pacific Northwest encompasses extensive forests, topography that creates abrupt changes in climate and ecosystems over short distances, with mountain and marine environments in close proximity. The Northwest provides about one-fourth of the nation's softwood lumber and plywood. The fertile lowlands of eastern Washington produce 60 percent of the nation's apples and large fractions of its other tree fruit. Population and economic growth of this area has been twice the national rate since 1970, its population nearly doubling during this period. The area provides moderate climate, a high quality of life, and outdoor recreational opportunities, which are becoming increasingly stressed because of the rapid development occurring in the area. Stresses are occurring today from dam operations, land-use conversion from natural ecosystems to metropolitan areas, intensively managed forests, agriculture, and grazing. The result has been in the loss of old-growth forests, wetlands, and native grasslands; increased urban air pollution; extreme reduction of salmon runs; and increasing numbers of threatened and endangered species.
Over the 20th century, the region has grown warmer and wetter. Annual average temperature has increased 1-3°F (0.5-1.5°C) over most of the region. Annual precipitation has also increased across the region by 10 percent on average, with increases reaching 30 to 40 percent in eastern Washington and northern Idaho. The region's climate also shows recurrent patterns of year-to-year variability. The warm years tend to be relatively dry with low stream flow and light snowpack, while cool ones tend to be relatively wet with high stream flow and heavy snow-pack. This variability has distinct effects on the ecosystems. Warmer, drier years have summer water shortages, less abundant salmon, and increased probability of forest fires.
The U.S. Global Change Research Program projects in their models that regional warming in the 21st century will be much greater than observed during the 20th century, with an average warming over the region of about 3°F (1.5°C) by the 2050s. By the 2090s, average summer temperatures are projected to rise by 7-8°F (4-4.5°C), and winter temperatures are expected to rise 8-11°F (4.5-6°C). Precipitation is expected to develop a pattern of wetter winters and drier summers, with an overall decrease in water supply. By the 2090s, however, the projected annual average precipitation is expected to increase, ranging from a few percentage points to 20 percent.
Warmer, wetter winters will increase flooding in the rain-fed rivers. Projected year-round warming and drier summers will probably increase summer water shortages in both rain-fed and snow-fed rivers, including the Columbia, because there would be less snowpack and because it would melt earlier. In the Columbia River, there are already serious water allocation conflicts, and the waterway is already vulnerable to water shortages.
In this region, evergreen coniferous forests dominate the landscape. Approximately 80 percent of the area west of the Cascades is covered with coniferous forests. These forests are sensitive to climate variation because warm, dry summers stress them directly by limiting seedling establishment and summer photosynthesis, as well as indirectly by creating conditions favorable to pests and wildfire. The extent, species mix, and productivity are expected to change under the influence of global warming. The amount of change will depend on the interaction of the amount of precipitation, the seasonal water-storage capacity of forest soils, and changes in the trees' water-use efficiency under elevated CO2. These factors will make proper land management and conservation practices important in order to maintain healthy landscapes and protect natural resources. Adaptation measures can include planting species adapted to the projected climate rather than the present climate; managing forest density to reduce susceptibility to drought stress and fire risk; and using prescribed burning to reduce the risk of large, high-intensity fires. Other practices such as reduced tree cutting, reduced road construction, and the establishment of large buffers around streams are some other ways to promote diversity of plant and animal species in the forest ecosystem.
Sea-level rise is also a concern in the Pacific Northwest. Areas such as Puget Sound, where coastal subsidence is occurring, are especially at risk. These areas are also at risk for winter landslides and increased erosion on sandy stretches of the Pacific Coast. Severe storm surges and erosion are presently associated with El Niño events, which raise sea level for several months and change the direction of prevailing winds.
Global warming is projected to bring similar shifts. Projected heavier winter rainfall is likely to increase soil saturation, landsliding, and winter flooding. All of these changes will likely increase the damage to property and infrastructure on bluffs and beachfronts, as well as beside rivers and waterways.
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