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Updated 12 October, 2003

US National Assessment of
the Potential Consequences
of Climate Variability and Change
Educational Resources
Regional Paper: Pacific Northwest


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Copyright 2002 Photodisc Inc

Seattle, Washington

The Pacific Northwest region includes the states of Washington, Oregon, and Idaho, and for assessing impacts in the Columbia River basin, some areas in adjoining states and the Canadian province of British Columbia. The region has a great diversity of resources and ecosystems, including spectacular forests containing some of the world's largest trees; mountain and marine environments so close together there are strong influences between them; and nearly all the volcanoes and glaciers in the contiguous US.

The region is divided climatically, ecologically, economically, and culturally by the Cascade Mountains. The low-lying areas west of the Cascades hold three quarters of the region's population. Here, once-dominant forestry, fishing, and agriculture have been overtaken by aerospace, computer software and hardware, trade, and services. The relatively declining natural resource sectors, though, remain economically and culturally important. For example, the Northwest still provides about a quarter of the nation's softwood lumber and plywood. Agriculture is now much more important in the east of the region. Thanks in part to massive water-management and irrigation projects, the fertile lowlands of eastern Washington are the Fruit Bowl -- of the nation, producing 60% of the nation's apples and large fractions of its other tree fruit. Idaho produces about a quarter of the nation's potatoes.

The region has seen several decades of rapid population and economic growth. Population has nearly doubled since 1970, a growth rate almost twice the national average. The region is projected to continue growing faster than the national average. Both recent and projected growth rates are similar east and west of the Cascades (very slightly higher on the west), so the west side is projected to continue to contain nearly three quarters of the region's population. Federal lands comprise roughly half the region's land area. The region's environment presents a great variety of outdoor recreational opportunities, and its moderate climate and quality of life contribute to its continuing attraction to so many newcomers.

The same environmental attractions that draw people and investment to the region are increasingly stressed by the region's rapid development. The predominant current stresses arise from direct human interactions with the land, through such activities as dam building, forestry (including replacement of natural forests by plantations), and land-use conversion from the original forests, wetlands, grasslands, and sagebrush to expansion of metropolitan areas, intensively managed forests, agriculture, and grazing. The consequences include loss of old-growth forests, wetlands, and native grass and steppe communities, increasing urban air pollution, extreme reduction of salmon runs, and increasing numbers of threatened and endangered species.

The Northwest has a great diversity of landscapes and ecosystems, reflecting the region's varied climate and topography. Dense, tall moist conifer forests cover about 80% of western Washington and Oregon. A century of commercial logging has cut nearly all this forest at some time, greatly altering its species and age distribution. Only 10 to 20% of the original extent remains as old-growth forest. The west portions of this region also include oak forests and grasslands in low-lying river valleys, coastal salt marshes and freshwater wetlands, and pockets of mixed forests of drought-resistant conifers and hardwoods.

Major Ecological Regions of the Pacific Northwest

Major Ecological Regions
of the Pacific Northwest

Source: United States National Atlas

East of the Cascade Mountains, the drier climate and more frequent fires generate an open, park-like forest of ponderosa pine and various firs and spruce, patches of alpine larch at higher elevations, and whitebark pine especially near the upper tree line. In the Rocky Mountains and the east slope of the Cascades, forest gives way at high elevations to alpine meadows, and at lower elevations to juniper woodlands, sagebrush steppe, and grasslands, as well as high desert and lava fields in Idaho.

As on the west side of the Cascades, human influence has greatly altered east-side ecosystems. Fire suppression, grazing, and selective cutting of forests have transformed all but a few percent of the original ponderosa pine forest into overstocked mixed-species forests that are highly susceptible to fire, insects, and disease. More than 99% of the prairie grasslands near the meeting point of Idaho, Oregon, and Washington have been converted to crops, mostly wheat, while about 90% of the sagebrush steppe on the Snake River plain in Idaho has been converted to agriculture. Grazing has transformed nearly all of the remaining grassland and sagebrush steppe, leading to expansion of Juniper woodlands into former rangeland. Grazing has also resulted in large-scale replacement of native perennials with invasive annuals such as Cheatgrass, Medusahead, and Yellow Starthistle that reduce grazing capacity.

An additional stress on inland forests is the devastation of the high elevation whitebark pine (Pinus albicaulis) by the introduced fungus, white pine blister rust. Throughout the species' range in northern Idaho more than half of the trees are dead, while infection rates of living trees are above 50% throughout Washington and Idaho, and above 20% in Oregon.

The region is rich in biodiversity. In the wet west-side forests, more than 150 species of terrestrial snails and slugs have been identified, and 527 species of fungi, of which 234 are rare and occur nowhere else. It is estimated that these forests could support 50,000 to 70,000 species of arthropods. Oregon contains between 3,000 and 4,000 identified species of vascular plants, putting Oregon in the top six states for plant diversity. Washington and Idaho are in the top 15 in the country. Although 67 populations of fish in the region are on either federal or state sensitive species lists, much more is known about salmon and trout, the most highly valued species in the region, than about other species.

Of 58 distinct salmonid stocks in the region, 26 are now listed as endangered or threatened under the Endangered Species Act (ESA), including the Puget Sound Chinook. This is the first ESA listing to affect a major metropolitan area. Of roughly 450 species of birds identified in five sub-regions of the Northwest by the Breeding Bird Survey, from 10 to 35 species per sub-region show decreased numbers since the 1960s. Three to 25 species per subregion show increases, with the largest net decreases in the coastal forests of southern Oregon. Seven carnivorous mammals -- the grizzly bear, gray wolf, lynx, wolverine, fisher, marten, and kit fox -- have small and threatened regional populations, principally due to loss of forest habitat, and the secondary effects of logging road construction.

Average Annual Precipitation, Pacific Northwest, 1961-1990

Average Annual Precipitation,
Pacific Northwest, 1961-1990

The Cascade mountains divide the wetter west from the drier east.

Source: Mapping by C. Daly, graphic by G. Taylor and J. Aiken, copyright 2000, Oregon State University.

Climate Variability and Change

West of the Cascades, the climate of the Northwest is maritime, with abundant winter rains, dry summers, and mild temperatures year-round. Temperatures are usually above freezing in winter, so snow seldom stays on the ground more than a few days. Most places west of the Cascades receive more than 30 inches of precipitation annually, while some westward mountain slopes of the Olympic and Cascade mountain ranges receive more than 200 inches. Although a mild maritime climate has prevailed in the region for several centuries, thousand-year records show substantial fluctuations. For example, from about 4,000 to 8,000 years ago in the Puget Sound area, dominance of dry vegetation types such as California chaparral suggests the region's climate was much warmer and drier, resembling the present climate of California's northern Central Valley.

East of the Cascade crest, the climate shifts sharply from abundant rainfall to abundant sunshine, with annual precipitation generally less than 20 inches, as little as 7 inches in some places. These precipitation differences are most pronounced in winter: summer precipitation in the west is only slightly higher than in the east, while winter precipitation is four to five times higher. Though average temperatures are similar east and west, the east has larger daily and annual ranges, with hotter summers and colder winters.

The El Niño Southern Oscillation (ENSO) Phenomenon

El Niño events occur when the trade winds that normally blow from east to west over the tropical Pacific Ocean diminish and the waters of the central and eastern tropical Pacific become warmer than normal. The ENSO cycle, normally about one-year in length, is a continuously changing pattern of ocean-atmosphere behavior. It often causes rainfall patterns and amounts to change in various locations around the globe.

For example, El Niño winters tend to be drier than normal (including reduced snowpack) in the Pacific Northwest and wetter than normal along the coasts to both the south and north of the region. In contrast, the La Niña phase of ENSO tends to bring winters that are cooler and wetter than normal in the Pacific Northwest. The major exception to this cool-wet versus warm-dry pattern occurs during the strongest El Niño events, such as that of 1998. During these strong events winters on the Northwest coast are warmer and wetter -- i.e., while moderate El Niños tend to make Northwest winters warmer and drier, the strongest El Niños reverse the effect on precipitation and make the region warmer but with near normal precipitation. Historically ENSO events have occurred approximately every 2-7 years.


Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a recently identified pattern of longer-term variability, described by changes in Pacific sea-surface temperature north of the equator (20 degrees latitude). Like the warm El Niño phase of ENSO, the warm or positive phase of PDO warms the Pacific near the equator and cools it at northern mid-latitudes. But unlike ENSO, PDO's effects are stronger in the central and northern Pacific than near the equator, and its irregular period is several decades, tending to stay in one phase or the other for 20 to 30 years at a time.

For example, the PDO was in its cool, or negative phase from the first sea-surface temperature records in 1900 (and possibly before) until 1925, then in warm or positive phase until 1945, cool phase again until 1977, and warm phase until the 1990s. Evidence is beginning to mount that another change to the cool phase of PDO likely occurred in the mid-1990s, but it is too early to tell with confidence.

The warm phase of PDO, like El Niño, brings warmer winter temperatures over western North America and warmer ocean temperatures along the coast. Because the storm track splits and carries the storms to the north and south of this region, these winters tend to be drier than normal in the Pacific Northwest (including less snowpack) and wetter than normal along the coasts to both the south and north. In contrast, years during the cool phase of PDO are like the cool La Niña phase of ENSO, tending to bring winters that are cooler and wetter than normal in the Pacific Northwest.


Northwest Average Temperature, Observed and Modeled

Northwest Average Temperature,
Observed and Modeled

The red line shows annual-average temperature in the Northwest in the 20th century, observed from 113 weather stations with long records. The blue line shows the historical Northwest average temperature calculated by the Canadian model from 1900 to 2000, and projected forward to 2100.

Source: Mote et al (1999), Summary (p. 6).

Historical Climate

Over the 20th century, the Northwest has grown warmer and wetter. The average trend in temperature is + 1.4F with nearly equal warming in summer and winter. Annual precipitation also increased nearly everywhere in the region, by 11% on average, with the largest increases of about 50% in northeastern Washington and southwestern Montana.

In addition to this trend toward a warmer, wetter climate, the Northwest's climate also exhibits significant recurring patterns of multi-year variability. The predominant pattern is that warm years tend to be relatively dry with low streamflow and light snowpack, while cool years tend to be relatively wet with high streamflow and heavy snowpack. Although the differences in temperature and precipitation are relatively small (differences in monthly-average temperature of up to 2 to 4F in winter), they have clearly recognizable effects on important regional resources. For example, warmer, drier years tend to have summer water shortages, less abundant salmon, and increased risk of forest fires.

These year-to-year variations in the region's climate are clearly related to two large-scale patterns of climate variation over the Pacific, one more and one less well known. The more well known pattern is the El Niño/ Southern Oscillation (ENSO) and the lesser known pattern is the Pacific Decadal Oscillation (PDO). While both of these patterns have an influence on the seasonal weather, for instance raising the average winter temperature by about 1 ˚F, they do not control it and there are other influences involved.

Weather and Climate

Distinguishing clearly between the terms weather and climate is important to understanding how to interpret the results of this section. Weather is the hour-to-hour and day-to-day state of the atmosphere, providing specific indications of whether, at a particular time, it is rainy or sunny, warm or cold, windy or calm. Climate is the average weather over time, and encompasses a locale's typical weather patterns, including frequency and intensity of storms, cold outbreaks, and heat waves.

Just as the weather varies naturally, the climate varies naturally in response to such factors as sunspots, volcano eruptions, and atmosphere-ocean interactions (e.g., El Niño events). Climate change is a shift in the climate that lasts a few decades or more. Human activities in the last two centuries have become important drivers of climatic change. For this paper, whether the cause of an impact is natural or anthropogenic (human) is less important than whether it has to do with long-term trends or shorter patterns of variation. Thus we use more intuitive definitions:

  1. variability refers to day-to-day, season-to-season, year-to-year, and decade-to-decade patterns of weather and climate;
  2. climate change refers to longer-term trends in the average weather and climate, usually measured and experienced by long-term changes in temperature, precipitation, and sea level.


Global Warming or Climate Change

The media often uses the term global warming when talking about changes to the global climate. The phrase climate change, however, actually encompasses the more intricate set of changes that scientists are projecting. For example, the world is not expected to warm uniformly and some areas actually could become cooler as other parts of the Earth warm. Although more rain and snowfall are expected as the globe warms, some areas will become drier at the same time that other areas become wetter. The phrase climate change is, therefore, a more accurate way to describe projected changes to the global environment.


Possible Future Climates

The climate model results selected for use in the National Assessment study (Hadley and Canadian) show regional warming continuing at an increased rate in the next century, in both summer and winter. Average warming over the region is projected to reach about +3F by the 2020s and +5F by the 2050s -- well outside the natural range of climate in the 20th century. Annual precipitation changes projected through 2050 over the region range from a small decrease (-7% or 2 -- ) to a slightly larger increase (+13% or 4 -- ). The projected changes in precipitation, unlike the projected temperature changes, are within the range of year-to-year variability that has been experienced over the past 100 years in the PNW. The models suggest small changes in yearly average precipitation, but the seasonal trends are larger: nearly all the climate models show wetter winters and some show drier summers in the future.

After 2050, the projected trend to a warmer, wetter regional climate continues with substantially more warming likely to occur in winter than in summer. By the 2090s, projected average summer temperatures rise by +7.3F to 8.3F, while winter temperatures rise +8.5F to 10.6F. Projected precipitation increases over the region range from 0 to 50% depending on the model. Warmer temperatures, though, mean less winter precipitation will fall as snow and therefore there will be less snowpack for later melting and use.

These projected changes are associated with large-scale shifts in atmospheric circulation over the Pacific, especially in winter, which resemble the changes that occur during the strongest El Niño events. As a result, winters are warmer and wetter -- both in total precipitation and in the amount of rainfall in heavy storms -- because warmer temperatures increase the quantity of water vapor

Temperature Change in the Pacific Northwest, 20th & 21st Centuries
Precipitation Change in the Pacific Northwest, 20th & 21st Centuries

Warming since 1900 in the Pacific Northwest ranges from 0 to 4F.  By 2100, both models project warming near 5F west of the Cascades, with much larger warming further east in the Canadian model.

Precipitation has increased over most of the Pacific Northwest since 1900.  Both climate models project continued precipitation increases, with the largest increases in the southern part of the region.

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