Climate Change in the Pacific Northwest
In the Pacific Northwest, we are collaborating with climate researchers at the University of Washington’s Climate Impacts Group (CIG), the U.S. Geological Survey, the U.S. Forest Service (USFS) and many others to develop an understanding of climate change effects in the Pacific Northwest, and how to manage fish and wildlife resources in light of these effects.
The assessment used 20 different global climate models for greenhouse gas emissions under a “medium” emissions scenario and 18 models for a “low” scenario. The assessment focuses on the “medium” scenario which is based on estimates that are lower than current rates of greenhouse gas emissions. In other words, the assessment is far from a “worse case” scenario or even a “business as usual” scenario.
Climate and ecology in the Pacific Northwest are largely influenced by the interactions between seasonally varying atmospheric circulation patterns, or weather, and the mountainous terrain within the region.
Naturally Occurring Variations in Pacific Northwest Climate
Temperature records indicate that Pacific Northwest temperatures increased 1.5°F since 1920. Regionally downscaled climate models project increases in annual temperature of, on average, 2.0°F by the 2020s, 3.2 °F by the 2040s, and 5.3°F by the 2080s (compared to the 1970-1999 period), averaged across all climate models. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project wetter autumns and winters and drier summers. Increases in extreme high precipitation (falling as rain) in the western Cascades and reductions in snowpack are key projections from high-resolution regional climate models. (Littell et-al., 2009)
Changes in temperature and precipitation will continue to decrease snow pack, and will affect stream flow and water quality throughout the Pacific Northwest region. Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing. This change will result in:
The decline of the region's snowpack is predicted to be greatest at low and middle elevations due to increases in air temperature and less precipitation falling as snow. The average decline in snowpack in the Cascade Mountains, for example, was about 25% over the last 40 to 70 years, with most of the decline due to the 2.5 degrees F increase in cool season air temperatures over that period. As a result, seasonal stream flow timing will likely shift significantly in sensitive watersheds. (Littell et-al., 2009)
Studies and the results of vegetation change modeling suggest that a number of different scenarios are possible for Pacific Northwest forests. These scenarios differ dramatically, ranging from projections of forest expansion to forest dieback, as a result of uncertainty regarding how projected temperature and precipitation changes will interact to affect drought stress in trees or otherwise modify total annual productivity. Other major uncertainties are whether increased levels of carbon dioxide (CO2) in the atmosphere would increase primary productivity or help trees withstand reduced soil moisture. The likeliest scenario seems to be that increased forest growth could occur during the next few decades, but that at some point temperature increases would overwhelm the ability of trees to make use of higher winter precipitation and higher CO2.
Virtually all future climate scenarios predict increases in wildfire in western North America, especially east of the Cascades, due to higher summer temperatures and earlier spring snowmelt. Fire frequency and intensity have already increased in the past 50 years, and most notably the past 15 years in the shrub steppe and forested regions of the West. The area burned by fire regionally is projected to double by the 2040s and triple by the 2080s. The probability that more than two million acres will burn in a given year is projected to increase from 5% (observed) to 33% by the 2080s. USFS and CIG researchers have linked these trends to climate changes. Drought and hotter temperatures have also led to an increase in outbreaks of insects, such as the mountain pine beetle, increasing the risk of fire. (Littell et al., 2009)
The melting of mountain glaciers and the Greenland and Antarctic ice sheets along with the thermal expansion of the oceans will likely continue to increase sea level for many hundreds of years into the future. The consensus estimate of sea level rise by 2100, published in the Intergovernmental Panel on Climate Change’s Fourth Assessment, was estimated at 0.6 to 2.0 ft. Improved estimates of the range of sea level rise by 2100, which now include estimated effects of ice dynamics, lie between 2.6 and 6.6 ft, a significantly higher estimate.
The ocean will eventually absorb most carbon dioxide released into the atmosphere as a result of the burning of fossil fuels. Dissolving of carbon dioxide into ocean surface waters will increase the acidity of ocean surface waters. Oceanic absorption of CO2 from fossil fuels may result in larger acidification changes over the next several centuries than any inferred from the geological record of the past 300 million years (with the possible exception of those resulting from rare, extreme events such as meteor impacts).
In addition to temperature and rainfall changes, researchers and others have observed rising sea levels and changes to ocean conditions. Some important climate-related factors to consider are sea level, air and sea surface temperatures, winter precipitation, and storminess. These factors influence coastal erosion, landslides, flooding and inundation, estuarine water quality, and invasion of exotic species. In particular, the following conditions increase the risk associated with various coastal hazards:
In the Pacific Northwest, climate change may affect the coastal marine environment by increasing ocean temperature, increasing the vertical stratification of the water column (reducing mixing which is important to the marine food chain), and changing the intensity and timing of coastal winds and upwelling. Wind-driven coastal upwelling and mixing are particularly important to productive marine ecosystems that support diverse marine life, major fisheries and seabirds. Upwelling usually brings cold, nutrient-rich water to the surface in nearshore areas, supporting highly productive food webs. However, too much wind may transport planktonic organisms offshore and away from coastal areas. These coastal systems are highly variable in both locality and time. Natural changes can occur daily, weekly, seasonally, yearly or even every ten years. And upwelling can vary greatly due to El Niño-Southern Oscillation events which occur on average every 2 to 7 years, as well as decadal shifts known as cool or warm phases of the Pacific Decadal Oscillation. For example, El Niño events often result in reduced upwelling and productivity.
The Fish and Wildlife Service’s 2009 5-year review of the Marbled Murrelet (pp. 42-45) contains a thorough evaluation of climate change affects to the marine environment. The review concludes that climate change is likely to result in changes to the murrelet’s marine environment. While physical changes to the near-shore environment appear likely, much remains to be learned about the magnitude, geographic extent, and temporal and spatial patterns of change, and their effects on coastal and marine species.
Climate change affects salmon throughout its life stages. Historically, warm periods in the coastal ocean have coincided with relatively low abundances of salmon, while cooler ocean periods have coincided with relatively high salmon numbers.
Salmon productivity in the Pacific Northwest is clearly sensitive to climate-related changes in stream, estuary, and ocean conditions. In the past century, most Pacific Northwest salmon populations have fared best in periods having high precipitation, deep mountain snowpack, cool air and water temperatures, cool coastal ocean temperatures, and abundant north-to-south "upwelling" winds in spring and summer.
Rising stream temperatures will likely reduce the quality and extent of freshwater salmon habitat. The duration of periods that cause thermal stress and migration barriers to salmon is projected to at least double and perhaps quadruple by the 2080s for most analyzed streams and lakes. The greatest increases in thermal stress (including diseases and parasites which thrive in warmer waters) would occur in the Interior Columbia River Basin and the Lake Washington Ship Canal. The combined effects of warming stream temperatures and altered stream flows will very likely reduce the reproductive success of many salmon populations in Washington watersheds, but impacts will vary according to different life-history types and watershed-types. As more winter precipitation falls as rain rather than snow, higher winter stream flows scour streambeds, damaging spawning nests and washing away incubating eggs for Pacific Northwest salmon. Earlier peak stream flows flush young salmon from rivers to estuaries before they are physically mature enough for transition, increasing a variety of stressors including the risk of being eaten by predators.
Littell, J.S., M. McGuire Elsner, L.C. Whitely Binder, and A.K. Snover (eds). 2009. The Washington Climate Change Impacts Assessment: Evaluating Washington's Future in a Changing Climate, (PDF 14.1 MB) Climate Impacts Group, University of Washington, Seattle, Washington.
Ocean Carbon and Biogeochemistry Program, Subcommittee on Ocean Acidification. December 2, 2008. Ocean Acidification- Recommended Strategy for a U.S. National Research Program.