About Pacific Northwest Climate

Climate Impacts in Brief

Climate Impacts on Pacific Northwest Coasts

Impacts of Climate Variability

Our analysis of the impacts of climate variability on the Pacific Northwest (PNW) coastal zone suggests that 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:

• Increased sea level (associated with El Niño events during winter and spring, Figure 1) increases the risk of coastal erosion,
• Increased winter precipitation (associated with La Niña years, Figure 2, and cool phase PDO years) increases the risk of coastal riverine flooding and of landslides,
• Southeasterly winter storms (associated with El Niño events during winter and spring) increase the risk of coastal erosion, and
• The co-occurrence of these three conditions increases the likelihood of large, damaging coastal erosion and flooding events.

Figure 1 Monthly mean sea levels during the 1997-1998 El Niño at Neah Bay, Washington and Crescent City, California compared to the average and maximum long-term (1975-1998) monthly mean sea levels (from NOAA tide gauge data).



Figure 2 Large numbers of landslides in the Seattle area have occurred during the winters of 1933-34, 1985-86, 1996-97 and 1998-99. All were La Niña winters, and all but 1985-86 were exceptionally wet winters.

Significant erosion can also cause coastal inlets to migrate and lagoons to fill, and often undermines the shoreline armoring constructed to protect against shoreline erosion. The number and length of shore protection works constructed on the Oregon coast tends to increase substantially in the year immediately after a severe El Niño event, and continues for 2-3 years thereafter (Figure 3).

Changes in ecosystem characteristics and functioning are less clearly associated with climate variability due to the increased complexity of these systems and the lack of long-term studies. Still, some pathways by which climate influences natural systems are clear, even if the linkages themselves are not yet quantifiable. Water quality, for example, is strongly influenced by freshwater input (strongly a function of winter precipitation in the PNW); stream, estuarine, and coastal ocean temperatures; and patterns of stratification and mixing between the estuaries and the coastal ocean.

Humans have played a large role in shaping the coastal environment through such activities as building walls and revetments; dredging, filling, and otherwise modifying the shoreline; introducing exotic and/or invasive species while harvesting native species. These activities have affected the distribution and resilience of coastal ecosystems and strongly shape the region’s vulnerability to past and future shifts in climate.

Pacific Northwest (PNW) coastal managers are familiar (to varying degrees) with repeating patterns of PNW climate variability but make little use of climate forecasts in their planning and operations (Johnson 1998). Uncertainty over the value that forecasts would add, as well as institutional, legal, financial, and technical constraints, have hindered managers’ consideration of climate forecasts and climate-based resource forecasts. The evolving integrative coastal management plans of Washington and Oregon (enacted under the U.S. Coastal Zone Management Act) are making headway at coordinating previously piecemeal management and planning, but could still go further in taking advantage of recent developments in climate and impacts research.

Climate Change Impacts

Global climate change is projected to exacerbate many of the same stresses and hazards currently facing the coastal zone. In general, the long-term effects of climate change on the coastal zone will likely be similar in nature, but greater in magnitude, to some of the effects of short-term climate variability. This is a result of the similarities between the regional climate shifts projected for anthropogenic climate change (warmer wetter winters, resulting in increased winter streamflow; warmer summers; and increased sea level) and some of those experienced during La Niña winters (increased precipitation and winter streamflow) and El Niño years (warmer winters, resulting in decreased spring and summer streamflow; increased sea level).

Projections of 21st century global sea level rise by the Intergovernmental Panel on Climate Change (IPCC) are 2.0-8.6 mm/year, compared to 1.0-2.5 mm/year observed during the last century (Intergovernmental Panel on Climate Change (IPCC), J. T. Houghton, L. G. Meira Fihlo, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell (eds.). 1996a. Climate Change 1995: The Science of Climate Change: Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press). On average (i.e., the IPCC's "best estimate"), global sea level is expected to be 19 inches higher by 2100, with a range of 6 to 37 inches. The sea level rise that will be experienced along the coasts of the PNW will also depend on circulation changes in the Northeast Pacific (in much the same way that El Niño elevates our sea levels) and on local vertical land movements. In some places (south Puget Sound and the northern Oregon coast), the fact that the ground is slowly sinking will exacerbate sea level rise; in other places (the Washington and southern Oregon coasts), uplift will ameliorate sea level rise.

The ultimate impacts of future climate change will reflect both changes in climate and the impacts of human choices and activities, both past and future, in the coastal zone. Some climate models project changes in sea-level pressure patterns that suggest a more southwesterly direction of future winter winds, much like the strong El Niño events of 1982-83 and 1997-98. Combined with higher sea levels, these changes suggest an acceleration of coastal erosion on the Pacific Ocean coast since erosion is sensitive to these two factors.

The heavier winter rainfall that is projected by nearly all of the climate models suggests an increase in saturated soils and therefore landslides. An increased frequency and/or magnitude of landsliding could be expected anywhere the geologic conditions are conducive to landsliding, and are likely to be even more severe in areas subject to intensive development on unstable slopes. Within Puget Sound the cyclic interaction of beach erosion and bluff landsliding is expected to be exacerbated by sea level rise (more beach erosion) and heavier winter rainfall.

Selected References

For more publications on climate impacts on PNW coastal environments, please see CIG Publications.

Canning, D. J., and P. W. Mote. (in review). Climate impacts on the coasts of the Pacific Northwest. Chapter 9 in E.L. Miles, A. K. Snover and The Climate Impacts Group, Rhythms of Change: An Integrated Assessment of Climate Impacts on the Pacific Northwest. Cambridge, Massachusetts: MIT Press.

Johnson, Z. P. 1998. Sensitivity of the coastal management system in Washington state to the incorporation of climate forecasts and projections. MMA thesis, School of Marine Affairs, University of Washington, Seattle.