HYDROCLIMATE
Since OCAR3 (Dalton et al. 2017), a new analysis of observed changes in snow resources in the west (Mote et al. 2018) show that nearly every location in Oregon experienced declines in spring snowpack since mid-20th century. Recently, an analysis of atmospheric variability (Siler et al 2018) indicates that the influence of regional warming on the west’s snowpack since 1985 has been largely masked by natural variability in ocean temperatures and atmospheric circulation patterns important during the cool season, effectively slowing the rate of spring snowpack decline. The authors expect greatly enhanced response in the snowpack to warming in coming decades as this pattern ebbs.
Change in (left) April 1 snow water equivalent and (right) summer soil moisture, for 2040-69 under the high-emissions RCP8.5 scenario, as a percentage of 1971-2000 baseline from a mean of 10 GCMs. Figure prepared using the NW Climate Toolbox, climatetoolbox.org. Data source: VIC hydrologic model.
The climate toolbox depicts how the region’s water variables will change. For example, Figure 6 shows the disappearing snowpack expected by the end of the century. Most of the Northwest will see decreases in April 1 snowpack in excess of 56% but the highest peaks in the Cascades are projected to decrease less, only in the 11-33% range. These reductions in snowpack will lead to wintertime increases but summertime decreases in soil moisture in most places (Figure 6). The increases in soil moisture in the driest parts of the region are also seen in the regional superensemble, and are confined to lower soil layers. Upper soil layers also dry substantially there, but the paucity of deep-rooted plants limit the depletion of lower soil layers.
Changes in rainfall will accentuate extremes.
Annual precipitation is not projected to change, but models generally suggest modest increases in winter precipitation and decreases in summer precipitation. Extreme precipitation may change more (~20%) in eastern Oregon than western Oregon (~10%) by mid-century. Heavy rainfall can lead to slope instability and landslides, and close important transportation corridors.
estimates of the maximum plausible sea level by the end of the century (2100) have increased to 8.2 feet.
However, even after global temperature stabilizes, ice sheets will continue melting irreversibly until they reach a new equilibrium which could take millennia. Warming beyond the global 2°C target could lead to irreversible melting of Greenland, highlighting the importance of global policy meant to limit warming. Stabilizing global climate soon could limit sea level rise to less than 3.3 feet even in 2300.
Monthly non-regulated streamflow in the Willamette River at Salem for 2040-2069 under high and low emission scenarios and over the 1971-2000 historical baseline. Shaded regions show the range from 10 climate models. Figure prepared with the NW Climate Toolbox, climatetoolbox.org, data source: streamflow routing of VIC hydrologic model.
In most basins, the changes in snowmelt timing also alter streamflow (Figure 7). The increases in average wintertime flow (owing to reduced snow accumulation and more rapid runoff) also correspond to increases in flood risk in those basins. Summertime flow is reduced in many basins, by as much as 50% (in June).
EMERGING ISSUES
Infrastructure managers are beginning to consolidate planning for the combined risks of sea level rise, flooding, and seismic hazards, as well as tsunami risks that can also arise from a major earthquake event. Going forward, it could be useful to identify strategies that enhance community resilience and emergency response capacity to many types of hazards and potential disruptions.
OCAR 4
FOURTH CLIMATE ASSESSMENT REPORT
