Climate impacts on the Pacific Northwest environment: Hydrology and water resources Dennis P. Lettenmaier Department of Civil and Environmental Engineering and Climate Impacts Group University of Washington

Characteristics of the current PNW climate

Annual PNW Precipitation (mm)

Hydrologic Characteristics of PNW Rivers

A history of the PDO A history of ENSO warm warm cool 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Columbia River Apr-Sep streamflow 5 4 3 105 cubic feet per second 2 1 1900 1920 1940 1960 1980 2000

Sensitivity of Snow-dominated and Transient Rivers to Changes in Temperature and Precipitation Temperature warms, precipitation unaltered: Streamflow timing is altered Annual volume stays about the same Precipitation increases, temperature unaltered: Streamflow timing stays about the same Annual volume is altered

1998 cool PDO/warm ENSO

1999 cool PDO/cool ENSO

2001 cool PDO/ENSO neutral

Temperature trends in the PNW US Historical Climate Network data 113 stations with long records Almost every station shows warming Urbanization not a major source of warming

Observed temperature trend

Historic Analogues for the Effects of Climate Change

Unusually Warm Year Ollalie Meadows (3700 ft elevation) WY 1992 Near Normal Precipitation Warm Temperatures ( + 3.5 F) normal precipitation normal snowpack

Effect of 1992 Winter Climate on Two PNW Rivers Cedar River Western Cascades (caused predominantly by warm temperatures) Columbia River at The Dalles (caused both by warm temperatures and decreased precipitation)

Climate change: Predictions for the next century

This figure shows 10-year average temperatures averaged for Washington, Oregon, and Idaho. The observed temperatures are compared with a simulation by one climate model (the CGCM1) for 1900-2050. This plot underscores that the 1990’s were the warmest decade of the 20th century in the Northwest (as well as globally), by almost 1 degree, and that the rate of warming from 1970 to 2000 has been roughly what the CGCM1 simulates. Looking ahead to the 2020s and 2040s, we examined output from a total of 8 climate models and the warmest, average, and coolest of these 8 models are shown as red, yellow, and green. The average rate of warming is a bit less than 1 degree per decade. Even the coolest scenario still shows about 2F warming by the 2040s.

Precipitation changes Most of the models we looked at also project modest increases in precipitation, mainly in winter. Estimated climate change from 20th c. to 2040s using 8 climate model scenarios.

Climate models: The current climate

Interpreting the hydrologic effects

To evaluate the effects of these temperature and precipitation changes on the Northwest’s water resources, we have used the Variable Infiltration Capacity (VIC) model, developed at the University of Washington. VIC includes multiple soil levels, energy and water balance at the surface, and a sub-grid distribution of land surface types and infiltration values. We have implemented VIC over the Columbia River Basin at 1/8 degree by 1/8 degree horizontal resolution. Source for this and the next 6 slides: Hamlet and Lettenmaier, 1999, J. Amer. Water Resources Assn.

The main impact: less snow April 1 Columbia Basin Snow Extent

Columbia Basin Average Snow Water Equivalent HadCM2 (Warm/Wet) and ECHAM4 (Warm/Dry) Scenarios Conclusion: Both Warm/Wet and Warm/Dry scenarios result in reduced SWE

Other sectors (forests, fish, …)

Forests Vegetation carbon Vegetation modeling for 2070-2100 including effects of changes in temperature precipitation (seasonality?) CO2 (uncertain) Longer, hotter summers likely to take a toll on Northwest forests even with CO2 fertilization This figure shows a simulation of North American vegetation using the MC1 model, a dynamic global vegetation model that includes equilibrium biogeography and disturbance components with related feedbacks. Assuming 35% increase in water use efficiency in an enriched-CO2 environment and significant benefits from increased winter precipitation, the model projects increases in vegetation carbon (biomass) for much of the region. However, the most heavily forested areas see decreases in this model. Big questions about WUE assumption. Courtesy Ron Neilson, OSU

???? and climate damage Floods Low summer streamflow, high temp Green arrows: improvements, red arrows: declines Decrease in floods in some basins, increase in others Warmer, shallower streams in summer bad for juveniles Estuary conditions: warming probably increases water column stability, decreases biological productivity ???? Estuary conditions: prey, predators, competitors

Impacts of climate change on the PNW Highest confidence: Models: warmer; higher snow line summer water supply, drought demand for water conflicts over water resources winter streamflow increases in snowmelt-driven basins coastal flooding, inundation salmon freshwater survival energy production Italics indicate an output from climate models Length of arrow indicates magnitude of socioeconomic and ecological impact Reduction in summer water supply: a direct consequence of snow line retreat, affects whole region Demand: to overcome increased evaporative loss (mainly ag, also urban) Conflicts: a consequence of reduced supply and increased demand Winter streamflow: flooding only significant on west side, only affects small areas, very weather-dependent (rather than climate dependent)

Are we prepared for a changing climate? Natural resource management presently assumes that Climate does not change If few of the region’s institutions are prepared to deal differently with a year whose climate is different from normal, how will they deal with a decade or a future whose climate is different from what they’ve experienced as normal? In other words, what will it take for the PNW to become climate-wise? But what if it does?

Adaptation pathways Assess the problem, identify vulnerabilities Long-term planning for reduced summer water Incorporate strategies into water policy These come from White Paper #3