Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group

1. Understanding the Effects of Climate Change and Climate Variability on the Water Cycle in the Pacific Northwest • Alan F. Hamlet, • Philip W. Mote, • Dennis P. Lettenmaier • JISAO/CSES Climate Impacts Group • Dept. of Civil and Environmental Engineering • University of Washington

2. Natural AND human influences explain the observations of global warming best. Natural Climate Influence Human Climate Influence All Climate Influences

3. +3.2°C °C +1.7°C +0.7°C 1.2-5.5°C 0.9-2.4°C Observed 20th century variability 0.4-1.0°C Pacific Northwest

4. % -1 to +3% +6% +2% +1% Observed 20th century variability -2 to +21% -1 to +9% Pacific Northwest

5. Local changes in climate may be larger than regional changes, but are also inherently more uncertain, particularly for precipitation.

6. Will Global Warming be “Warm and Wet” or “Warm and Dry”? Answer: Probably BOTH!

7. Regionally Averaged Cool Season Precipitation Anomalies PRECIP

8. Projected Hydrologic Changes Associated with Warming

9. For areas that accumulate substantial snowpack, the areas close to freezing in mid-winter are most sensitive to warming. ~2060 +2.3C, +4.5% winter precip

10. Changes in Simulated April 1 Snowpack for the Canadian and U.S. portions of the Columbia River basin (% change relative to current climate) 20th Century Climate “2040s” (+1.7 C) “2060s” (+ 2.25 C) -3.6% -11.5% -21.4% -34.8% April 1 SWE (mm)

11. Simulated Changes in Natural Runoff Timing in the Naches River Basin Associated with 2 C Warming • Impacts: • Increased winter flow • Earlier and reduced peak flows • Reduced summer flow volume • Reduced late summer low flow

12. Is there evidence of change in historic records?

13. Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49

14. As the West warms, spring flows rise and summer flows drop Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8): 1136-1155

15. Wide-Spread Glacial Retreat has Accompanied 20th Century Warming. Loss of glacial mass may increase summer flow in the short term and decrease summer flow in the long term. 1902 2002 The recession of the Illecillewaet Glacier at Rogers Pass between 1902 and 2002. Photographs courtesy of the Whyte Museum of the Canadian Rockies & Dr. Henry Vaux.

16. Impact Pathways Associated with Hydrologic Changes • Changes in water quantity and timing • Reductions in summer flow and water supply • Increases in drought frequency and severity • Changes in hydrologic extremes • Changing flood risk (up or down) • Summer low flows • Changes in groundwater supplies • Changes in water quality • Increasing water temperature • Changes in sediment loading (up or down) • Changes in nutrient loadings (up or down) • Changes in land cover via disturbance • Forest fire • Insects • Disease • Invasive species

17. Impact Pathways Associated with Hydrologic Changes • Changes in outdoor recreation • Tourism • Skiing • Camping • Boating • Changes in engineering design standards • Road construction • Storm water systems • Flood plain definitions • Changes in transportation corridors • Changing risk of avalanche or debris flows • Human health risks • Temperature-related health risks

18. Approaches to Adaptation and Planning • Anticipate changes. Accept that the future climate will be substantially different than the past. • Use scenario based planning to evaluate options rather than the historic record. • Expect surprises and plan for flexibility and robustness in the face of uncertain changes rather than counting on one approach. • Plan for the long haul. Where possible, make adaptive responses and agreements “self tending” to avoid repetitive costs of intervention as impacts increase over time.

19. Conclusions • Climate change will result in significant hydrologic changes in western North America, including reduced natural storage as mountain snowpack, increased flow in winter, and reduced flow in summer. Changes in extremes (droughts and floods) are likely to occur. • Impacts will not be equally distributed, and areas near freezing in mid-winter will be the most sensitive to warming related losses of snowpack and streamflow timing shifts. • A number of impact pathways related to water resources , water quality, and ecosystem disturbance and function are likely to be activated by these changes. • There is a wide-spread need to incorporate expected changes in climate into long-range planning at all levels of governance.