Understanding the Water Resources Impacts of Climate Change 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

Recession of the Muir Glacier On the left is a photograph of Muir Glacier taken on August 13, 1941, by glaciologist William O. Field; on the right, a photograph taken from the same vantage on August 31, 2004, by geologist Bruce F. Molnia of the United States Geological Survey (USGS). According to Molnia, between 1941 and 2004 the glacier retreated more than twelve kilometers (seven miles) and thinned by more than 800 meters (875 yards). Ocean water has filled the valley, replacing the ice of Muir Glacier; the end of the glacier has retreated out of the field of view. The glacier’s absence reveals scars where glacier ice once scraped high up against the hillside. In 2004, trees and shrubs grow thickly in the foreground, where in 1941 there was only bare rock. Aug, 13, 1941 Aug, 31, 2004 Image Credit: National Snow and Ice Data Center, W. O. Field, B. F. Molnia http://nsidc.org/data/glacier_photo/special_high_res.html

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

Natural AND human influences explain the observations of global warming best. Natural Climate Influence Human Climate Influence Red: observations of global average temperature Grey: simulations with a climate model (huge computer program, like weather prediction model only run for hundreds of years) Natural influence: volcanoes, solar variations – guesses before ~1970, better since then Human influence: greenhouse gases, sulfate aerosols it is possible to simulate the climate of the last 100 years, and the conclusion is that humans didn’t matter much before 1960 – the early warming and the cooling were largely natural, and the late-century warming was largely human-caused All Climate Influences

Observed 20th century variability Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed) Warming ranges are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 0.7C by 2020s, 1.7C by 2040s, 3.2C by 2090s. Pink box shows +/- 2 sigma for annual average temperature (sigma=0.6C). Red lines show previous generation of change scenarios. Until mid-century, emissions scenarios play a minor role in the temperature impacts. Towards the end of the century they play a big role. Conclusions: 1) Adaptation will be an essential component of the response to warming over the next 50 years. 2) Mitigation of greenhouse gas emissions will play an important role in determining the scope of late 21st century impacts. 0.4-1.0°C Pacific Northwest

Observed 20th century variability % -1 to +3% +6% +2% +1% Curves are fits to ln(CO2) for A2 (solid) and B1 (dashed) Precip changes are shown for 2020s, 2040s and 2090s relative to 1990s. Central estimates: 1% by 2020s, 3% by 2040s, 6% by 2090s. Pink bar shows +/- 2 sigma for PNW annual precip. Observed 20th century variability -1 to +9% -2 to +21% Pacific Northwest

The warmest locations that accumulate snowpack are most sensitive to warming +6.8% winter precip

Simulated Changes in Natural Runoff Timing in the Nooksack 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

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 extremes Changing flood risk (up or down) Summer low flows Changes in groundwater Changes in Energy Supply and Demand Increased demand and reduced supply in summer Changes in water quality Increasing water temperature Changes in sediment and nutrient loading (up or down) Changes in land cover Forest fire Insects Disease Invasive species Changes in outdoor recreation Skiing Camping Boating

In sensitive areas, systematic reductions in summer water availability will decrease the yield of water supply systems. Master's Thesis: Wiley, M.W. (2004). "Analysis Techniques to Incorporate Climate Change Information into Seattle’s Long Range Water Supply Planning," University of Washington

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.

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 (e.g. western WA) will be the most sensitive to warming related losses of snowpack and streamflow timing shifts. A number of impact pathways related to water resources, land use planning, 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.