2014/07/16 Council Agenda Packet

21: NORTHWEST Observed Climate Change Temperatures increased across the region from 1895 to 2011, none find statistically significant changes in the Northwest.11 with a regionally averaged warming of about 1.3°F.10 While These and other climate trends include contributions from precipitation has generally increased, trends are small as both human influences (chiefly heat -trapping gas emisions) compared to natural variability. Both increasing and decreasing and natural climate variability, and consequently are not trends are observed among various locations, seasons, and projected to be uniform or smooth across the country or over time periods of analysis (Ch. 2: Our Changing Climate, Figure time (Ch. 2: Our Changing Climate, Key Message 3). They are 2.12). Studies of observed changes in extreme precipitation also consistent with expected changes due to human activities use different time periods and definitions of "extreme," but (Ch. 2: Our Changing Climate, Key Message 1). Projected Climate Change An increase in average annual temperature of 3.3°F to 9.7°F emissions, summer precipitation is projected to decrease by is projected by 2070 to 2099 (compared to the period 1970 as much a°s130% by the end of the century (Ch. 2: Our Changing to 1999), depending largely on total global emissions of heat- Climate).2 Northwest summers are already dry and although trapping gases. The increases are projected to be largest in a 10% reduction (the average projected change for summer) is summer. This chapter examines a range of scenarios, including a small amount of precipitation, unusually dry summers have ones where emissions increase and then decline, leading to many noticeable consequences, including low streamflow west lower (B1 and RCP 4.5) and medium (A1B) total emissions, of the Cascades13 and greater extent of wildfires throughout and scenarios where emissions continue to rise with higher the region.14 Note that while projected temperature increases totals (A2, A1FI, and RCP 8.5 scenarios). Change in annual are large relative to natural variability, the relatively small average precipitation in the Northwest is projected to be projected changes in precipitation are likely to be masked by within a range of an 11% decrease to a 12% increase for 2030 natural variability for much of the century.15 to 2059 and a 10% decrease to an 18% increase for 2070 to 209912 for the B1, A1B, and A2 scenarios (Ch. 2: Our Changing Ongoing research on the implications of these and other Climate). For every season, some models project decreases changes largely confirms projections and analyses made over and some project increases (Ch. 2: Our Changing Climate, the last decade, while providing more information about how lo,1z aspect of seasonal changes in climate impacts are likely to vary from place to place within Key Message 5), yet one precipitation is largely consistent across climate models: for the region. In addition, new areas of concern, such as ocean scenarios of continued growth in global heat -trapping gas acidification, have arisen. Key Message 1: Water -related Challenges Changes in the timing of streamflow related to changing snowmelt have been observed and will continue, reducing the supply of water for many competing demands and causing far- reaching ecological and socioeconomic consequences. Description of Observed and Projected Changes Observed regional warming has been linked to changes in the timing and amount of water availability in basins with significant snowmelt contributions to streamflow. Since around 1950, area -averaged snowpack on April 1 in the Cascade Mountains decreased about 20%,19 spring snowmelt occurred 0 to 30 days earlier depending on location,'' late winter/early spring streamflow increases ranged from 0% to greater than 20% as a fraction of annual flow,18'19 and summer flow decreased 0% to 15% as a fraction of annual flow,17 with exceptions in smaller areas and shorter time periods.20 Hydrologic response to climate change will depend upon the dominant form of precipitation in a particular watershed, as well as other local characteristics including elevation, aspect, geology, vegetation, and changing land use.22 The largest re- sponses are expected to occur in basins with significant snow accumulation, where warming increases winter flows and ad- vances the timing of spring melt.1823 By 2050, snowmelt is pro- U.S. GLOBAL CHANGE RESEARCH PROGRAM 489 CLIMATE CHANGE IMPACTS IN THE UNITED STATES