Michael Dettinger US Geological Scripps Institution of Oceanography, La Jolla, CA Dan Cayan, USGS/SIO Iris Stewart, SIO Noah Knowles, USGS Recent hydro- and pheno-climatic trends & Projections of 21st Century climatic trends for the US

In recent decades, winter and spring temperatures have warmed significantly across the West. Cayan et al., 2001 Observed

Stewart et al., 2005 Spring-pulse dates Centers of Mass …and, by several measures, Western streamflow has been arriving earlier in the year in recent decades. in recent decades. Also: Dettinger and Cayan, 1995; Cayan et al., 2001

The warming trends have lengthened growing seasons and hasten green-up dates. Start Date = First day in longest run of days each year with Tavg > 5C (Cayan et al., 2001)

How much of the observed trend is due to natural, decades-long fluctuations of the climate system? The primary natural candidate to explain these trends is the multi-decadal Pacific Decadal Oscillation, which makes the West warm in its “El Nino-like” phase. Warm Cool Dettinger et al., 2001

Lilac/Honeysuckle Phenology (departures from average phenological dates) Cayan et al, 2001

PDO = -0.8 PDO = -1.0 PDO = -0.4

The recent reversal of PDO has NOT slowed the progress of timing trends. The average streamflow timing in the years after the 1998 transition to a “La Nina-like” PDO state have been even earlier than timings in the preceding “El Nino-like” PDO epoch. Stewart et al., 2005

Under varied greenhouse forcings, climate models yield a fairly narrow range of warming scenarios and (amidst a broad overall range) a tendency for little precipitation change in California and most of the West. Greenhouse forcings Dettinger, of 23 in this range 19 of 23 in this range cm/mont h

Across most of the conterminous states, current climate models project small changes in precipitation. Northernmost states consistently wetter. Zero-change contour thru middle of country. Globally, poles and tropical oceans get wetter. Subtropics get drier.

These projected changes in mean temperatures translate into large changes in occurrence of extreme temperatures. from ACPI Parallel-Climate model simulations,

Influence of a +3ºC warming on FREEZING SEASON & GROWING SEASON LENGTHS Using the historical ( ) record: Days longer Freezing seasons Growing seasons

Under even the coolest of these scenarios, with little change in precipitation, streamflow-timing trends in snowfed streams are projected to continue through the 21st Century. Stewart et al., 2004 Center-of-Mass Streamflow Timing

Observations of late 20th Century Hydroclimatic Change in the West Warmer winter & springs by ~ +2C Warmer winter & springs by ~ +2C More precipitation as rain More precipitation as rain Less snowpack by April 1 (except where Less snowpack by April 1 (except where there has been more El Nino precipitation) Earlier snowmelt and snowfed streamflow Earlier snowmelt and snowfed streamflow Earlier greenup and growing seasons Earlier greenup and growing seasonsProjections… Warming by +2 to +6ºC, perhaps more so at higher altitudes Warming by +2 to +6ºC, perhaps more so at higher altitudes Uncertain & small precipitation changes, maybe more--maybe less, Uncertain & small precipitation changes, maybe more--maybe less, except in North Significant changes in snowpack amounts, growing seasons, Significant changes in snowpack amounts, growing seasons, & soil moisture even if only temperature changes A couple of months change in freezing and growing seasonA couple of months change in freezing and growing season lengths would result from even modest +3ºC warming.

REFERENCES Cayan, D.R., Kammerdiener, S., Dettinger, M.D., Caprio, J.M., and Peterson, D.H., 2001, Changes in the onset of spring in the western United States: Bulletin, American Meteorological Society, 82, Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21 st Century California: San Francisco Estuary and Watershed Science, 3(1), Dettinger, M.D., and Cayan, D.R., 1995, Large-scale atmospheric forcing of recent trends toward early snowmelt in California: Journal of Climate 8(3), Dettinger, M.D., Cayan, D.R., Meyer, M.K., and Jeton, A.E., 2004, Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, : Climatic Change, 62, Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and Watershed. Climatic Change, 62, Meehl, G.A., Washington WM, Ammann C, Arblaster JM, Wigley TML, and Tebaldi C., 2004, Combinations of natural and anthropogenic forcings and 20th century climate. J. Climate. Mote, P.W., 2003, Trends in snow water equivalent in the Pacific Northwest and their climatic causes. Geophysical Research Letters, 30, DOI /2003GL Nijssen, B.N., R. Schnur and D.P. Lettenmaier, 2001, Global retrospective estimation of soil moisture using the VIC land surface model, , J. Clim. 14, Stewart, I., Cayan, D.R., and Dettinger, M.D., 2004, Changes in snowmelt runoff timing in western North America under a ‘Business as Usual’ climate change scenario: Climatic Change, 62, Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal of Climate 18,