Michael Dettinger US Geological Survey @ Scripps Institution of Oceanography, La Jolla, CA

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

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

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

4. 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)

5. 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

6. Lilac/Honeysuckle Phenology (departures from 1983-1994 average phenological dates) Cayan et al, 2001

7. 1999 2000 2001 2002 2003

8. PDO = -0.8 PDO = -1.0 PDO = -0.4 PDO = -0.4

9. 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

10. 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. 20 of 23 in this range 19 of 23 in this range cm/month cm/month Greenhouse forcings 1900-2100 Dettinger, 2005

11. 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.

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

13. Using the historical (1950-99) record: Influence of a +3ºC warming on FREEZING SEASON & GROWING SEASON LENGTHS Freezing seasons Days longer 0 80 Growing seasons 0 90 Days longer

14. 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. Center-of-Mass Streamflow Timing Stewart et al., 2004

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

16. 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, 399-415. Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21st Century California: San Francisco Estuary and Watershed Science, 3(1), http://repositories.cdlib.org/jmie/sfews/vol3/iss1/art4 . 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), 606-623. 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, 1900-2099: Climatic Change, 62, 283-317. Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and Watershed. Climatic Change, 62, 319-336. 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 10.1029/2003GL0172588. Nijssen, B.N., R. Schnur and D.P. Lettenmaier, 2001, Global retrospective estimation of soil moisture using the VIC land surface model, 1980-1993, J. Clim. 14, 1790-1808. 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, 217-232. Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal of Climate 18, 1136-1155.