Dan Cayan Scripps Institution of Oceanography, Climate Research Division and US Geological

1. Climate Change challenges looming for western water resouces Western States Water Council, Irvine, CA Oct 15, 2006 Dan Cayan Scripps Institution of Oceanography, Climate Research Division and US Geological Survey with input from Mike Dettinger Iris Stewart Noah Knowles sponsors: NOAA OGP RISA element California Energy Comm PIER program http://meteora.ucsd.edu/cap

2. What changes are evident from observations?

3. The result of the natural Greenhouse effect can be seen by comparing the temperature of the moon with that of the earth which are about the same distance from sun but moon is 33C (60F) cooler The average surface temperature of the moon is -18°C. The moon, of course, has no atmosphere. . By contrast, average surface temperature of the Earth is 15°C.

4. Humans have greatly Influenced the Atmosphere during the Industrial Era Source: IPCC TAR 2001 While these are relatively low concentrations, these trace gases are critical because they absorb infrared radiation and thus change the temperature of the atmosphere and the earth’s surface

5. Observations suggest that global temperatures have already risen at a extremely rapid pace. Northern Hemisphere temperatures Mann et al., 2000 1990’s warmest decade in instrumental record (NASA/NOAA) 1. 1998 warmest year 2. 2002 3. 2003 4. 2004 **2005 2nd (or 1st) warmest

6. Very broad winter and spring warming 1950-1997 California warmed 0.5-1.5C

7. To put these projected levels of of warming into perspective:SENSITIVITY OF HYDROCLIMATE TO A +3ºC WARMING… What fraction of each year’s precipitation historically fell on days with average temperatures just below freezing? YOSEMITE Less vulnerable More vulnerable “Rain vs Snow” In Yosemite, detailed simulations suggest: 30% historically --> 45% by 2100 Computed by Mike Dettinger from gridded historical US weather data (from Bates et al, in rev)

8. historically: “Cool” storms contribute immediate runoff from smaller areas of the river basin (the rest goes into snowpack for later) runoff In a warmer climate: Warm storms contribute immediate runoff from larger areas of the river basin runoff

9. Earlier spring flows last 2-3 decades center time “Center Timing” of snowmelt watersheds have advanced by 1-5 weeks earlier across West Iris Stewart

10. Downward Trends in April 1 Snow Water Equivalent 1950-1997 Source: Mote et al. (2004)

11. Snow losses (1950-2000) have been greatest in lower and middle elevation zones where warming is most effective Phil Mote, U. Washington

12. More Rain Less Snow WY 1949-2004 Winter (Nov-Mar) SFE/P trends at western US weather stations: symbol area is proportional to study-period changes, measured in standard deviations as indicated; circles indicate high trend significance (p<0.05), squares indicate lower trend significance (p>0.05). Noah Knowles et al. 2006 in press J. Climate

13. since 1985the number of large wildfires in western U.S. increased by 4X Anthony Westerling et al. Science August 2006

14. Large wildfire threat is aggravated by warmer springs and summers Anthony Westerling et al. Science August 2006

15. Springs with earlier snowmelt have produced summers with higher wildfire

16. What are recent climate model simulations telling us?

17. But, while future emissions will continue, their levels could be could be more or could be less…, and climate would respond accordinglyGlobal CO2 Emissions ScenariosIntergovernmental Panel on Climate Change (IPCC) Higher (A1fi) Previous (IS92a) Lower (B1) PNAS study Hayhoe et al Summer 2004

18. We face significant losses of spring snowpack • Less snow, more rain • Particularly at lower elevations • Earlier run-off • More floods • Less stored water By the end of the century California could lose half of its late spring snow pack due to climate warming. This simulation by Noah Knowles is guided by temperature changes from PCM’s Business-as-usual coupled climate simulation. (a middle of the road emissions scenario) better understanding of present day and future snow accumulation and melt processes is needed

19. 6 different climate models Strong concensus for warming But large uncertainty with precipitation

20. Projections of Precipitation Trends Precipitation generally projected to increase at high latitudes; decrease in subtropics; patchy & uncertain in between (e.g., thruout lower 48)

21. Observed SFO (left) and modeled Global (right). Sea level rise estimates based upon an envelope of output from several GHG emission scenarios Projected envelope of global s.l. rise observed Climate models Only provide loose guidance on The amount of sea level rise, but It is very likely that rates will increase

22. Seasonally intensified warming some models suggest amplified summer warming— does this bear out in land surface models? great implications for Ag!

23. Climate models project ocean warming by end of century of 1.5-2.C greater warming on land than oceans would amplifythermal gradient across California coast-interior In GCMs, Summer land-surface warming is accentuated—can we resolve this and understand changes in sea-land circulations and associated phenomena? GFDL CM2.1 Jun-Aug air temp change 2070-2099 minus 1961-1990

24. OBSERVATIONS AND MODELS INDICATE: Humans have altered atmospheric composition and thus are altering the earth’s climate; GH gases have long lifetimes, so choices made now and in future will determine future climate. Warming already underway and coming fast. Western temperature projections are broadly in consensus (+2 to +6ºC by 2100), Warming would produce more rain, less snow, earlier flows, more floods, higher sea level, & drier summers. California precipitation projections scattered, with MOST showing small (drier?) changes but a couple of wet outliers. “Shoulders” of watershed elevations at 6000-8000’ would generate more immediate runoff Better monitoring and modeling crucially needed.

26. The extinction of the cool part of the temperature distribution

27. Douglas Alden UCSD-SIO 2005

28. Ocean Beach , February 1983 Extreme storm-forced sea levels during an extreme tide

29. Projected total exceedances of San Francisco hourly sea level height (SLH) above historical 99.99 percentile (black), and number that are coincident with sea level pressure anomalies less than -7mb, Projected sea level from GFDL model weather and Nino3.4 SST with a linear trend of 30cm over 2000-2100.

30. Changes in Sierra Nevada Outflow, 2060 –2100 …and, in many settings, more severe winter floods… Knowles and Cayan, 2004; http://www.cgd.ucar.edu/cas/ACACIA/workshops/precip/dettinger.pdf

31. Historical Floods in Yosemite Winter warm storms have produced largest floods

32. Other major floods New Years 1997 Large dots: Major winter floods Open circles: Warm-wet storms that didn’t yield floods (mostly, soils too dry) NOTE: About half of the extremely warm-wet winter storms have yielded major floods. From http://www.cgd.ucar.edu/cas/ ACACIA/workshops/precip/dettinger.pdf

33. Simulated Merced River Flood Responses to Global Warming--using a climate model that has unusually (among climate models) large precipitation events in response to global warming ---> A whole “new” population of winter floods! Observed timing & size of floods, 84 yrs Simulated HISTORICAL timing & size of floods, 400 yrs Simulated FUTURE timing & size of floods, 400 yrs From Wilby & Dettinger, 2001

34. Flood Responses to Global Warming? Broad mid-altitude shoulders of the range are going to be critical! Warm storms put rain up onto extremely flat, broad mid-altitude shoulders. 6500-7500’ 1980-2300 m 7500-8500’ 2300-2600 m Hetch Hetchy Mono Lake Tioga Rd (120) Yosemite Valley You are here

35. Tenaya Lake Clouds Rest Half Dome Yosemite Valley 400,000 years of temperatures, carbon dioxide and methane Pushing a climate system well beyond its norms….. From Hansen, Climatic Change 2005, based on Petit, Nature 1999 (slide borrowed from Brad Udall, CIRES Western Water Assessment)

36. Global Energy Balance: The overall state of the global climate is determined by the balance between energy the Earth receives from the Sun and the energy which the Earth releases back to space. Absorption of long wave Terrestrial radiation by gasses in the atmosphere causes a greenhouse effect.

37. The Greenhouse Effect