1. Micro-Rings and Megadroughts - What Can Tree Rings Tell Us About Recent Extreme Droughts? Jeff Lukas Western Water Assessment (WWA) CIRES, University of Colorado 2013 Weather and Climate Summit January 17, 2013 – Breckenridge, CO

2. WWA is part of the NOAA RISA program network 11 regional, university- based research programs Help NOAA interface between climate science and society

3. Acknowledgements Overall: Connie Woodhouse (U. of Arizona) Other Collaborators: Steve Gray (USGS); Dave Meko, Dan Griffin, Henry Adams (U. of Arizona); Balaji Rajagopalan (U. of Colorado); Brad Udall, Joe Barsugli, Klaus Wolter (WWA); Robin Webb (NOAA); Bob Thompson, Lesleigh Anderson (USGS); Ben Harding (AMEC); Jim Prairie, S. Gangopadhyay (USBR); Roger Kjelgren, Tammy Rittenour, Eric Allen (Utah State); Matt Bekker (BYU); Ed Cook (Columbia U.) Agency Partners: US Bureau of Reclamation, Denver Water, Colorado River District, Northern Colorado Water Conservancy District, Rio Grande Water Conservation District, California Dept. of Water Resources, City of Westminster (CO), NM Interstate Stream Commission, Salt River Project (AZ), and others Funding: NOAA Climate Program Office: Western Water Assessment and Climate Change Data and Detection (GC02-046); Denver Water; USGS; US Bureau of Reclamation

4. Outline 1977 1983 -8 -6 -4 -2 0 2 4 6 125013001350140014501500155016001650170017501800185019001950 2000 Summer PDSI How do tree rings record hydroclimatic variability? How do recent extreme droughts compare with these records of paleo-variability? What are megadroughts? How might drought risk change in the future?

5. What is drought?

6. Drought: driven by precip deficit, exacerbated by heat Modified from National Drought Mitigation Center, U. Nebraska Lower soil moisture Earlier peak flow, Reduced streamflows and reservoir inflows Precipitation deficit Higher temperatures, more sunshine, lower humidity Increased ET More plant stress, drier fuels Reduced snowpack, groundwater recharge

7. Recent extreme drought: Three case studies Colorado River Basin (2000-2012+) Texas (2011) US Heartland (2012+)

9. How can we get more context for “unprecedented” events? 2002 0 100 200 300 400 500 600 700 800 1915193019451960197519902005 A n n u a l F l o w ( 1 0 0 0 a c r e - f e e t ) South Platte R., Colorado - annual streamflows

10. Paleoclimatology: analysis and reconstruction of pre- instrumental climate, mainly using environmental proxies Lake sediments Packrat middens (vegetation) Tree rings (Dendrochronology) Pollen Ice cores Corals Speleothems Ocean sediments

11. Key attributes of tree rings as a paleo-proxy for climate and hydrology Annual resolution Absolute dating to calendar year Long, continuous records (200 to 10,000 yrs) Widespread distribution Straightforward translation into climate variables

12. The methods for using tree rings to examine past climate are not new Schulman, E. 1942. A tree-ring history of runoff of the Colorado River, 1366- 1941. Report to the Los Angeles Bureau of Power and Light. Douglass, A. E. 1909. "Weather Cycles in the Growth of Trees." Monthly Weather Review, 37.

13. Climate is typically the main factor limiting tree growth At the highest elevations and latitudes: energy availability (warmth) At lower elevations and mid- latitudes: moisture availability

14. Moisture availability varies greatly from year to year 8 10 12 14 16 18 20 22 24 190019101920193019401950196019701980199020002010 Annual precipitation, in. Annual precipitation, western Colorado

15. 19771983 Douglas-fir, south-central CO So, for most trees across the US: Dry conditions = Narrow ring Wet conditions = Wide ring This moisture signal integrates both precipitation and evapotranspiration

16. Annual growth often closely tracks annual moisture availability Western CO August-July Precipitation vs. Pinyon ring width (WIL 731) 10 12 14 16 18 20 22 24 26 19301940195019601970198019902000 Precipitation (in.) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ring Width (mm) 1977 1983

17. Same climate influences the growth of all trees at a site = cross-dating 1900191019201930 Two Douglas-fir trees near Boulder, CO

18. 1900 1910 1920 1930 Two Douglas-fir trees near Boulder, Colorado 1925 Same climate influences the growth of all trees at a site = cross-dating

19. Ponderosa pine near Pagosa Springs, Colorado Micro-ring: a really dry year 1900 1904 Image: Peter Brown, Rocky Mountain Tree-Ring Research (http://www.rmtrr.org)

20. This moisture signal in trees can serve as proxy for multiple moisture-related variables Annual or seasonal precipitation Spring snow-water equivalent (SWE) Annual (water-year) streamflow Drought indices (PDSI, SPI)

21. Annual growth (ring-width) is not the only tree-ring indicator of climate Stable isotopes of carbon ( 12 C, 13 C) reflect carbon assimilation and thus moisture status Stable isotope of oxygen ( 18 O) reflects temperature of the source water taken up by the tree Density of latewood reflects summer warmth in energy- limited trees

22. Dry sites, in nearly all cases Stands of old conifers (pine, Douglas-fir, juniper, etc.), or long-lived hardwoods like oaks Collect cores and cross sections from >15 trees (same species) Cross-date and measure their rings, compile into a site chronology Collecting moisture-sensitive tree-ring records (chronologies)

23. Compilation of the site chronology enhances the common (hydroclimatic) signal Ring-width index Van Bibber site, near Golden, Colorado (ponderosa pine) Robust averaging

24. Green Mountain Reservoir (GMR) Douglas-fir chronology (588-2005 ) (north of Breckenridge) Living trees back to 1300s AD Dead wood back to 500s AD

25. Moisture-sensitive tree-ring chronologies developed at CU from 2000 to 2009 (INSTAAR Dendrochronology Lab)

26. Three pinyon pine chronologies in SW Colorado 1900s 1800s 1700s 0.0 0.5 1.0 1.5 2.0 18001810182018301840185018601870188018901900 Tree-ring index MCP NMV GVR 0.0 0.5 1.0 1.5 2.0 19001910192019301940195019601970198019902000 Tree-ring index MCP NMV GVR 0.0 0.5 1.0 1.5 2.0 17001710172017301740175017601770178017901800 Tree-ring index MCP NMV GVR 1900s 1800s 1700s

27. >1800 moisture-sensitive tree-ring chronologies across North America as of 2009 Figure: Cook et al. (2009), J. Quaternary Science

28. Generating tree-ring reconstructions 0 5 10 15 20 25 30 19001915193019451960197519902005 annual flow, MAF 0 5 10 15 20 25 30 150016001700180019002000 Annual Flow (MAF) Observed hydroclimate record Subset of tree-ring chronologies Best-fit statistical relationship between the tree rings and observations during overlap period Tree-ring reconstruction of hydroclimate

29. Uncertainty in the reconstructions A reconstruction is a plausible estimate of past climate The distribution of model errors can be used to generate confidence intervals (gray bands, above) But this doesn’t capture uncertainties originating from subjective modeling choices 0 200,000 400,000 600,000 800,000 1,000,000 192519351945195519651975198519952005 Annual flow, acre-feet

30. Case #1: Colorado River Basin, 2000-2012 Lees Ferry, AZ Breck US Drought Monitor August 13, 2002

31. Colorado River Basin Drought (2000-2012+) Record lowest observed precipitation over Upper Colorado Basin (since 1895) 5-year period (2000-2004) 10-year period (2000-2009) 13-year period (2000-2012) US Drought Monitor August 13, 2002

32. Observed (naturalized) annual flow, Colorado River at Lees Ferry, AZ since 1906 0 5 10 15 20 25 30 190019101920193019401950196019701980199020002010 Water Year Flow, MAF Data: Reclamation (1906-2008); 2009-12 values estimated from preliminary Reclamation data 2002 2012 Record lowest streamflows in Upper Colorado Basin (since 1906): 5-year period (2000-2004) 10-year period (2000-2009) 13-year period (2000-2012)

33. Figure and data: TreeFlow website, http://treeflow.info/upco/coloradoleesmeko.htmlhttp://treeflow.info/upco/coloradoleesmeko.html Reference: Meko et al. 2007. Medieval Drought in the Upper Colorado River Basin, Geophysical Research Letters Tree-ring reconstructed and observed annual flow, Colorado River at Lees Ferry, AZ since 1906 R 2 = 0.75

34. 0 5 10 15 20 25 30 75010001250150017502000 Annual Flow, MAF Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, AZ, 762-2005 2002 reconstructed flow 2002: in the lowest 10 reconstructed annual flows since 762 Figure and data: TreeFlow website, http://treeflow.info/upco/coloradoleesmeko.htmlhttp://treeflow.info/upco/coloradoleesmeko.html Reference: Meko et al. 2007. Medieval Drought in the Upper Colorado River Basin, Geophysical Research Letters

35. Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 10-year running mean 2000-2009 mean observed flow

36. Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 20-year running mean 10 11 12 13 14 15 16 17 18 75010001250150017502000 Water Year Annual Flow, MAF Mid-1100s megadrought 46 dry years out of 57

37. Water managers around the US are using tree-ring reconstructions to assess and plan for future droughts Colorado Water Conservation Board* Denver Water City of Boulder, Colorado* New Mexico Interstate Stream Commission Salt River Project, Arizona California Department of Water Resources Wyoming Water Development Commission Utah Dept. of Water Resources U.S. Bureau of Reclamation - Lower Colorado Region* Oklahoma Water Resources Board Georgia Soil and Water Conservation Commission Suwanee and St. Johns Water Management Districts, Florida *in conjunction with climate change projections

38. “Direct Paleo” sequence based on Meko Lees Ferry reconstruction (1130-1182) Modeled Lake Powell (orange) and Lake Mead (green) year-end elevations Bureau of Reclamation: Powell & Mead Operations EIS, 2007 No power from Powell

39. TreeFlow web resource - western US paleo-streamflow http://treeflow.info Access to data for over 60 flow reconstructions Descriptions of applications Technical workshop presentations Resources and references Colorado River Streamflow: A Paleo Perspective

40. North America gridded summer Palmer Drought Index reconstructions – Ed Cook et al. (2004, 2008) <- 835 moisture- sensitive tree-ring chronologies Data: http://www.ncdc.noaa.gov/paleo/pdsi.html <- 286 point reconstructions on a 2.5-degree grid

41. Case #2: Texas Drought (2011) Record lowest observed statewide 12-month precipitation (1895- ) (Sep 2010-Aug 2011) Record lowest statewide summer (JJA) PDSI (1895- ) US Drought Monitor August 30, 2011

42. August 2011: 9 of 10 Texas climate divisions <-4 PDSI

43. Tree-ring reconstructed and observed Texas summer PDSI, since 1900 -8 -6 -4 -2 0 2 4 6 190019101920193019401950196019701980199020002010 Summer PDSI Reconstructed Observed R 2 = 0.68 Observed Data: NOAA NCDC: http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txthttp://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txt Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html

44. -8 -6 -4 -2 0 2 4 6 13001400150016001700180019002000 Summer PDSI Tree-ring reconstructed Texas summer PDSI since 1250 2012 observed Observed Data: NOAA NCDC: http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txthttp://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txt Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html

45. -2 0 1 2 13001400150016001700180019002000 Summer PDSI Tree-ring reconstructed statewide Texas summer PDSI since 1250 with 20-year running mean Observed Data: NOAA NCDC: http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txthttp://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txt Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html Late 1500s megadrought: 20 of 24 years with negative statewide PDSI

46. Case #3: US Heartland Drought (2012-) Record lowest summer (JJA) precipitation in Wyoming and Nebraska (1895- ) Highest % area of contiguous US in moderate or worse summer drought (PDSI) since 1934 US Drought Monitor: highest % area of contiguous US in drought (1999 - ) July - Warmest month on record for contiguous US (1895-) 2012 – Warmest year on record for contiguous US US Drought Monitor July 24, 2012

47. July 2012: ~60% of contiguous US <-2 PDSI

48. 0 10 20 30 40 50 60 70 80 90 190019101920193019401950196019701980199020002010 Percent Area <-2 PDSI Observed Reconstructed Tree-ring reconstructed and observed percent area of contiguous US with summer PDSI <-2 since 1900 Observed Data: NOAA NCDC: http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txt http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txt Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html Paleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch R 2 = 0.80 2012 DRIER 1934

49. 0 10 20 30 40 50 60 70 80 10001100120013001400150016001700180019002000 Percent area PDSI <-2 Tree-ring reconstructed percent area of contiguous US with PDSI <-2 since 1000 AD Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html Paleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch 2012 observed DRIER

50. Tree-ring reconstructed percent area of contiguous US with PDSI <-2 since 1000 AD, 20-year running mean Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlhttp://www.ncdc.noaa.gov/paleo/pdsi.html Paleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch 2012 observed DRIER

51. General messages from the three case droughts in the context of the tree-ring record The one-year events of the last decade are within the bounds of paleo-natural variability—but among the most severe of the last millennium Longer recent droughts tend to pale in comparison to the paleo-reconstructed extended droughts, including the 1100s and 1500s megadroughts 20 th century was generally wetter than previous centuries

52. What conditions lead to megadroughts? Severe and extensive N.A. droughts since 1850 are strongly associated with persistent cool conditions in E. tropical Pacific (~La Niña) Limited evidence suggests cooler E tropical Pacific during the Medieval Climate Anomaly (c. 900-1400) MCA was relatively warm, globally and regionally Mean SST anomalies (blue-orange) and land precipitation anomalies (brown-green) from 1948-1957

53. 1130-1170: 37 of 41 years with negative PDSI across western US, including 23 years in a row Figure: Herwiejer et al. 2007, J. Climate Tree-ring reconstructed Medieval megadroughts (Cook et al. PDSI) 1021-1051: 27 of 31 years with negative PDSI across western US 1240-1265: 21 of 25 years with negative PDSI across western US

54. Figure: Kleppe et al. 2011, Quaternary Science Reviews Independent paleoevidence of Medieval megadroughts in western US Upright trees rooted at the bottom of E. Sierra lakes indicate low stands >200 years long Widespread dune activation in western Great Plains Increased frequency of fire, seen in tree fire-scar and charcoal records across West Greater salinity in Pacific coast estuaries Medieval-era trees in Fallen Leaf Lake, Tahoe Basin, California

55. Even if anthropogenic climate change were not occurring, we’d want to prepare for droughts worse than any modern (>1900) droughts 10 11 12 13 14 15 16 17 18 75010001250150017502000 Water Year Annual Flow, MAF

56. What can we say about drought risk, given the changing climate? 1950200020502100 50°F 60°F Colorado mean annual temperature - ensemble of 16 GCMs, medium emissions scenario Observed temp.

57. It is likely that anthropogenic warming to date has worsened the impacts/indicators of recent US droughts 0.5 -1.0°F additional anthropogenic heating? 2011 Lower Soil moisture PDSI Fuel moisture Streamflows Analysis: John Nielsen-Gammon, Texas A&M, TX State Climatologist http://blog.chron.com/climateabyss/2011/09/texas-drought-and-global-warming/

58. Increasing temperatures will exacerbate any future drought, and push dry (low-precip) conditions into “drought” more often 0 5 10 15 20 25 30 200020102020203020402050206020702080209021002110 Water Year Flow, MAF Colorado River at Lees Ferry observed flows (gray), and future warming- forced decrease consistent with mid-range of recent modeling studies (red) -10% -20% -30% Percentage declines in flow based on ensemble mean from projections in Colorado River Basin Water Supply & Demand Study, US Bureau of Reclamation, 2012

59. Broad-scale precipitation patterns are likely to change too (drier south, wetter north) Projected seasonal precipitation, 2080-2099, relative to 1961- 1979, Mean of 15 GCMs (Karl et al. 2009)

60. Anthropogenic climate change will also likely alter the modes of climate variability (e.g., ENSO) that drive long- term US droughts However, the direction of future change is very uncertain (More La Niña? More El Niño?) GCMs do not capture well the observed persistence of tropical Pacific conditions Is the Medieval period a potential analog for the future (warmer) climate?

61. Drought under anthropogenic climate change Modified from National Drought Mitigation Center, U. Nebraska Lower soil moisture Earlier peak flow, Reduced streamflows and reservoir inflows Precipitation deficit Higher temperatures, more sunshine, lower humidity Increased ET More plant stress, drier fuels Reduced snowpack, groundwater recharge Climate change

62. Recap 1977 1983 -8 -6 -4 -2 0 2 4 6 125013001350140014501500155016001650170017501800185019001950 2000 Summer PDSI Tree rings robustly capture past hydroclimatic variability Recent extreme droughts are still within the bounds of the paleo-variability Persistent “megadroughts” such as in the mid-1100s have no modern analog The future will bring generally increased drought risk for the US, if not megadrought risk

63. Links to resources in this presentation http://wwa.colorado.edu/events/wxsummit

64. Please contact me (lukas@colorado.edu) if you have questions or want images or data