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 SummitJanuary 17, 2013 – Breckenridge, CO
WWA is part of the NOAA RISA program network
• 11 regional, university-based research programs
• Help NOAA interface between climate science and society
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
Outline
1977 1983
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1250130013501400145015001550160016501700175018001850190019502000
Sum
mer
PD
SI
• 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?
What is drought?
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
Recent extreme drought: Three case studies
• Colorado River Basin (2000-2012+)
• Texas (2011)
• US Heartland (2012+)
How can we get more context for “unprecedented” events?
2002
0
100
200
300
400
500
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700
800
1915 1930 1945 1960 1975 1990 2005
An
nu
al
Flo
w
(10
00
acr e
-fe
et)
South Platte R., Colorado - annual streamflows
Paleoclimatology: analysis and reconstruction of pre-instrumental climate, mainly using environmental proxies
Lake sediments
Packrat middens
(vegetation) Tree rings(Dendrochronology)
Pollen
Ice coresCorals
Speleothems
Ocean sediments
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
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.
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
Moisture availability varies greatly from year to year
81012141618202224
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Annu
al p
reci
pita
tion,
in.
Annual precipitation, western Colorado
1977 1983
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
Annual growth often closely tracks annual moisture availability
Western CO August-July Precipitation vs. Pinyon ring width (WIL 731)
101214161820222426
1930 1940 1950 1960 1970 1980 1990 2000
Prec
ipita
tion
(in.)
00.10.20.30.40.50.60.70.8
Ring
Wid
th (m
m)
1977
1983
Same climate influences the growth of all trees at a site = cross-dating
1900 1910 1920 1930
Two Douglas-fir trees near Boulder, CO
1900 1910 1920 1930
Two Douglas-fir trees near Boulder, Colorado
1925
1925
Same climate influences the growth of all trees at a site = cross-dating
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)
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)
Annual growth (ring-width) is not the only tree-ring indicator of climate
• Stable isotopes of carbon (12C, 13C) reflect carbon assimilation and thus moisture status
• Stable isotope of oxygen (18O) reflects temperature of the source water taken up by the tree
• Density of latewood reflects summer warmth in energy-limited trees
• 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)
Compilation of the site chronology enhances the common (hydroclimatic) signal
Ring
-wid
th in
dex
Van Bibber site, near Golden, Colorado (ponderosa pine)
Robust averaging
Green Mountain Reservoir (GMR) Douglas-fir chronology (588-2005) (north of Breckenridge)
Living trees back to 1300s AD
Dead wood back to 500s AD
Moisture-sensitive tree-ring chronologies developed at CU from 2000 to 2009 (INSTAAR Dendrochronology Lab)
Three pinyon pine chronologies in SW Colorado
1900s
1800s
1700s
0.00.51.01.52.0
1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900
Tree
-rin
g in
dex
MCPNMVGVR
0.00.51.01.52.0
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Tree
-rin
g in
dex
MCPNMVGVR
0.00.51.01.52.0
1700 1710 1720 1730 1740 1750 1760 1770 1780 1790 1800
Tree
-rin
g in
dex
MCPNMVGVR
1900s
1800s
1700s
>1800 moisture-sensitive tree-ring chronologies across North America as of 2009
Figure: Cook et al. (2009), J. Quaternary Science
Generating tree-ring reconstructions
05
1015202530
1900 1915 1930 1945 1960 1975 1990 2005
annu
al fl
ow, M
AF
05
1015202530
1500 1600 1700 1800 1900 2000
Annu
al F
low
(MAF
)
Observed hydroclimate recordSubset of tree-ring chronologies
Best-fit statistical relationship between the tree rings and observations during overlap period
Tree-ring reconstruction of hydroclimate
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
1925 1935 1945 1955 1965 1975 1985 1995 2005
Annu
al fl
ow, a
cre-
feet
Case #1:
Colorado River Basin, 2000-2012
Lees Ferry, AZ
Breck
US Drought Monitor August 13, 2002
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
Observed (naturalized) annual flow, Colorado River at Lees Ferry, AZ since 1906
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5
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1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Wat
er Y
ear F
low
, MAF
Data: Reclamation (1906-2008); 2009-12 values estimated from preliminary Reclamation data
20022012
Record lowest streamflows in Upper Colorado Basin (since 1906):• 5-year period (2000-2004)• 10-year period (2000-2009)• 13-year period (2000-2012)
Figure and data: TreeFlow website, http://treeflow.info/upco/coloradoleesmeko.htmlReference: 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
R2 = 0.75
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750 1000 1250 1500 1750 2000
An
nu
al F
low
, MA
FTree-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.htmlReference: Meko et al. 2007. Medieval Drought in the Upper Colorado River Basin, Geophysical Research Letters
Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 10-year running mean
101112131415
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750 1000 1250 1500 1750 2000
An
nu
al F
low
, M
AF
2000-2009 mean observed flow
Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 20-year running mean
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Water Year
An
nu
al F
low
, M
AF
0
5
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20
25
1120 1130 1140 1150 1160 1170 1180
Ann
ual f
low
, MA
F
Mid-1100s megadrought 46 dry years out of 57
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
“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
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
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
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
August 2011: 9 of 10 Texas climate divisions <-4 PDSI
Tree-ring reconstructed and observed Texas summer PDSI, since 1900
-8-6-4-20246
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Sum
mer
PD
SI
ReconstructedObserved
R2 = 0.68
Observed Data: NOAA NCDC: http://www1.ncdc.noaa.gov/pub/data/cmb/sotc/drought/2012/13/uspctarea-wetdry-mod.txtPaleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.html
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1300 1400 1500 1600 1700 1800 1900 2000
Sum
mer
PD
SITree-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.txtPaleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.html
-2
-1
0
1
2
1300 1400 1500 1600 1700 1800 1900 2000
Sum
mer
PD
SITree-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.txtPaleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.html
• Late 1500s megadrought: 20 of 24 years with negative statewide PDSI
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
July 2012: ~60% of contiguous US <-2 PDSI
010203040
5060708090
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Perc
ent A
rea
<-2
PDSI
ObservedReconstructed
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
Paleo Data: Cook et al. 2008: http://www.ncdc.noaa.gov/paleo/pdsi.htmlPaleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch
R2 = 0.80
2012
DRIER 1934
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1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Perc
ent a
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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.htmlPaleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch
2012 observedDRIER
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.htmlPaleo Analysis: Eugene Wahl, NOAA NCDC Paleoclimatology Branch
2012 observed
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1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Perc
ent a
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PDSI
<-2
DRIER
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
• 20th century was generally wetter than previous centuries
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
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
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
Even if anthropogenic climate change were not occurring, we’d want to prepare for droughts worse than any modern (>1900) droughts
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Water Year
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AF
What can we say about drought risk, given the changing climate?
1950 2000 2050 2100
50°F
60°F
Colorado mean annual temperature - ensemble of 16 GCMs, medium emissions scenario
Observed temp.
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 Climatologisthttp://blog.chron.com/climateabyss/2011/09/texas-drought-and-global-warming/
Increasing temperatures will exacerbate any future drought, and push dry (low-precip) conditions into “drought” more often
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2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110
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ear F
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, 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
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)
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?
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
Recap
1977 1983
-8-6-4-20246
1250130013501400145015001550160016501700175018001850190019502000
Sum
mer
PD
SI
• 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
Links to resources in this presentation
http://wwa.colorado.edu/events/wxsummit
• Please contact me ([email protected]) if you have questions or want images or data
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