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

-8-6-4-20246

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

600

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

0

5

10

15

20

25

30

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

0

5

10

15

20

25

30

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

1617181920

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

10

11

12

13

14

15

16

17

18

750 1000 1250 1500 1750 2000

Water Year

An

nu

al F

low

, M

AF

0

5

10

15

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

-8-6-4-20246

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

0

10

20

30

40

50

60

70

80

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Perc

ent a

rea

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.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

05

101520253035

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Perc

ent a

rea

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

10

11

12

13

14

15

16

17

18

750 1000 1250 1500 1750 2000

Water Year

An

nu

al F

low

, M

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

0

5

10

15

20

25

30

2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110

Wat

er Y

ear F

low

, 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 (lukas@colorado.edu) if you have questions or want images or data