Paleo Perspectives on Extreme Climatic Variability
In the Mountains of Western North America
Stephen Gray1, Julio Betancourt1, Connie
Woodhouse2, Dave Meko3 and Lisa
Graumlich4
USGS Desert Laboratory
2NOAA Paleoclimatology Program
3Laboratory of Tree-Ring Research
Univ. of Arizona
4Big Sky Institute, Montana State Univ.
1
Western Drought:
1999-2005
Lake Powell Delta
June 02
March 03
Using the Tree-ring Record to
Understand Western US Drought
- Current events in the context of
natural variability
- Structure/geography of extreme
climatic events
- Ecological impacts
- Implications for management
- What about wet events?
Upper Colorado River Flow Reconstructions
Woodhouse, Meko and Gray (in prep)
Tree rings capture both interannual
and interdecadal variations in climate
Lee’s Ferry
Current Drought in a Long-term Context
Lee’s Ferry
p = 0.10
Average 1999-2004
20th Century Wetness in a Long-term Context
Lee’s Ferry
Average 1983-1987
p = 0.10
Upper Colorado: Frequency Characteristics of
Hydroclimatic Variability
Strong Decadal to Multidecadal Variability
Decadal to Multidecadal Variability
Lee’s Ferry
25 and 50 yr splines
Low-frequency Hydroclimatic Variability
Lee’s Ferry
25 and 50 yr splines
Non-stationary behavior
Ecological Impacts
Extreme droughts reset demographic clocks
& open niches for regeneration or new invasion
Late 16th Cent.
Megadrought
1976-1995 shift to warm wet
Living trees
Died in 1950s drought
Died in other years
Sevilleta LTER, NM
Pinyon pine
N > 3000 trees
Betancourt, Swetnam & Gottfried (2003)
Pinyon pine series for last 40 ka along transect ~31-41°N
Drought
regulates
expansion
over last
millennium
Betancourt & Holmgren in prep
Switching between dry/wet regimes drives
disturbance/succession cycles
Dry & Hot
Regimes
Disturbances
Broadscale
Mortality
Hydrologic
Effects??
Lots of
open niches
Increased
Synchrony
Rapid
Regeneration
Wet &
Cool Regimes
Switching between dry/wet regimes drives
disturbance/succession cycles
Dry & Hot
Regimes
Disturbances
Broadscale
Mortality
Increased
Synchrony
Hydrologic
Rapid Regeneration
Effects?? Successful Colonization
Lots of
open niches
Wet &
Cool
New
Species
Decadal to Multidecadal Regimes are Synchronous at
Regional to Sub-continental Scales
Pederson, Gray, Graumlich and Fagre (in review)
Regional Synchrony and Cross-scale Interactions
Ignition- single tree
Large Area: feedbacks and
non-linear interactions
Predictability?
Spread within patch
Spread among patches
Peters et al (2004) PNAS
Regional Synchrony and Cross-scale Interactions
Large Area: feedbacks and
non-linear interactions
Photos Tom Swetnam and Neil Cobb
Ecological Impacts
• Regime-like behavior is a defining
characteristic of western US climate
• Persistent wet/dry regimes are a
primary driver of ecological change
• Promotes regional (sub-continental)
synchrony
• Potential for cross-scale interactions
complicates forecasting
Management, Society and Natural Resources
UPCO Basin: Multidecadal Variability
Lee’s Ferry
50 yr spline
UPCO Basin: Multidecadal Variability
(Should this really be our reference period?)
Lee’s Ferry
50 yr spline
Population Growth: Colorado River Basin
CO, NV, NM, WY, UT and AZ
Includes California
3rd Wettest in 500 yr
4th (?) Wettest in 1000 yr
Implications for
Management and Society
• Decisions based on mean or short-term
conditions (e.g., 30 yr) can result in chronic
shortfalls
• Step-like changes in climate may be
misinterpreted as impacts of management
• Persistent climatic regimes solidify
institutional rules and policies
• But, regime shifts challenge our abilities to
adapt
Current Drought in a Long-term Context
Lee’s Ferry
p = 0.10
Average 1999-2004
Population Growth: Colorado River Basin
CO, NV, NM, WY, UT and AZ
The rules have
changed…
More people
More Demand
Longer, hotter growing seasons accelerate disturbance/
succession cycles and produce greater ecological synchrony
Dry & Hot
Disturbances
Broadscale
Mortality
Hydrologic
Effects??
Lots of
open niches
Increased
Synchrony
Rapid
Regeneration
Cool &
Wet
Effects of a longer, warmer growing season coupled
with the introduction of new species
Dry & Hot
Disturbances
Broadscale
Mortality
Hydrologic
Effects??
Lots of
open niches
Increased
Synchrony
Accelerated
Regeneration
Invasion by
Native &
Non-native
Species
Warmer &
Wet
Reconstructed July-July precipitation
Use of the tree-ring
record to understand
structure of droughts
NE Utah reconstruction
40
1578-1579
30
20
10
0
1550
1560
1570
1580
1590
Extended droughts
interspersed with
short wet episodes
1600
WGA’s Drought Response
Plan was shelved during
1997-1998 El Niño
Precipitation (in)
25
20
15
10
Southwestern U.S.
Precipitation
1997-1998
200405?
5
0
1990 1992 1994 1996 1998 2000 2002 2004
We tend to celebrate
too early, and let a little
rain spoil our resolve