Ancient trees, climate models, and the future of drought in western Colorado

Jeff Lukas - Lukas@colorado.edu

Western Water AssessmentCIRES, University of Colorado

38th Annual Colorado Water Workshop Western State Colorado University, Gunnison, COJuly 17, 2013

2013

Parsing our changing climate: a conceptual model

Internal (“natural, unforced”) climate variability

+External climate forcing (Greenhouse gases, etc.)

Changing future climate

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PAST PRESENTPRESENT FUTURE

PRESENT FUTURE

change

Drought: driven by precipitation deficit, exacerbated by high temperatures

Middle photo: Jane Stulp

Need to look back and forwards to assess future drought risk

1900 2013

Instrumental record

Climate model projections

Tree-ring record

Western Colorado annual (water year) precipitation vs. Western Colorado naturalized runoff, 1900-2012

Precip Data: NOAA NCDC, CO Division 2 data

Precipitation deficits and hydrological drought

Streamflow data: Reclamation (1906-2010); 2011-12 values estimated from preliminary Reclamation data

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02468101214161820

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amflo

w, M

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PrecipitationStreamflow

Western Colorado’s annual water balance – average year

Image: D. Meko, U. of Arizona

Precip: 40 MAF

ET : -30 MAF

Runoff: 10 MAF

(MAF = million acre-feet)

Western Colorado’s annual water balance – drought year

Image: D. Meko, U. of Arizona

Precip: 30 MAF

ET : -24 MAF

Runoff: 6 MAF

While precipitation declines only 25% vs. average year, runoff declines 40% due to ET component increasing as proportion of precipitation

(MAF = million acre-feet)

Using tree rings to reconstruct past droughts

Douglas-fir, south San Juans, CO

1977 1983

Tree growth is typically limited by moisture availability

So:– a dry year leads to a narrow growth ring– a wet year leads to a wide growth ring

Ancient trees and wood are abundant across western Colorado

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Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005

2002

1977, 2002: in the lowest 10 reconstructed annual flows since 762

Meko et al. 2007. Medieval Drought in the Upper Colorado River Basin, Geophysical Research Letters; data available at http://treeflow.info

Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005, with 20-year running mean

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Instrumental record

Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005, with 20-year running mean

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Mid-1100s megadrought

46 of 57 years were dry

Future projections from global climate models

Jan. 2070

Temperatures – Going up!

Source: Marty Hoerling, NOAA ESRL PSD; data available from http://gdo-dcp.ucllnl.org/

1950 2000 2050 2100

50°F

60°F

Western Colorado annual temperatures - ensemble of 16 GCMs, medium emissions scenario

Observed temp.

+4ºF by 2050

1950 2000 2050 2100

32”

16”

Western Colorado annual precipitation – ensemble of 16 GCMs, medium emissions

Precipitation – Up? Down?

Observed precip.

Source: Marty Hoerling, NOAA ESRL PSD

Runoff – Probably down, but continued high variability!

Observed runoff

Data: http://gis.usbr.gov/Streamflow_Projections/

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Upper Colorado River Basin annual runoff – ensemble of 16 GCMs, medium emissions

Western Colorado’s annual water balance – average year, median 2050 projection (+4oF, no precip change)

Image: D. Meko, U. of Arizona

Precip: 40 MAF

ET : -31 MAF

Runoff: 9 MAF

Even though precipitation doesn’t change, runoff declines ~10% to 9 MAF due to increasing ET

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Projections for Western Colorado from all GCM runs used in the most recent studies

Source: Denver Water & Joint Front Range Climate Change Vulnerability Study, as printed in Kerr, Science, 25 Nov 2011

More runoff

Less runoff

Median 2050

The “new normal” in a (yet) warmer climate

• Increased evaporation and transpiration• More rain and less snow in fall and spring• Reduced spring snowpack• Earlier peak runoff • Reduced annual flows (unless precip gains cancel out)• Reduced summer soil moisture• Greater plant moisture stress and crop water use

• These are all impacts currently associated with drought…they will occur more often in a warming climate

• Any future drought will be more severe, and will tend to persist longer, for a given precipitation deficit

Don’t fixate on trend: impacts will still be most deeply felt in low-precipitation (drought) years

Middle photo: Jane Stulp

Dust-on-snow also makes snow melt sooner, and probably reduces overall runoff

• Already has significant hydrologic impact; likely to get worse in future with warming/drying of source regions

• Impacts are in same direction as those caused by warming

Photos by Chris Landry, CSAS

Future drought: expectations from tree rings, instrumental record, and climate model projections

750 1000 1250 1500 1750 2000 1950 2000 2050 21001900 2000

Take-home

• Ancient trees tell us that the natural hydroclimatic variability is larger than the last 100 years would suggest (extreme short-term droughts & megadroughts)

• The instrumental record tells us precipitation drives drought, but temperature (ET) is also a big factor

• The climate models tell us that the temperature (ET) dial will continue to get turned up, but much more uncertain about precipitation

• Drought conditions will likely be more frequent and severe in the future

• Given the uncertainties in future climate, “scenario-based” planning is more appropriate than traditional “single-target” planning

• Please contact me (lukas@colorado.edu) with questions or requests for information