Ancient trees, climate models, and the future of drought in western Colorado 38th Annual Colorado Water Workshop Western State Colorado University, Gunnison, CO July.
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2013 Ancient trees, climate models, and the future of drought in western Colorado 38th Annual Colorado Water Workshop Western State Colorado University, Gunnison, CO July 17, 2013 Jeff Lukas - [email protected] Western Water Assessment CIRES, University of Colorado Parsing our changing climate: a conceptual model + PAST PRESENT Internal (“natural, unforced”) climate variability PRESENT FUTURE External climate forcing (Greenhouse gases, etc.) change = PRESENT Changing future climate FUTURE Drought: driven by precipitation deficit, exacerbated by high temperatures Middle photo: Jane Stulp Need to look back and forwards to assess future drought risk Instrumental record Climate model projections Tree-ring record 1900 2013 Precipitation deficits and hydrological drought 24 22 20 18 16 14 12 10 8 6 Precipitation Streamflow 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Precip Data: NOAA NCDC, CO Division 2 data Streamflow data: Reclamation (1906-2010); 2011-12 values estimated from preliminary Reclamation data 20 18 16 14 12 10 8 6 4 2 0 Streamflow, MAF Precipitation, in. Western Colorado annual (water year) precipitation vs. Western Colorado naturalized runoff, 1900-2012 Western Colorado’s annual water balance – average year (MAF = million acre-feet) Precip: 40 MAF ET : -30 MAF Runoff: 10 MAF Image: D. Meko, U. of Arizona Western Colorado’s annual water balance – drought year (MAF = million acre-feet) 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 Image: D. Meko, U. of Arizona Using tree rings to reconstruct past droughts 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 1977 Douglas-fir, south San Juans, CO 1983 Ancient trees and wood are abundant across western Colorado Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005 Annual Flow, MAF 30 25 20 15 10 5 2002 0 750 1000 1250 1500 1750 2000 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 Instrumental record 18 17 Annual Flow, MAF 16 15 14 13 12 11 10 750 1000 1250 1500 Water Year 1750 2000 Tree-ring reconstructed annual flows, Colorado River at Lees Ferry, 762-2005, with 20-year running mean 18 17 15 14 13 12 11 25 10 20 750 1000 Annual flow, MAF Annual Flow, MAF 16 Mid-1100s megadrought 1250 1500 1750 2000 15 Water Year 10 5 46 of 57 years were dry 0 1120 1130 1140 1150 1160 1170 1180 Future projections from global climate models Jan. 2070 Temperatures – Going up! Western Colorado annual temperatures ensemble of 16 GCMs, medium emissions scenario 60°F +4ºF by 2050 50° F Observed temp. 1950 2000 2050 Source: Marty Hoerling, NOAA ESRL PSD; data available from http://gdo-dcp.ucllnl.org/ 2100 Precipitation – Up? Down? 32” Western Colorado annual precipitation – ensemble of 16 GCMs, medium emissions 16” Observed precip. 1950 2000 2050 2100 Source: Marty Hoerling, NOAA ESRL PSD Runoff – Probably down, but continued high variability! Annual Runoff, MAF 50 Upper Colorado River Basin annual runoff – ensemble of 16 GCMs, medium emissions 40 30 20 10 0 Observed runoff 1950 2000 Data: http://gis.usbr.gov/Streamflow_Projections/ 2050 2100 Western Colorado’s annual water balance – average year, median 2050 projection (+4oF, no precip change) Precip: 40 MAF ET : + Runoff: 9 MAF Image: D. Meko, U. of Arizona -31 MAF Even though precipitation doesn’t change, runoff declines ~10% to 9 MAF due to increasing ET Projections for Western Colorado from all GCM runs used in the most recent studies More runoff Less runoff Median 2050 Source: Denver Water & Joint Front Range Climate Change Vulnerability Study, as printed in Kerr, Science, 25 Nov 2011 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 190 0 200 0 1950 2000 2050 2100 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 ([email protected]) with questions or requests for information