Human-induced changes in the hydrological cycle of the western United States

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Human-induced changes in the hydrological cycle of the western United States. Tim Barnett 1 , David Pierce 1 , Hugo Hildalgo 1 , Tapash Das 1 , Celine Bonfils 2 , Ben Santer 2 , G. Bala 2 , Art Mirin 2 , Andy Wood 4 , Toru Nozawa 3 , Dan Cayan 1 , Mike Dettinger 1
Human-induced changes in the hydrological cycle of the western United States Tim Barnett1, David Pierce1, Hugo Hildalgo1, Tapash Das1, Celine Bonfils2, Ben Santer2, G. Bala2, Art Mirin2, Andy Wood4, Toru Nozawa3, Dan Cayan1, Mike Dettinger1 1Scripps Institution of Oceanography 2Lawrence Livermore National Lab 3National Inst. Environmental Sciences (Japan) 4Univ. Washington Changing hydrological cycle western United States The hydrological cycle is changing over the western United States
  • Examples of such changes are well documented:
  • Changes in snowfall & snow pack
  • e.g., Mote 2003; Mote et al. 2005; Knowles et al. 2006
  • Changes in streamflow
  • e.g., Cayan et al. 2001; Stewart et al. 2005; Maurer et al. 2007
  • Warmer air temperatures
  • e.g., Dettinger et al. 1995; Easterling 2002 WHY? Anthro causes? Natural variability? Solar / Volcanic forcing changes?
  • WHY? Detection and Attribution (D&A) United States
  • Detection: are the changes inconsistent with natural variability?
  • Attribution: are the changes consistent with anthropogenic (or other) forcing?
  • Generate a “fingerprint” that encapsulates changes expected (from model runs)
  • Match fingerprint in obs and forced models (with and without anthropogenic forcings)
  • Novel aspects United States
  • Multivariate Detection and Attribution (D&A) Analyze snowpack, river flow and air temperature simultaneously
  • Regional
  • Have to address problems of large amplitude natural variability
  • Related to the hydrological cycle
  • Rare in formal D&A work
  • People can immediately relate to it
  • Detection & Attribution: Overall scheme United States
  • Start with observations
  • SWE/P (1 April Snow Water Equv. / Oct-Mar precip)
  • Temperature (examined JFM daily minimum temperature)
  • River flow (examined JFM fraction and CT, center of timing)
  • Next, use global GCMs: control and anthropogenically forced runs
  • Downscale to region of interest
  • Run hydrological model w/ downscaled data for hydrologic variables (SWE/P, flow)
  • Detection and attribution analysis
  • Models and data United States
  • Control model GCM runs
  • 850 yrs CCSM3-FV (1.25Ox1O; finer resolution than T85)
  • 750 yrs PCM (T42)
  • Anthropogenically forced GCM runs, 1900-1999
  • PCM (4 members)
  • MIROC (10 members)
  • Regional statistical downscaling of GCM forcing
  • 2 methods, 12 km resolution, western US domain
  • VIC hydrological model (1/8 deg resolution)
  • Observations, 1950-1999
  • Snow courses for SWE
  • UW, Maurer, PRISM for T and P
  • Naturalized flow from Colorado R. (Lee’s Ferry), Columbia R. (Dalles), Sacramento and San Joaquin river
  • Climate Model Selection United States Data United States SWE/P TREND COMPONENT United States These time series are the basis for the fingerprint Model based Obs snow course P affecting SWE/P? United States Dividing by P removes majority of correlation between SWE and P Trend in P (blue) vs. SWE (red), 1950-1999 60% of stations show increasing P, but 71% show decreasing SWE The SWE/P fingerprint United States Trend in the leading EOF of 9 SWE/P timeseries (74%) D&A United States PDFs are assembed by resampling control climate runs S/N ratio is essentially the standard normal deviate of the fingerprint given control run statistics. SWE/P D&A United States Detection Time Scale United States Extension to Multivariate - Time series of key variables (obs.) All variables have been normalized (fractionalized) by dividing by the CCSM3-FV control run mean over first 300 yrs. Necessary for the multivariate detection and attribution (D&A), so have same variance in each variable (the “units problem”). Multivariate fingerprint: PCM (obs.) Multivariate D&A: PCM (obs.) Ensemble signal strength & significance (obs.) Fingerprint signal strength Significance Attribution: Why did it take so long? (obs.) Detection time D&A summary (obs.)
  • Natural variability cannot explain obs.
  • Solar/volcanic forcing cannot explain obs
  • Changes in precipitation cannot explain obs
  • ANTHROPOGENIC warming CAN explain obs. changes very well Q: WHY? ANS: It is ‘US’!
  • Conclusions (obs.)
  • Much previous work noting changes in snow cover, temperature, and river flow over the western U.S. -- but no formal D&A, nor multi-variate analysis
  • We have performed a formal multivariate detection and attribution analysis of SWE/P, JFM temperatures, and river flow
  • The changes in western hydrology over 1950-99 are largely human-induced; PCM captures 74% of low frequency signal
  • The PCM, run in forecast mode, shows a grim view of western U.S. water supplies within the next 30 years. If it worked so well over the last 50 years, we have good reason to believe these predictions
  • References (a few) (obs.) Hidalgo, H. G., M. D. Dettinger, and D. R. Cayan, 2007: Downscaling daily precipitation and temperature fields over the U.S. with constructed analogues. J. Clim., in review. Wood, A. W., L. R. Leung, V. Sridhar, and D. P. Lettenmaier, 2004: Hydrologic implications of dynamical and statistical approaches to downscaling climate model outputs. Climatic Change, 62, 189-216. Barnett, T. P., D. W. Pierce, H. G. Hidalgo, C. Bonfils, B. D. Santer, T. Das, G. Bala, A. W. Wood, T. Nazawa, A. Mirin, D. R. Cayan, M. D. Dettinger, 2008: Human-induced changes in the hydrology of the western United States. Science, published online 31 Jan 2008, doi:10.1126/science.1152538. Bonfils, C., B. D. Santer, D. W. Pierce, H. G. Hidalgo, G. Bala, T. Das, T. P. Barnett, C. Doutriaux, A. W. Wood, A. Mirin, T. Nozawa, 2008: Detection and attribution of temperature changes in the mountainous western United States. J. Climate, in review. Hidalgo, H. G., T. Das, M. D. Dettinger, D. R. Cayan, D. W. Pierce, T. P. Barnett, G. Bala, A. Mirin, A. W. Wood, C. Bonfils, B. D. Santer, and T. Nozawa, 2008: Detection and attribution of climate change in streamflow timing of the western United States. J. Climate,iIn review. Pierce D.W., T.P. Barnett, H.G. Hidalgo. T. Das, C. Bonfils, B. Sander, G. Bala, M. Dettinger, D. Cayan and A. Mirin. 2008. Attribution of declining western US snowpack to human effects. J Climate, in review. Do we have time to change directions?? (obs.) Where are we going? And why are we in this hand-basket? Roger Pulwarty
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