<<

R EPORTS Fig. 4. Isobars for hydrogen de- 4. C. R. Reid, K. M. Thomas, J. Phys. Chem. B 105,10619 sorption from adsorbents AC 100 E (2001). (squares), E (diamonds), M (tri- C 5. J. L. C. Rowsell, A. R. Millward, K. S. Park, O. M. Yaghi, angles), and C (circles) at 1 bar M J. Am. Chem. Soc. 126, 5666 (2004). 6. G. Ferey et al., Chem. Commun. 2003, 2976 (2003). hydrogen pressure. 80 AC 7. D. N. Dybtsev, H. Chun, S.-H. Yoon, D. Kim, K. Kim, J. Am. Chem. Soc. 126, 32 (2004). 8. L. Pan et al., J. Am. Chem. Soc. 126, 1308 (2004). 9. O. M. Yaghi, H. Li, C. Davis, D. Richardson, T. L. Groy, 60 Acc. Chem. Res. 31, 474 (1998). 10. R. Kitaura, K. Seki, G. Akiyama, S. Kitagawa, Angew. Chem. Int. Ed. Engl. 42, 428 (2003). 40 11. M. Eddaoudi et al., Acc. Chem. Res. 34, 319 (2001). 12. S. L. James, Chem. Soc. Rev. 32, 276 (2003). 13. K. Endo et al., J. Am. Chem. Soc. 119, 4117 (1997). Amount Adsorbed (%) 20 14. M. Kondo, T. Yoshitomi, K. Seki, H. Matsuzaka, S. Kitagawa, Angew. Chem. Int. Ed. Engl. 36, 1725 (1997). 15. P. Losier, M. J. Zaworotko, Angew. Chem. Int. Ed. 0 Engl. 35, 2779 (1996). -200 -180 -160 -140 -120 -100 -80 16. K. N. Power, T. L. Hennigar, M. J. Zaworotko, New J. T (°C) Chem. 22, 177 (1998). 17. C. J. Kepert, M. J. Rosseinsky, Chem. Commun. 1999, 375 (1999). 18. A. J. Fletcher, E. J. Cussen, C. J. Kepert, M. J. Rosseinsky, K. M. Thomas, J. Am. Chem. Soc. 123, 10001 (2001). at low temperature compared with C,at- in turn are both close in size to H2 and have tributed to adsorption in the ultranano- sufficient flexibility due to framework dy- 19. E. J. Cussen, J. B. Claridge, M. J. Rosseinsky, C. J. Kepert, G J. Am. Chem. Soc. 124, 9574 (2002). porosity ( 0.5 nm). The isosteric enthalpy namics to allow kinetic trapping of the 20.D.Bradshaw,T.J.Prior,E.J.Cussen,J.B.Claridge, for H2 adsorption on AC at zero surface guest. The current materials are still cer- M. J. Rosseinsky, J. Am. Chem. Soc. 126, 6106 coverage is low, at 4.03(7) kJ mol–1 (tem- tainly not practical storage materials with (2004). - - 21. Materials and methods are available as supporting perature range –196 C to –159 C) (figs. S5 respect to U.S. Department of Energy guide- material on Science Online. to S6) (21). This result is entirely consist- lines (6 weight percent being a target for 22. K. S. W. Sing et al., Pure Appl. Chem. 57, 603 (1985). ent with desorption isobar characteristics 2010, versus È1.0 for our best material). 23. A. J. Fletcher, E. J. Cussen, D. Bradshaw, M. J. Rosseinsky, of H from AC decreasing rapidly with in- However, the design of porous MOF mate- K. M. Thomas, J. Am. Chem. Soc. 126, 9750 (2004). 2 24. This work was partly supported by the Carbon Trust, creasing temperature. rials to include thermally activated windows the Engineering and Physical Sciences Research Porous structures E and M have quite in the open channel structure offers the Council, and the Northwest Molecular Materials different isobar T dependence compared with possibility of modifying their desorption Centre. X-ray coordinates have been deposited with the Cambridge Crystallographic Data Centre AC and C. The lack of desorption of H2 up kinetics to improve their hydrogen storage (M.2CH OH: 248270; E.2C H OH: 114887; - 3 2 5 to about –160 C is consistent with the characteristics. C.3C3H6(OH)2: 225153; and C: 225154). isotherm hysteresis shown in Fig. 2. De- sorption from E and M is thus kinetically References and Notes Supporting Online Text www.sciencemag.org/cgi/content/full/1101982/DC1 limited at low T.AsT is increased, thermal 1. ‘‘Grand challenge for basic and applied research on Materials and Methods motion allows desorption of the trapped H hydrogen storage: statement of objectives,’’ available 2 Tables S1 and S2 at www.eere.energy.gov/hydrogenandfuelcells/docs/ guest to occur via window opening, which Figs. S1 to S9 - gc_h2_storage.doc. becomes noticeable at –160 C, with com- 2. G. Gundiah, A. Govindaraj, N. Rajalakshmi, K. S. plete desorption at about –80-C. The narrow Dhathathreyan, C. N. R. Rao, J. Mater. Sci. 85,209 25 June 2004; accepted 4 October 2004 window systems thus outperform the larger (2003). Published online 14 October 2004; 3. A. J. Fletcher, K. M. Thomas, Langmuir 16,6253 10.1126/science.1101982 pore materials AC and C at higher tempera- (2000). Include this information when citing this paper. tures because of kinetic trapping of H2. Hydrogen adsorption on widely differ- ent porous structures is very sensitive to T, consistent with thermodynamics for H2 ad- Long-Term Aridity Changes in the sorption on activated carbon Eenthalpy of adsorption È4 kJ mol–1 (figs. S3 to S6)^, driving desorption well below room temper- ature. Given the intrinsic difficulty in tuning Edward R. Cook,1* Connie A. Woodhouse,2 C. Mark Eakin,2 3 4 the carbon surface–H2 van der Waals inter- David M. Meko, David W. Stahle action to compete with thermal energies, the development of carbon-based adsorbents for The western United States is experiencing a severe multiyear drought that is

H2 storage is extremely challenging. There unprecedented in some hydroclimatic records. Using gridded drought is an intrinsic conflict between the large reconstructions that cover most of the western United States over the past pore volume required to enhance H2 storage 1200 years, we show that this drought pales in comparison to an earlier capacity and the resulting decrease in the period of elevated aridity and epic drought in AD 900 to 1300, an interval strength of the interaction in wider pores. broadly consistent with the Medieval Warm Period. If elevated aridity in the

The hysteresis in H2 uptake in porous MOF western United States is a natural response to climate warming, then any materials, where the pore window dimen- trend toward warmer temperatures in the future could lead to a serious long- sions are similar to the kinetic diameter of term increase in aridity over western .

H2, differs qualitatively from more rigid clas- sical sorbents. Hydrogen can be loaded under More than 50% of the coterminous United near-record precipitation deficits throughout high P andstoredatlowP if the cavities States experienced moderate to severe the western United States (1). Large portions are larger than the windows (Fig. 1D), which drought conditions in 2002, with record or of the Canadian Prairie provinces also suf-

www.sciencemag.org SCIENCE VOL 306 5 NOVEMBER 2004 1015 R EPORTS fered from severe drought, as well as ex- structions that extend back to AD 800. These United States (8, 9), but now with a much tensive areas of , particularly in the reconstructions were produced with a well- denser network of 602 centuries- to millennia- northern and western parts of the . In tested principal components regression pro- long tree-ring chronologies used as predic- many of these areas, the 2002 drought was cedure developed previously to robustly tors of PDSI (7). As before (8), a split cal- actually part of an ongoing drought that started reconstruct drought across the coterminous ibration and verification scheme was used to in late 1999 or before, with widespread drought conditions already persisting for È3 Fig. 1. The North years. Drought abated in many areas by late American summer 2002 to early 2003, but severe drought drought reconstruction conditions have continued to affect the in- grid and the (mostly) western U.S. , terior western United States throughout the circumscribed by the 2004 summer (2). thick black irregular This drought highlights both the extreme polygon. Out of the vulnerability of the semi-arid western United 286 total grid points, States to precipitation deficits and the need 103 are contained with- to better understand long-term drought var- in the Western region. Each of the 103 grid iability and its causes in North America. To points has an annu- this end, we have used centuries-long, an- ally resolved drought nually resolved tree-ring records to recon- reconstruction that struct annual changes in both drought and extends back to at wetness over large portions of North Amer- least AD 1380, with ica. The reconstructed drought metric is the 68 extending back to AD 800. These form summer-season Palmer Drought Severity In- the basis for our recon- dex (PDSI) (3), a widely used measure of struction of area af- relative drought and wetness over the United fected by drought in States (4) and other global land areas (5, 6). the West (7). New PDSI reconstructions have been pro- duced on a 286-point 2.5- by 2.5- regular grid Fig. 2. (A)Smoothed LONG-TERM CHANGES IN DROUGHT AREA IN THE WEST (Fig. 1) (7). This grid covers most of North DAI reconstruction 100 (solid black curve) for A America and is a substantial expansion of an THE CENTRAL DATES OF THE the West, showing 80 1150 earlier 155-point 2- by 3- drought reconstruc- 936 1034 1253 SIGNIFICANT (p<0.05) EPOCHS tion grid that covered only the coterminous two-tailed 95% boot- ARE INDICATED WITH ARROWS strap confidence inter- 60 United States (8). In addition, our drought vals (dashed black reconstructions are 600 to 1200 years long curves) and the long- over much of the U.S. portion of the North term mean (thin hor- 40 American grid (particularly in the western izontal black line). Sixty- DRIER United States), a substantial increase over the year smoothing was % DROUGHT AREA 20 È applied to highlight 300 years available from the previously the multidecadal to 0 1613 1829 published reconstructions, which all began in WETTER 1321 1915 centennial changes in

1700. Finally, the variance restoration we aridity. The four driest 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 apply to the grid point reconstructions (7) epochs (P G 0.05, those YEAR allows for updates of those records to AD with confidence limits 2003 with instrumental PDSI data. Together, above the long-term mean in Fig. 2A) are be- DROUGHT TREND: 1900 - 2003 these attributes enable us to compare the 100 fore AD 1300, whereas current western U.S. drought to those that the four wettest (P G B are reconstructed to have occurred as far back 80 CURRENT 0.05) epochs occur af- DROUGHT as AD 800, a time period that includes the so- ter that date. The dif- called Medieval Warm Period (MWP). ference between the 60 means of the AD 900 The region of interest here is contained 40 within an irregular black polygon on the to1300period(redline, 42.4%) and AD 1900 to

North American grid (Fig. 1), an area that 2003 period (blue line, % DROUGHT AREA 20 we henceforth refer to simply as the West. 30%) are also appar- Each of the 103 grid points in the West has a ent. The 12.4% differ- 0 1900 1920 1940 1960 1980 2000 summer PDSI reconstruction that covers the ence between the two YEAR periods translates into common interval AD 1380 to 1978, with a G minimum of 68 grid points having recon- an average drought area (PDSI –1) increase of 41.3% in the West during the earlier period. This difference is statistically significant (P G 0.001) given an equality-of-means t test with degrees of freedom corrected for first-order autocorrelation. Even so, some of the AD 900 to 1300 period 1Lamont-Doherty Observatory, Palisades, NY PDSI estimates are extrapolations, because they fall outside the range of the instrumental PDSI 10964, USA. 2National Oceanic and Atmospheric data in the AD 1928 to 1978 calibration period (7). As regression-based estimates, these Administration (NOAA), National Climatic Data extrapolations have greater uncertainty compared to those that fall within the range of the Center, Boulder, CO 80305, USA. 3Laboratory of calibration period. However, they are still based on the actual growth histories of highly drought- Tree-Ring Research, University of , Tucson, 4 sensitive trees. Therefore, we argue that our DAI reconstruction is indicative of what really AZ 85721, USA. Department of Geosciences, Uni- happened in the West, even during the AD 900 to 1300 period of elevated aridity (7). (B)The versity of Arkansas, Fayetteville, AR 72701, USA. annually resolved AD 1900 to 2003 portion, which more clearly reveals the severity of the *To whom correspondence should be addressed. current drought relative to others in the 20th century and an irregular trend (red smoothed E-mail: [email protected] curve) toward increasing aridity since 1900.

1016 5 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org R EPORTS develop and test the grid point regression point PDSI reconstruction to that of the area. Doing so emphasizes some earlier models, with time-varying subsets of tree- instrumental data (7). Doing so corrected for increases in aridity that dwarf the compara- ring predictors used this time at each grid temporal and spatial variations in explained tively short-duration current drought in the point to maximize the lengths of the PDSI PDSI variance (i.e., the regression coefficient West. Expressing the DAI reconstruction in reconstructions (7)(fig.S1).Testsofthetime- R2) over the drought grid, which provided DAI this filtered way does not minimize the effect varying subset approach (7) (fig. S3) indicate estimates based on the full amplitude of re- of the current multiyear drought (Fig. 2B). that this method produces reconstructions with constructed PDSI variability. Restoring the Rather, it emphasizes the fact that more highly significant hindcast skill, especially lost variance had no meaningful effect on the intense droughts of longer duration have when the reconstructions are examined as a relative changes in aridity presented here (7). occurred in the past and could occur in the Western regional average (10). Consequently, variance restoration mainly op- future. The four driest epochs (P G 0.05), At this spatial scale, it is possible to erates here as a bias correction that increases centered on AD 936, 1034, 1150, and 1253, critically examine the long-term history of the DAI to that which is presumably closer to all occurred during a È400-year interval of drought in the West over the past 1200 years. what really happened in the past. overall elevated aridity from AD 900 to 1300. For this purpose, we used the Drought Area After restoring the lost variance to the This time period is broadly consistent with the Index (DAI) (7). The DAI is a simple count PDSI reconstructions, we updated them from MWP, an epoch noted for both anomalous of the number of grid point reconstructions 1979 to 2003 with appended, gridded instru- warmth (11) and hydroclimatic variability (12) that exceed a given threshold of PDSI (e.g., mental data (7) (fig. S4). This allowed the in parts of the West. The DAI reconstruction PDSI G –1) in any given year, with the current drought to be put in its proper long- also reveals an abrupt change to persistently number of grid points exceeding the thresh- term perspective. We also conducted a Bfrozen less arid conditions after AD 1300 (13, 14) old divided by the total number of grid grid[ analysis of the DAI to determine if the that lasted for È600 years, up to about AD points to provide an estimate of percent area decline in available grid points from 103 in 1920. The four lowest aridity epochs (P G affected by drought. As such, it is a AD 1380 to 68 in AD 800 (7) (figs. S2 and S5) 0.05), which culminate with the early 20th nonparametric expression of the area affect- might have a negative effect on the fidelity of century Bpluvial[ (15), are found during this ed by drought over a given domain. Because the DAI reconstruction. The loss of spatial post–AD 1300 interval. any grid point can potentially contribute to information in the Western DAI record Since the turn of the 20th century, overall the total area affected by drought, the DAI before AD 1380 was found to be negligible. aridity in the West has increased in an also implicitly absorbs the natural spatial The Western DAI reconstruction (Fig. 2A) irregular manner. The high-resolution seg- variability of the expressed PDSI patterns is based on a PDSI G –1 threshold with 2- ment of the reconstruction (Fig. 2B) shows from year to year. tailed 95% bootstrap confidence limits (7). the classic drought of the 1930s Before calculating the Western DAI recon- The annual DAI estimates have been smoothed and the severe 1950s_ drought. Although the struction, we restored the lost variance due to with a 60-year low-pass filter to highlight current drought does not differ greatly from regression in each time-varying subset grid multidecadal or longer changes in drought those past episodes in terms of maximum annual percent area affected, its 4-year dura- Fig. 3. Nine indepen- RECORDS OF LONG-TERM ARIDITY CHANGES OVER THE WEST tion appears to be unusual over the past 104 dent records of long- years. Compared to the earlier Bmegadroughts[ term aridity changes 2 CHARCOAL, CA (18) that are reconstructed to have occurred in of the 0 around AD 936, 1034, 1150, and 1253, how- West (12, 16–23)com- -2 ever, the current drought does not stand out pared to the DAI re- construction. All of 2 SEQUOIA FIRE SCARS, CA (16) as an extreme event, because it has not yet the time series records 0 lasted nearly as long. This finding shows that have been smoothed -2 the West can experience far more severe to highlight multi- droughts than any found in the 20th century decadal to centennial 2 PYRAMID LAKE ∆δO18/∆t, NV (22) instrumental climate record, including the S variations and are E 0

T current one. A

expressed as standard I

V -2 With this in mind, it is important to validate normal deviates for E D the occurrence of our AD 900 to 1300 elevated comparison purposes. L BURNT KNOB LAKE CHARCOAL, ID (19) A 2

The yellow bars are M aridity period. There are numerous indepen- R 0 dated intervals of O dent indicators of past aridity that have been N -2 aridity. The break in D described from various subregions in the West. R

the yellow bar for the A

D 2 CYGNET LAKE CHARCOAL, WY (17) These indicators include in situ tree stumps in

‘‘lake bottom stumps’’ N A indicates two dry pe- T 0 lakebeds and river channels (in ) S (12), an above-average frequency of fire scars riods separated by a -2 wetter interval. The on trees (in California) (16), elevated char- shaded tan area high- 2 MOON LAKE SALINITY, ND (20) coal in lake sediments (in California, , lights the period of 0 and ) (17–19), increased lake salin- persistently elevated -2 ity (in ) (20), sand dune activa- aridity in the DAI re- cord over the AD 900 LAKE BOTTOM STUMPS, CA (12) tion (in ) (21), changes in lake to 1300 period. These 2 WEST DROUGHT (DAI<-1) sediment oxygen isotope ratios (in Nevada) independent records 0 (22), and lichen oxalate residue (in ) all show multidecadal -2 SAND DUNE ACTIVITY, NE (21) (23). Although these records vary in dating intervals of elevated INFERRED WEST TX ARIDITY (23) accuracy and resolution, they provide in- aridity during that -4 800 1000 1200 1400 1600 1800 2000 dependent direct or inferred evidence of ele- time, which collective- YEARS ly support our DAI re- vated aridity in the AD 900 to 1300 period construction based on long, drought-sensitive, tree-ring records. Numbers in parentheses are (Fig. 3), which collectively supports the more citations. temporally and spatially complete DAI evi-

www.sciencemag.org SCIENCE VOL 306 5 NOVEMBER 2004 1017 R EPORTS dence for our 400-year epoch of unprece- Additional support for this mechanism of tics, as measures of explained variance, are strongly dented drought severity and duration in the drought development in the West has recently dependent on the number of tree-ring chronologies available for PDSI reconstruction at each grid point West. emerged by radiatively forcing the Zebiak- (7) (fig. S3). When these statistics are calculated for Given this result and the effect of the Cane (Z-C) ENSO model with changing vol- the 103–grid point West regional average PDSI current drought on the West, it is critically canic forcing and solar irradiance over the past reconstruction, CRSQ, VRSQ, RE, and CE are 0.86, 0.73, 0.78, and 0.72, respectively, for the most highly important to understand how the AD 900 to 1000 years (31). The volcanic signal consists replicated post-1800 period of the reconstruction, 1300 megadrought epoch could have devel- of a series of impulsive radiative forcing based on a median of 41 tree-ring predictors per grid oped and persisted for so long. In so doing, events from explosive eruptions in equatorial point reconstruction, and 0.68, 0.54, 0.64, and 0.53, respectively, when based on the smallest subset of it might be possible to determine the like- , that affect tropical Pacific SSTs on tree-ring predictors available at the start of each grid lihood that we could return to a similar time the relatively short El NiDo–La NiDa time point reconstruction (a median of 2 per grid point) of more intense and prolonged aridity. The scale. In contrast, lower frequency solar (7) (fig. S3). Although there is indeed a decline in calibration and verification skill as the number of overall coincidence between our megadrought forcing results in a weaker, more equilibrated, tree-ring chronologies available for reconstruction epoch and the MWP suggests that anoma- SST response on multidecadal and longer time declines, the latter set of statistics still indicates lously warm climate conditions during that scales. highly significant hindcast skill. 11. V. C. LaMarche Jr., Science 183, 1043 (1974). time may have contributed to the develop- When the Z-C model is forced in this 12. S. Stine, Nature 369, 546 (1994). ment of more frequent and persistent droughts way, eastern tropical Pacific SSTs tend to- 13. Other high-resolution paleoclimate records from lakes in the West. Coupled -atmosphere model ward a cool, La NiDa–like base state during scattered across the northern prairies of North runs with elevated atmospheric greenhouse the model run_s early period (circa AD 1100 America also show abrupt moisture regime shifts around the AD 1300 period. gases also suggest that anomalous surface to 1250) of high solar irradiance and reduced 14. K. R. Laird et al., Proc. Natl. Acad. Sci. U.S.A. 100, warming over interior North America will volcanism. This result is supported by 2483 (2003). lead to increased summer dryness from ele- evidence of extreme drought in Peru during 15. F. K. Fye, D. W. Stahle, E. R. Cook, Bull. Am. Metereol. Soc. 84, 901 (2003). vated evapotranspiration demand and reduced medieval times (AD 800 to 1250) (32), an 16. T. W. Swetnam, Science 262, 885 (1993). soil moisture content (24). interval that is very similar to our AD 800 to 17. S. H. Millspaugh, C. Whitlock, P. J. Bartlein, Geology variability also modulates 1300 epoch of elevated aridity. Thus, the Z- 28, 211 (2000). 18. J. A. Mohr, C. Whitlock, C. N. Skinner, Holocene 10, moisture transport into the West on interan- C model results (31) provide a plausible 587 (2000). nual to multidecadal time scales (25). These physical mechanism for how protracted La 19. A. Brunelle, C. Whitlock, Quat. Res. 60, 307 (2003). teleconnected influences on drought and wet- NiDa–like conditions could have developed 20. K. R. Laird, S. C. Fritz, K. A. Maasch, B. F. Cumming, Nature 384, 552 (1996). ness have been linked to a variety of cli- during the MWP, a drought-inducing condi- 21. J. A. Mason, J. B. Swinehart, R. J. Goble, D. B. Loope, mate modes, including the El NiDo–Southern tion for both Peru (32) and the West (26). Holocene 14, 209 (2004). Oscillation (ENSO) and Pacific decadal vari- If the Z-C modeling results hold up, it is 22. L. Benson et al., Quat. Sci. Rev. 21, 659 (2002). 23. J. Russ, D. H. Loyd, T. W. Boutton, Quat. Int. 67,29 ability (e.g., the Pacific Decadal Oscilla- plausible that continued warming over the (2000). tion) (26–28), which can act together to tropical Pacific, whether natural or anthro- 24. R. T. Wetherald, S. Manabe, Clim. Change 43, 495 enhance drought development in the West. pogenically forced, will promote the devel- (1999). Decadal variability in the has opment of persistent drought-inducing La 25. A. Gershunov, T. P. Barnett, Bull. Am. Meteorol. Soc. 79, 2715 (1998). also been associated with large-scale U.S. NiDa–like conditions. Should this situation 26. J. E. Cole, J. T. Overpeck, E. R. Cook, Geophys. Res. droughts (28), but no causal mechanism for occur, especially in tandem with midconti- Lett. 29, 10.1029/2001GL013561 (2002). this has yet been presented, particularly west nental drying over North America, the epoch 27. M. Barlow, S. Nigam, E. H. Berbery, J. Clim. 14, 2105 (2001). of the Great . There is also evidence of unprecedented aridity revealed in the DAI 28. G. J. McCabe, M. A. Palecki, J. L. Betancourt, Proc. for a 20th-century trend in U.S. drought reconstruction might truly be a harbinger of Natl. Acad. Sci. U.S.A. 101, 4136 (2004). frequency being associated with increasing things to come in the West. 29. M. Hoerling, A. Kumar, Science 299, 691 (2003). 30. M. A. Cane et al., Science 275, 957 (1997). temperatures (28), 31. M. E. Mann, M. A. Cane, S. E. Zebiak, A. Clement, J. with anomalous late 20th-century warming References and Notes Clim., in press. of western Pacific and sur- 1. J. Lawrimore, S. Stephens, ‘‘Climate of 2002 annual 32. B. Rein, A. Lu¨ckge, F. Sirocko, Geophys. Res. Lett. 31, face temperatures (SSTs) also contributing review’’ (NOAA National Climatic Data Center, L17211, 10.1029/2004GL020161 (2004). available at http://lwf.ncdc.noaa.gov/oa/climate/ 33. We gratefully acknowledge the NOAA International since 1998 to the development of droughts research/2002/ann/events.html), 2003. Tree-Ring Data Bank (www.ncdc.noaa.gov/paleo/ over mid-latitude areas of the Northern Hemi- 2. D. Miskus, U.S. Drought Monitor (National Centers for treering.html) and its many contributors for much sphere (29). Thus, large-scale warming, such Environmental Prediction// of the tree-ring data used in this work. In addition, NOAA, Climate Prediction Center, available at www. many tree-ring records not yet in the public domain as what plausibly occurred during the MWP, drought.unl.edu/dm/monitor.html), 2004. were kindly contributed to our drought reconstruc- is again suggested as a contributor to the AD 3. W. C. Palmer, ‘‘Meteorological drought’’ (Res. Pap. tion project by a number of tree-ring scientists doing 900 to 1300 epoch of elevated aridity and No. 45, U.S. Department of Commerce Weather work in the United States, , and Mexico. All Bureau, , DC, 1965). are gratefully appreciated. We also thank M. A. Cane, epic drought in the West. 4. R. R. Heim Jr., Bull. Am. Meteorol. Soc. 83, 1149 (2002). R. Seager, M. E. Mann, and two anonymous reviewers Anomalous warming over the tropical 5. A. Dai, K. E. Trenberth, T. R. Karl, Geophys. Res. Lett., for helpful comments. Drought reconstructions dis- Pacific Ocean may also lead to increased 25, 3367 (1998). cussed in this paper can be found at www.ncdc. 6. A. Dai, K. E. Trenberth, T. Qian, J. Hydrometeorol., noaa.gov/paleo/pdsi.html. Supported by NOAA grant drought frequency in the West through the in press. no. NA06GP0450 and NSF grant no. ATM 03-22403. way that it promotes enhanced upwelling in 7. Materials and methods are available as supporting This is Lamont-Doherty Earth Observatory contribu- the eastern Pacific Bcold tongue[ region. material on Science Online. tion no. 6681. This Bjerknes-style dynamical response of 8. E. R. Cook, D. M. Meko, D. W. Stahle, M. K. Cleaveland, J. Clim. 12, 1145 (1999). Supporting Online Material the coupled ocean-atmosphere system to 9. Z. Zhang, M. E. Mann, E. R. Cook, Holocene 14, 502 www.sciencemag.org/cgi/content/full/1102586/DC1 (2004). exogenous heating of the tropical atmo- SOM Text 10. The calibration and verification statistics used to sphere has been argued on both observation- Figs. S1 to S5 assess the goodness-of-fit and validity of the PDSI References and Notes al and theoretical grounds (30). The resulting reconstructions are (i) the calibration period coeffi- enhanced upwelling promotes the develop- cient of multiple determination or regression R2 ment of La NiDa–like, cool SSTs over the (CRSQ), (ii) the verification period square of the 9 July 2004; accepted 17 September 2004 Pearson correlation coefficient or r2 (VRSQ), (iii) the Published online 7 October 2004; eastern tropical Pacific, a condition that is reduction of error (RE), and (iv) the coefficient of 10.1126/science.1102586 associated with drought in the West (26). efficiency (CE) (8). The magnitudes of these statis- Include this information when citing this paper.

1018 5 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org