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Long-Term Aridity Changes in the Western United States

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Long-Term Aridity Changes in the Western United States 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)^, Western United States 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 North America. 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 Mexico, particularly in the reconstructions were produced with a well- denser network of 602 centuries- to millennia- northern and western parts of the country. 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. region, 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).
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