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Wanli Wu, Amanda H. Lynch, Sheldon Drobot, James Maslanik, A. David McGuire, and Ute Herzfeld

variability in terms of its seasonal cycle and spatial patterns. Overall, the RCM simulation exhibits relatively larger variations (2.90 hPa) than ERA-40 (2.63 hPa). Arctic climate has been characterized by substantial changes in recent decades. The SLP trends for each month from 1992 to 2001 are shown in Figure 7 based on the RCM simulation and the large-scale reanalysis. The trends in SLP have seasonal variation, and the trends simulated by the RCM are similar to the trends derived from the reanalyses

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Joy Clein, A. David McGuire, Eugenie S. Euskirchen, and Monika Calef

. 2007 ). In contrast, a recent remote sensing study has found that photosynthetic activity in the boreal forest of North America unaffected by fire has declined in the last two decades ( Goetz et al. 2005 ). While process-based biogeochemical models generally simulate a small net carbon sink in the recent past for the distribution of Arctic tundra ( Sitch et al. 2007 ), there is much spatial variability in estimated net carbon balance by these models (e.g., see McGuire et al. 2000a ; Euskirchen et

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A. D. McGuire, J. E. Walsh, J. S. Kimball, J. S. Clein, S. E. Euskirchen, S. Drobot, U. C. Herzfeld, J. Maslanik, R. B. Lammers, M. A. Rawlins, C. J. Vorosmarty, T. S. Rupp, W. Wu, and M. Calef

simulation of regional hydrology was sensitive to precipitation and water vapor differences among the driving datasets and that accurate simulation of regional water balance is limited by biases in the forcing data. Satellite-based analyses for the region indicate that vegetation productivity of the region increased during the last two decades of the twentieth century because of earlier spring thaw, and the temporal variability of vegetation productivity simulated by the models from 1980 to 2000 was

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J. S. Kimball, M. Zhao, A. D. McGuire, F. A. Heinsch, J. Clein, M. Calef, W. M. Jolly, S. Kang, S. E. Euskirchen, K. C. McDonald, and S. W. Running

, with comparatively small direct anthropogenic impacts to land cover and NPP. Plant biophysical processes in these regions are also strongly limited by seasonal frozen temperatures, while weather conditions show large interannual variability with an accelerated warming trend over the past several decades relative to the rest of the globe ( Serreze et al. 2000 ; Chapin et al. 2005 ). However, fire mortality and insect defoliations are major disturbances and drivers of forest succession in boreal

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J. S. Kimball, K. C. McDonald, and M. Zhao

SSM/I time series to determine the spatial pattern and annual variability of the primary springtime thaw event for Alaska and northwest Canada from 1988 to 2000. We also applied a biome-specific production efficiency model (PEM) driven by daily surface meteorological inputs from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis ( Kistler et al. 2001 ) and National Oceanic and Atmospheric Administration (NOAA) Advanced Very High

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T. Scott Rupp, Xi Chen, Mark Olson, and A. David McGuire

the effects of anthropogenic climate warming will occur first and most dramatically at high latitudes ( Houghton 2001 ; ACIA 2005 ). Surface air temperatures measured across high-latitude sites of western North America show an increase of approximately 0.3°C decade −1 over the past century ( Keyser et al. 2000 ) and show that current temperatures are the highest experienced in the last 400 yr ( Jacoby and D’Arrigo 1995 ; Jacoby et al. 1996 ; Overpeck et al. 1997 ). Precipitation has increased

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