Climate Observations and Projections
Observations
Observations of temperature and precipitation (including snowfall) were derived from the most recent version of the U.S. Historical Climate Network database. We used the version of the HCN data set that contains monthly mean values of temperature and precipitation computed from a data set in which missing daily values were filled in using surrounding stations (Williams et al. 2005). We do not use the version that corrects for urban heat island effects. We present time series beginning as early as 1900 and ending as late 2003, depending on data availability. Click here to view these data. Linear, least-squares fits as well as running averages are presented with these data. We also present time-series of extreme temperature and precipitation computed from the same daily HCN data set that was used to compute monthly means. It also includes linear fits and running averages. View maps of linear trends in mean temperature and precipitation and number of days above 90°F. Mean annual cycles of temperature and precipitation based on data from NCDC’s 1971-2000 climate normals at the HCN stations can be viewed here.
Averages of temperature and precipitation by climate division were computed as follows. From the monthly values, we computed seasonal means for each year at each station. Seasonal means for each year were computed only if all months in the season had defined means. We then computed seasonal, 30-year means for the first three time periods noted above if, for a given season, at least 25 of the 30 years had seasonal means. Differences between 30-year periods at a given station and for a given season were computed only if there were defined values for each of the 30-year periods. These differences were then averaged by climate division to produce the maps. Click to view maps of temperature and precipitation for the 1971-2000 period, as well as differences with respect to this period.
Time series of temperature and precipitation averaged over the CARA region were made first by binning data into 2.5° × 3.5° grid cells, following NCDC’s recommendation for regional averaging, and then by averaging the cells together. This figure can be found here and a figure comparing the CARA to the contiguous U.S. (computed in the same manner) can be found here.
Climate Model Output
Climate model output was obtained from the online data archive associated with the IPCC (Intergovernmental Panel on Climate Change) 2000 assessment report (Cubasch et al., 2001). We used monthly 2-m temperature and surface precipitation output for the period 1911-2100 from seven models run under the A2 and B2 greenhouse gas emission scenarios. These scenarios were chosen because they were available for the greatest number of models and because they bracket the middle range of the scenarios used for the 2000 assessment. A2 and B2 correspond to atmospheric CO2 levels of about 850 and 620 ppm, respectively, in 2100, which can be compared to the preindustrial (prior to 1800) level of 280 ppm and present-day (2000) level of 370 ppm (Prentice et al., 2001). Global mean temperature increases for A2 and B2 with respect to 1990 are 3.8 and 2.7 °C, respectively, which are averages over all of the IPCC models (Cubasch et al., 2001). Click here for more details on the scenarios.
Following Hewitson (2003), we smoothed the model output to obtain a coarse resolution data set that more closely matches the skill resolution of the models. The data were then fit with a splined surface at 1/8° resolution for mapping and to obtain model output at the HCN stations in the CARA region. 30-year averages of these maps at seasonal resolution can be found for historical and projected climate. The 30-year periods begin with the years 1911, 1941, 1971, 2010, 2040 and 2070, and were chosen to be consistent with observed climatologies computed by the National Climatic Data Center and climatologies of model projections by the IPCC.
From the 1/8° maps, spatial averages were made over each of the CARA states and over major watersheds. For each of these subregions, and for each US HCN station, we computed seasonal and annual averages for every year in the climate model output archive between 1911 and 2100. From these seasonal and annual averages, means and standard deviations were computed for the 30-year periods discussed above. Figures displaying these data can be found for the states, watersheds, and HCN stations.
References
Cubasch, U., G. A. Meehl, G. J. Boer, R. J. Stouffer, M. Dix, A. Noda, C. A. Senior, Saper, and K. S. Yap, 2001. Chapter 9: Projections of future climate change. in J.T. Houghton et al. (eds.) Climate Change 2001: The Scientific Basis. Cambridge University Press, New York, NY, 525-582.
Hewitson, B. C., 2003, Developing perturbations for climate impact assessment, Eos, Trans. Amer. Geophys. Union, Vol. 84, No. 35, 337-348.
Prentice, I. C., G.D. Farquhar, M.J.R. Fasham, M.L. Goulden, M. Heimann, V.J. Jaramillo, H.S. Kheshgi, C. Le Quéré, R.J. Scholes and D.W.R. Wallace, 2001. Chapter 3. The Carbon Cycle and Atmospheric Carbon Dioxide. In: Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds., Cambridge University Press, New York, NY, 183-237.
Williams, C.N., Jr., M.J. Menne, R.S. Vose, and D.R. Easterling. 2005. United States Historical Climatology Network Monthly Temperature and Precipitation Data. ORNL/CDIAC-118, NDP-019. Available on-line [http://cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html] from the Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.