Karl et al. (1996) developed the Climate Extremes Index (CEI) to monitor observed climate variations over the contiguous United States. The components of this index use frequency-based statistics at each location and estimate the fraction of the US experiencing extreme values during a given period. The components of the CEI are related to the variations of extreme maximum temperatures, minimum temperatures, drought, heavy rainfall and the number of rain days. Extreme is used to mean either much above normal (above the ninetieth percentile) or much below normal (below the tenth percentile). For the component related to heavy rainfall, only the number of days with a much above normal proportion of precipitation due to heavy (much above normal) rainfall is considered. Problems within the definitions of the index are investigated and a modified version of the CEI is proposed.

An assessment of the modified version of the CEI has been undertaken over the twentieth century using observations from the United States Historical Climatology Network (USHCN). This has been compared with variations of the index using data from global climate model simulations for the twentieth century that include increasing concentrations of greenhouse gases and aerosols. Model data from the IPCC AR4 20C3M simulations with the NCAR PCM and CCSM3.0, GFDL CM2.0 and CM2.1, CCSR/NIES/FRCGC MIROC3.2, and CCCMA CGCM3.1 models have been used.

An increase in warm temperature extremes and a decrease in cold extremes are found using the modified CEI both in the observations and in the model simulations. Both trends in maximum and minimum temperatures are significant when compared with trends of the index calculated from pre-industrial control simulations. Precipitation trends within the index show a greater proportion of heavy rainfall events. Variations in the number of days with rainfall are difficult to separate from internal climate variability when compared with control model simulations. Trends of drought within the model simulations show large variability and no clear trend over the twentieth century. The combination of all the steps in the modified CEI shows a significant increase in warm extremes, a reduction in cold extremes, and to a lesser degree, an increase in heavy rainfall extremes over the twentieth century.