Effect of Baseline Period on Quantification of Climate Extremes Over the United States

Extreme climate events are societally harmful and have increased in frequency and intensity in recent decades. Indices based on temperature and precipitation are a valuable way to quantify climate extremes. Certain indices are defined relative to percentiles, which are dependent on a climatological baseline period. In this study, indices computed using temperature and precipitation from the Modern Era Retrospective Analysis for Research and Applications, Version 2 are calculated using percentiles from three baseline periods: 1981-2010, 1991-2020 and 1981-2020. Updating the baseline period from 1981 to 2010 to 1991-2020 leads to significant changes in the quantification of temperature and precipitation extremes over the United States over 1980-2021. Using the later baseline period indicates more cold extremes, fewer warm extremes, and fewer but more intense precipitation extremes throughout the US, with regional variation. Changing the baseline period can mislead the public and decision makers, potentially undermining the appropriate response to climate-related health risks. Indices computed using 2-m air temperature and precipitation are used to represent extreme climate events such as heat waves, cold waves, heavy precipitation, and drought. Some indices are defined relative to percentile-based thresholds, which are computed using a baseline climatology period. The baseline climatology is typically a thirty-year period and is updated every ten years. This study examines how updating the baseline climatology period from 1981-2010 to 1991-2020 affects the quantification of climate extremes in the United States over 1980-2021. In general, since the 1991-2020 period is warmer than 1981-2010 throughout the United States, there are fewer warm extremes detected and more cold extremes detected when it is used as the baseline. The differences are most notable in the southwest and northeast United States. The changes in the precipitation indices vary throughout the country, but in certain parts of the southern and central United States, updating the baseline period leads to the detection of fewer but heavier extreme precipitation events. It is important to communicate the choice of baseline climatology period to prevent misinterpretation of the extreme climate indices and the comparison of different studies. Updating the baseline period from 1981-2010 to 1991-2020 leads to significant changes in percentile-based extreme climate indices in the USTemperature indices show generally increased cold extremes and decreased warm extremes across the US when the baseline period is updatedFor precipitation indices, the later baseline period indicates fewer but more intense extreme events in the south and central US