Developing Record Temperature Ratio Indices for the United States and the Globe

Effective communication of climate indicators to the public requires using constructs that are intuitive. One such construct is a temperature record: the warmest or coldest condition experienced for a given temporal scale. Two major problems with diagnosing meaningful temperature records are (1) the need for a long period of record and (2) the need for the time series to be free from artificial shifts due to changes in observing practices. The recent development of high-quality grids of daily minimum temperature (tmin) and daily maximum temperature (tmax) over the contiguous U.S. (CONUS) provides an opportunity to create intuitive indicators of climate change based on record warm and record cold events. Here we describe the results from the development of (1) a U.S. Record Temperature Ratio Index based on tmax and tmin, and (2) a Global Record Temperature Ratio Index based on monthly mean tmax and mean tmin. Results are based on analyses of the period 1951-2017.

For the U.S. product, records are identified for various spatial scales from climate divisions to CONUS-wide. For example, the annual ratio of warm records to cold records from 1998-2017 has averaged about 4.4 to 1 across Michigan, about 4.1 to 1 across the Midwest National Climate Assessment Region, and 3.3 to 1 across CONUS. The most lopsided warm-records-to-cold-records ratios (17.3 to 1, 19.9 to 1, and 8.4 to 1, for Michigan, the Midwest, and the CONUS, respectively) occurred in 2012, which also ranks as the warmest year on record for the CONUS.

Looking at monthly temperature records from the gridded version of the NOAAGlobalTemp dataset from 1998-2017, we are not able to provide meaningful ratios on a monthly or annual basis because the prevalence of cold records is much smaller than the frequency of warm records. In fact, there were no cold monthly records for any grid cell in all of 2017. However, over the full 20-year period, there were 21.8 times as many warm records as there were cold records. The percentage of the globe experiencing extreme monthly temperatures has averaged about 6.8% for warm records in any given month from 1998-2017, whereas for cold events it has been only 0.3%.