SCIPP will build on existing partnerships with Gulf Coast water utilities, the USGS National Wetland and Aquatic Research Center, and Louisiana Sea Grant to quantify how extreme low temperatures and freeze days have changed since the early 1950s. A manuscript titled “Tropicalization of the Temperate Zone: Warming Winter Temperatures and Fewer Freeze Days across the Conterminous United States” has been submitted for review to Environmental Research: Climate with the following abstract:
“Here, we investigated changes in cool season and winter temperatures, along with the occurrence of freeze days, between 1952–2024 across the conterminous United States (CONUS). Many of our analyses span the CONUS, with emphasis placed on the tropical-temperate transition zone (TTTz) in the southeastern U.S. Since 1952, daily Tmin has increased at a higher rate and over a larger portion of the CONUS than daily maximum temperature (Tmax), particularly during winter. Annual (winter) Tmax significantly increased across 79% (53%) of the land area of the CONUS, while Tmin increased over 99% (70%). During winter, ~70% of the land area of CONUS observed Tmin warming rates that exceeded Tmax. The countywide average coldest Tmin during the cool season became milder across 57% of the CONUS, while the coldest Tmax showed little change and even cooled east of the Rocky Mountains in the central U.S. The timing of occurrence in the coldest Tmin and coldest Tmax, assessed from the single date of each year’s event, displayed no substantial change. Roughly 85% of the CONUS experienced a significant decline in cool-season freeze days, with the largest relative declines in regions where winter average Tmins are above freezing. An analysis of unique freeze day isopleths (30, 45, 60, and 75 days) across twenty-year periods shows that the average latitude of freeze days has migrated poleward substantially. Between 101°W and 79°W in the TTTz, the 30 freeze-day isopleth for the late period (2005–2024) was, on average, 122 km farther north than in the early period (1952–1971). Generally, the largest latitudinal shifts in freeze-day isopleths between early and late periods occurred in regions near the freezing threshold with low topographic relief, where freeze days are both climatologically marginal and often physiographically unprotected (e.g., the Mississippi River Basin).”
Following the manuscript, we created a tool (accessible at https://cmintemp.scipp.lsu.edu/) to make the results, data, and time series available to our stakeholders.