First, let’s consider air temperature.
The Wisconsin Initiative for Climate Change Impacts, or WICCI, analyzed data from 1950 to 2006, a 57-year period. Wisconsin has warmed during this period.
The observed warming since 1950 has been greatest in winter, with an average increase of 2.5 degrees across Wisconsin. Winter temperatures in northwestern Wisconsin have increased by 3.5 to 4.5 degrees.
The WICCI report also showed that from 1950 to 2006, the frequency at which daily low temperatures have fallen below zero has diminished by about four to five days across southern Wisconsin and 14 to 20 days across northwestern and central Wisconsin.
A separate report that compared minimum temperatures averaged from 1961 to 1990 with the average minimum temperature during the period 1971 to 2000 demonstrated that the average minimum temperatures in winter have increased at nearly all locations in the continental U.S.
Given these observed warming temperatures, we might hypothesize that the ground would show a warming trend due to energy changes between the air and ground.
In addition to measuring air temperatures, there are locations that also measure ground temperature. One study explored ground temperatures for counties in Wisconsin.
The winter conditions in Wisconsin between 1948 and 2012 showed reductions in the length of the frozen ground season, defined as the start of frozen ground to end of frozen ground. There is a two- to three-week shortening of frozen ground conditions.
The logging industry is impacted by this. Frozen ground makes it easier for harvesting trees; a shorter frozen ground season could mean a muddier season making logging difficult as well as damaging forest roadways.