And we can. The chart below shows the statewide average annual temperature since 1895, overlaid with a linear trend line. The increase is only about a degree, but it's quite clear.
(Click for larger chart)
Now, folks who don't believe in global warming will probably look at the chart above and say it doesn't prove anything. That the relatively small increase it shows could be due to the "heat island" effect, bad temperature stations, the sun, or perhaps even astrology, as the rather conservative South Dakota legislature seems to believe. But, with the exception of astrology, we have enough data to examine those possibilities. Climate models predict that human-caused temperature increases in Wisconsin should be seen mostly in the winter, but biased temperature measurements or changes in solar output should affect all four seasons more or less equally. As you can see from the next chart, almost all the warming in Wisconsin has indeed occurred during the winter, exactly as one would expect from anthropomorphic global warming.
(Click for larger chart)
While a one-degree increase in average yearly Wisconsin temperature over a hundred years isn't much, the three-degree winter increase, which has occurred mostly in the past thirty years, is a significant and noticeable change!
So, is this the final nail in the coffin of Wisconsin global warming deniers? Unfortunately, no. Their beliefs are based on ideology, not science or data. No amount of evidence will change their minds because they simply ignore or distort anything that doesn't fit their preconceived notions.
The data I used in the charts came from the Wisconsin State Climatology Office. It's a great resource for looking at local climate issues and concerns.
