Scientific evidence for slow, ongoing, systematic warming of Earth’s atmosphere is unequivocal. This conclusion comes from evidence-based science and a physical understanding of that evidence. Regular scientific assessments of global and regional climate began in the 1970s. These assessments, along with a physical understanding of the atmosphere, show that the impact of human activities on this warming has evolved from theory to established fact. This is not a radical political statement; it is a firm conclusion based on the analysis of carefully considered observations.

Burning fossil fuels generates greenhouse gases, which are transparent to solar radiation but absorb large amounts of terrestrial infrared radiation that results in warming the atmosphere. The planet’s average surface temperature has risen about 2 degrees Fahrenheit since the late 1800s. Most of that warming has occurred in the past 40 years.
The warming trends are manifest in a number of ways. For example, the agricultural growing season can be defined as the time between a region’s last frost and the first frost. The growing season in the contiguous 48 states has increased by more than two weeks since the beginning of the 20th century. The Plant Hardiness Zone Map (PHZM) was developed by the U.S. Department of Agriculture and first published in 1960. The PHZM was recently revised and reflects the observed changes in our climate. In Wisconsin, some zones have shifted north and the cold zone (3b) no longer resides in Wisconsin. As noted in this column before, the areal extent of air colder than 23 degrees F one mile above the ground during Northern Hemisphere winter has also systematically decreased since at least 1948.
Global warming is occurring, and its impacts are increasingly difficult to overlook. At such a juncture, the least prudent move would be to halt research on the problem. And yet, because too many of them want to pretend there is no problem, our leaders in Washington D.C. are conspiring to do just that. History is an uncompromising judge and will not remember these leaders well.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.