Average annual temperature has warmed about 2°F since the beginning of the 20th century. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century. Extreme heat is of particular concern for densely populated urban areas like Milwaukee, where high temperatures and high humidity can cause dangerous conditions.
Wisconsin has experienced an increase in both annual precipitation and heavy rain events, trends which are projected to continue. Projected increases in winter and spring precipitation will pose a continuing risk of spring planting delays, as well as an increased risk of flooding. Snowfall is projected to decline due to warmer temperatures.
Severe drought, a natural part of the Wisconsin climate, is a risk to this agriculture-dependent state. Increased rate of soil moisture depletion during dry spells due to higher temperatures, along with earlier snowmelt and a greater frequency of dry days, may increase the intensity of future naturally-occurring droughts.
Changes in seasonal and multi-year precipitation, evaporation, and temperature can affect water levels in the Great Lakes, causing serious environmental and socioeconomic impacts.
Wisconsin is located in the interior of North America, exposing it to a climate with large ranges in temperature. The southern part of the state experiences cold winters and mild to hot summers, while the north experiences frigid winters and generally cool summers with brief bouts of excessive heat. The lack of mountains to the north or south allows for incursions of bitterly cold air masses from the Arctic, as well as warm and humid air masses from the Gulf of Mexico, further increasing the range of conditions that can affect the state. The winter season is dominated by dry and cold air with occasional intrusions of milder air from the west and south. The summer is characterized by frequent warm air masses, either hot and dry continental air masses from the arid west and southwest, or warm and moist air from the south. However, periodic intrusions of cooler air from Canada provide breaks from summer heat. The state has borders along Lake Superior to the north and Lake Michigan to the east, and the proximity to the lakes provides a moderating effect on temperature for locations along the shorelines. Average annual temperatures vary from 39°F in the north to 50°F in the south.
Since the beginning of the 20th century, temperatures have risen approximately 2°F and temperatures in the 2000s have been warmer than any other historical period (Figure 1). The period of 2000-2004 was the hottest 5-year period in history. 2012 was the hottest year on record, with a statewide average temperature of 47.4°F, 5°F above the long-term average. This warming has been concentrated in the winter and spring, while summers have warmed less; this is a feature characteristic of much of the Midwest (Figure 2a). Warmer temperatures occurring earlier in the spring presents the additional threat of frost-freeze damage to early budding of fruit trees. In 2012 a “killer frost” closely followed an abnormally warm March resulting in significant damage to fruit crops. The lack of summer warming is reflected in a below average occurrence of very hot days (days with maximum temperature above 95°F) (Figure 2b) and no overall trend in warm nights (days with minimum temperature above 70°F) (Figure 2c). The winter warming trend is reflected in a below average number of very cold days (days with maximum temperature less than 0°F) since 2000 (Figure 3). The increase in winter temperatures has caused reduced lake ice cover. In the Great Lakes, ice coverage in the Great Lakes has been declining since the 1970s. For example, the average annual maximum ice coverage from 2003–2013 was less than 43%, compared to the 1962–2013 average of 52%. Ice cover duration on Lake Mendota has exhibited a consistent downward trend since the late 19th century (Figure 4).
Precipitation varies widely from year to year (Figure 5), and most of the state’s precipitation falls during the warmer half of the year. Statewide annual precipitation has ranged from a low of 20.53 inches in 1910 to a high of 41.28 inches in 1938. Recently, Wisconsin has experienced some unusually wet years; 2010 was the second wettest year on record (39.02 inches), and 2014 was the seventh wettest (37.07 inches). The driest multi-year periods were in the 1890s, 1930s, and mid-1950s, and the wettest in the 1990s and 2000s (Figure 5). The driest 5-yr period was 1929–1933 and the wettest was 1982–1986. Both winter and summer precipitation have been mostly above average over the last 20 years (Figure 2d). The frequency of heavy rain events has increased, with the highest number of 2-inch rain events occurring during the period of 2010–2014 (Figure 6). Snowfall varies from about 30 inches annually in the south to over 100 inches along the Gogebic Range. The heavy snowfall along the Gogebic Range is partially due to lake effect snow events along the south shore of Lake Superior. The shoreline of Lake Superior has experienced significant upward trends in annual snowfall totals. These upward trends are attributed to warmer air temperatures, which results in more moisture availability due to warmer surface water temperatures and reduced lake ice coverage. Annual snowfall totals have also increased over the rest of Wisconsin since 1930.
Wisconsin is susceptible to both groundwater flooding, and river flooding, both from ice jams and heavy precipitation, in the many rivers bordering and running through the state. Heavy rain and snow during fall and winter 2007–2008 led to elevated water tables by summer 2008. The elevated water table, combined with enhanced summer precipitation, caused flooding to persist for 6 months, causing approximately $17 mmillion in agricultural and property damage from groundwater flooding alone. In addition, over June 5–12, 2008, a series of storms caused heavy rain to fall across southern Wisconsin, with multiple stations reporting over 10 inches of rain. The rain caused severe flooding, totaling more than $1.2 billion in damages.
Severe winter storms are a regular occurrence due to the state’s northerly location and proximity to the winter storm track. Over February 1–2, 2011, southern Wisconsin experienced blizzard conditions due to a powerful storm tracking south of the state. Snow accumulations ranged from 12–26 inches with wind gusts from 45–60 mph. One of Wisconsin’s worst natural disasters was a devastating ice storm in the south central and southeastern portion of the state over March 4–5, 1976. Ice accumulations of up to 5 inches were reported, downing thousands of power lines and snapping many trees and utility poles. Some rural areas were without power for 10 days.
Severe thunderstorms are a threat to the state, particularly during the spring and summer months. A strong derecho on May 30–31, 1998 caused wind gusts of 70–100 mph, with some areas reporting winds of up to 128 mph. Over 250,000 people lost power, and damages were estimated at over $60 million. Although tornadoes are not as common as in other Midwestern states, they can occasionally occur and cause loss of life. On April 10, 2011, severe thunderstorms caused an outbreak of 15 tornadoes, 4 of which were classified as strong (EF-2 and EF-3). Fortunately, the tornadoes impacted relatively rural areas, limiting damages to just over $10 million.
Water levels in the Great Lakes have fluctuated over a range of three to six feet since the late 19th century (Figure 7). Higher lake levels were generally noted in the latter part of the 19th century and early 20th century, the 1940s and 1950s, and the 1980s. Lower lake levels were observed in the 1920s and 1930s and again in the 1960s. For Lakes Superior and Michigan-Huron, the first part of the 21st century has also seen lower levels. However, rapid increases in water levels have been observed in the Great Lakes following historic lows in 2013. In fact, Lakes Michigan and Huron have experienced a remarkable recovery with a rise of more than 3 feet from low levels in 2013.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records (Figure 1). Extreme heat is of particular concern when high temperatures, combined with high humidity, can cause dangerous heat index values with resulting risk to human health. Urban areas are especially vulnerable to extreme heat due to the urban heat island effect and high social vulnerability, which can lead to exacerbated health risks. Future heat waves are projected to be more intense; however, cold waves are projected to be less intense. Winter ice cover on the Great Lakes is projected to decrease.
Precipitation is projected to increase for Wisconsin, with the most likely increases occurring during the winter and spring (Figure 8), but snowfall is projected to decline due to the warmer temperatures. Additionally, extreme precipitation is projected to increase, potentially increasing the frequency and intensity of floods. Above normal precipitation raises the risk of springtime flooding, which could pose a threat to Wisconsin’s agricultural industry by delaying planting and resulting in loss of yield.
The intensity of future droughts is projected to increase. Even if precipitation increases in the future, increases in temperature will increase the rate of loss of soil moisture during dry periods. Thus, future summer droughts, a natural part of the Wisconsin climate, are likely to be more intense.
Changes in seasonal and multi-year precipitation, evaporation, and temperature can affect water levels in the Great Lakes, causing serious environmental and socioeconomic impacts. During the 1980s high lake levels resulted in the destruction of beaches, erosion of shorelines, and the flooding and destruction of near-shore structures. Low lake levels can affect the supply and quality of water, restrict shipping, and result in the loss of wetlands. Future changes in lake levels are uncertain and the subject of research. Reduced winter ice cover from warmer temperatures leaves shores vulnerable to erosion and flooding.