One of the major impacts of climate change will be alterations of the hydrologic cycle, but most global climate models have difficulty estimating future changes in precipitation patterns and rainfall intensities. In general, forecasted precipitation trends for the Great Lakes region are similar regardless of the emissions scenario input. By the end of the century, most models indicate an increase of up to 20% in annual precipitation across the Great Lakes region⁵² but published ranges have varied from a five cm decrease to a 20 cm increase in annual precipitation.⁵³ Most likely, the Great Lakes region will experience wetter than average conditions by 2100.

The seasonality and form of precipitation is expected to be altered by climate change. In the future, more precipitation is expected to fall in the form of rain and less as snow. Since 1980, almost 75% of the winters have seen below historical averages for snowfall.⁵⁴ Over the next few decades, the more southerly states could lose between two to four snow days each year. By the end of the century, the entire Great Lakes region could have 30-60% fewer snow days annually depending on the emissions scenario.⁵⁵ In addition, models project a shift in timing of precipitation from summer to spring.⁵⁶

In the future, more precipitation is expected to fall as rain and in heavy downpours, particularly during winter and spring seasons. Warmer air can hold more water vapor, therefore precipitation events are expected to become more intense and frequent. Heavy downpours, defined as the 10% largest amount of rainfall over a 24-hour period, are now twice as frequent as they were a century ago.⁵⁷ An increase in intense rainfall will increase the likelihood of flooding across the Midwest as precipitation patterns directly influence regional runoff and stream flow.

Secondary Impacts

• Increased flood risk: More intense but less frequent rainfall could enhance flooding regionally. Landscapes will not be able to absorb the waters from a deluge, causing flooding after intense rainstorms. Existing municipal stormwater infrastructure may be overwhelmed during periods of heavy rain, causing increased combined sewer overflows.
   
• Increased runoff and erosion rates: Rainfall generates surface runoff when it falls upon impervious surfaces or soils that are already saturated with water. More intense rainstorms could enhance surface runoff, causing localized erosion as dislodged sediments and topsoil become suspended in the runoff water. As runoff waters flow over various surfaces, chemicals, pollutants, sediments, and nutrients will be transported and deposited into regional streams and rivers, which may degrade water quality and enhance algae blooms.
   
• Increased stream flow: Precipitation runoff supplies roughly 50% of the freshwater stored in the Great Lakes. Water is delivered to the lakes through a vast network of streams and rivers found throughout the drainage basin. On average, annual total runoff is expected to increase by up to 7-9% across all future emissions scenarios. Maximum seasonal runoff is expected to occur in the winter and spring and will have increased average runoff by 20- 60%. Winter and spring will experience an increase in the number of days with high flows, autumn will experience more low-flow days, and summer will have a varied response.⁵⁸
   
• Soil moisture: Warmer summertime weather and increased transpiration coupled with altered precipitation frequency, intensity, and duration could lower summertime soil moisture levels. This will affect agricultural production as well as precipitation runoff generation throughout the year.⁵⁹